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Many mammals on land seem to be quite dangerous to humans: e.g., tigers, lions, elephants, hypos etc. However, their sea-dwelling counterparts (e.g., orcas, dolphins, sea lions, whales etc.) are relatively friendly to humans (and in contrast to say sharks). Is there a simple reason for this?
More encounters on land
There are more land animals that meet humans walking around than there are sea animals that meet humans swimming around. If fewer attacks happen on humans in the sea environment, it might just be because there are fewer encounters with marine mammals than with land mammals.
Greater species diversity on land
In your list used as example, you cherry-picked a few aggressive(-ish) land mammals but the vast majority of land mammals are not aggressive toward humans. The fact that there are no marine mammal that are aggressive toward humans might just be because there are too few fewer marine mammal species.
Pseudoreplication and phylogenetic signal
I would argue that your sample size is not as large as you might think. There are only four lineages of mammals that evolve to live in the seas. There is therefore a phylogenetic signal and ignoring it would be a problem of pseudo replication.
For example, I could very well think of territorial behaviour (which is where a lot of aggressive behaviour comes from) as having a strong phylogenetic signal.
Why considering only humans?
Your post title talks about aggressiveness in general and the content of your post talks about attacks against humans. It sounds that considering only attacks against humans to be very unfair. Orcas, typically, are fierce and violent predators although no orcas have ever attacked a human in the wild.
Are they really that friendly to humans?
There has been a number of attacks in captivity (well known cases with orcas). Also, there have been a number of whales attacks against boats (according to this BBC article).
In short, I think your comparison is unfair. I think you are looking for an explanation for a pattern that does not really exist.
A cave is a hollow place in the ground, usually large enough for an adult human to enter. Caves are formed by natural processes of weathering and might extend quite deep underground. Smaller openings on the ground like rock shelters, sea caves, and grottos are also designated as caves.
Caves are found throughout the world in all the continents including the frozen continent of Antarctica. People exploring caves are called cavers, and the amateur activity of exploring caves and cave animals is called caving. A large number of caves have been documented in countries across the world where caving is a popular activity. Speleology is the science of cave exploration and study. In areas where caves are located in inaccessible locations like those in the Amazon rainforest of Brazil or underneath the ice caps of Antarctica, little is known about the caves and the cave dwelling animals in such caves.
- OrderSirenia: sirenians
- Family Trichechidae: manatees
- (Trichechus inunguis) (Trichechus senegalensis) (Trichechus pygmaeus) validity questionable
- Suborder Whippomorpha
- Family Platanistidae
- (Platanista gangetica) with two subspecies
- , or susu (Platanista gangetica gangetica) , or bhulan (Platanista gangetica minor)
- , or boto (Inia geoffrensis) (Inia araguaiaensis)
- , or baiji (Lipotes vexillifer) functionally extinct since December 2006
- , or franciscana (Pontoporia blainvillei)
- (Hippopotamus amphibius) (Choeropsis liberiensis)
- Family Mustelidae
- Subfamily Lutrinae
- (Lutra lutra) (Lutra sumatrana) (Hydrictis maculicollis) (Lutrogale perspicillata) (Lontra canadensis) (Lontra provocax) (Lontra longicaudis) (Pteronura brasiliensis) (Aonyx capensis) (Aonyx cinerea)
- (Mustela lutreola) (Neovison vison)
- Genus Pusa
- (Pusa sibirica) (Pusa hispida ladogensis) (Pusa hispida saimensis)
- Suborder Hystricomorpha
- (Hydrochoerus hydrochaeris) (Hydrochoerus isthmius) (Myocastor coypus)
- (Castor canadensis) (Castor fiber)
- (Ondatra zibethicus) (Arvicola amphibius)
- (Ornithorhynchus anatinus)
- Family Rhinocerotidae: rhinoceroses
- (Rhinoceros sondaicus) (Rhinoceros unicornis)
- (Potamogale velox)
- Family Soricidae: shrews
- (Chimarrogale hantu) (Chimarrogale himalayica) (Chimarrogale phaeura) (Chimarrogale platycephala) (Chimarrogale styani) (Chimarrogale sumatrana) (Nectogale elegans) (Neomys anomalus) (Neomoys fodiens) (Neomys teres) (Sorex alaskanus) (Sorex palustris) , or marsh shrew (Sorex bendirii)
- (Desmana moschata)
- Family Didelphidae: opossums
- (Lutreolina crassicaudata) (Chironectes minimus)
One of the first known proto-mammals similar to modern placentals was aquatic, the Jurassic therapsid Castorocauda. It seems to have been adapted to water much like a beaver, with teeth different in many ways from all other docodonts, presumably due to a difference in diet. Most docodonts had teeth specialized for an omnivorous diet. The teeth of Castorocauda suggest that the animal was a piscivore, feeding on fish and small invertebrates. The first two molars had cusps in a straight row, eliminating the grinding function suggesting that they were strictly for gripping and not for chewing. This feature of three cusps in a row is similar to the ancestral condition in mammal relatives (as seen in triconodonts), but is almost certainly a derived character in Castorocauda. These first molars were also recurved in a manner adapted to hold slippery prey once grasped. These teeth are very similar to the teeth seen in mesonychids, an extinct group of semiaquatic carnivorous ungulates, and resemble, to a lesser degree, the teeth of seals. 
Another docodontan, the Late Jurassic Haldanodon, has been suggested to be a platypus or desman-like swimmer and burrower, being well adapted to dig and swim and occurring in a wetland environment. 
The tritylodontid Kayentatherium has been suggested to be semiaquatic. Unlike Castorocauda and Haldanodon, it was an herbivore, being probably beaver or capybara-like in habits. 
Another lineage of Mesozoic mammals, the eutriconodonts, have been suggested to be aquatic animals with mixed results. Astroconodon occurred abundantly in freshwater lacustrine deposits and its molars were originally interpreted as being similar to those of piscivorous mammals like cetaceans and pinnipeds by extension some researchers considered the possibility that all eutriconodonts were aquatic piscivores.  However, Zofia Kielan-Jaworowska and other researchers have latter found that the triconodont molars of eutriconodonts were more suited for a carnassial-like shearing action than the piercing and gripping function of piscivorous mammal molars, occluding instead of interlocking, and that Astroconodon's aquatic occurrences may be of little significance when most terrestrial tetrapod fossils are found in lacustrine environments anyway. 
However, two other eutriconodonts, Dyskritodon and Ichthyoconodon, occur in marine deposits with virtually no dental erosion, implying that they died in situ and are thus truly aquatic mammals.  Nonetheless, Ichthyoconodon may not be aquatic, but instead a gliding or even flying mammal.   More recently, Yanoconodon and Liaoconodon have been interpreted as semiaquatic, bearing a long body and paddle-like limbs. 
A metatherian, the stagodontid Didelphodon, has been suggested to be aquatic, due to molar and skeleton similarities to sea otters. 
An extinct genus, Satherium, is believed to be ancestral to South American river otters, having migrated to the New World during the Pliocene or early Pleistocene.  The South American continent houses the otter genus Lontra: the giant otter, the neotropical river otter, the southern river otter, and the marine otter.  The smooth-coated otter (Lutrogale perspicillata) of Asia may be its closest extant relative similar behaviour, vocalizations, and skull morphology have been noted. 
The most popular theory of the origins of Hippopotamidae suggests that hippos and whales shared a common ancestor that branched off from other artiodactyls around 60 million years ago (mya).   This hypothesized ancestral group likely split into two branches around 54 mya.  One branch would evolve into cetaceans, possibly beginning about 52 mya, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans.  The other branch became the anthracotheres, and all branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.  River dolphins are thought to have relictual distributions, that is, their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages.   Many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution thus, a grouping of all river dolphins is paraphyletic. For example, Amazon river dolphins are actually more closely related to oceanic dolphins than to South Asian river dolphins. 
Sirenians, along with Proboscidea (elephants), group together with the extinct Desmostylia and likely the extinct Embrithopoda to form the Tethytheria. Tethytheria is thought to have evolved from primitive hoofed mammals ("condylarths") along the shores of the ancient Tethys Ocean. Tethytheria, combined with Hyracoidea (hyraxes), forms a clade called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations. That is, elephants, hyraxes, and manatees share a common ancestry.  The ancestry of Sirenia is distinct from that of Cetacea and Pinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time. 
The oldest fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period. The extinct monotremes Teinolophos and Steropodon were once thought to be closely related to the modern platypus,  but more recent studies show that platypi are more related to the modern echidnas than to these ancient forms and that at least Teinolophos was a rather different mammal lacking several speciations seen in platypi.  However, the last common ancestor between platypi and echidnas probably was aquatic, and echidnas thus secondarily became terrestrial.  Monotrematum sudamericanum is currently the oldest aquatic monotreme known. It has been found in Argentina, indicating monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica, or that monotremes existed along the shorelines of Antarctica in the early Cenozoic. 
Marine mammals Edit
Marine mammals are aquatic mammals that rely on the ocean for their existence. They include animals such as sea lions, whales, dugongs, sea otters and polar bears. Like other aquatic mammals, they do not represent a biological grouping. 
Marine mammal adaptation to an aquatic lifestyle vary considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semiaquatic they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living.  Likewise, their diet ranges considerably as well some may eat zooplankton,  others may eat small fish,  and a few may eat other mammals.  While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large. They, namely sea otters and polar bears, play important roles in maintaining marine ecosystems, especially through regulation of prey populations.   Their role in maintaining ecosystems makes them of particular concern considering 23% of marine mammal species are currently threatened. 
Marine mammals were first hunted by aboriginal peoples for food and other resources.  They were also the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as whales and seals. Commercial hunting lead to the extinction of Steller's sea cow and the Caribbean monk seal.   After commercial hunting ended, some species, such as the gray whale and northern elephant seal,   have rebounded in numbers, however the northern elephant seal has a genetic bottleneck  conversely, other species, such as the North Atlantic right whale, are critically endangered.  Other than hunting, marine mammals, dolphins especially, can be killed as bycatch from fisheries, where they become entangled in fixed netting and drown or starve.  Increased ocean traffic causes collisions between fast ocean vessels and large marine mammals.  Habitat degradation also threatens marine mammals and their ability to find and catch food. Noise pollution, for example, may adversely affect echolocating mammals,  and the ongoing effects of global warming degrades arctic environments. 
Mammals evolved on land, so all aquatic and semiaquatic mammals have brought many terrestrial adaptations into the waters. They do not breathe underwater as fish do, so their respiratory systems had to protect the body from the surrounding water valvular nostrils and an intranarial larynx exclude water while breathing and swallowing. To navigate and detect prey in murky and turbid waters, aquatic mammals have developed a variety of sensory organs: for example, manatees have elongated and highly sensitive whiskers which are used to detect food and other vegetation directly front of them,  and toothed whales have evolved echolocation. 
Aquatic mammals also display a variety of locomotion styles. Cetaceans excel in streamlined body shape and the up-and-down movements of their flukes make them fast swimmers the tucuxi, for example, can reach speeds of 14 miles per hour (23 km/h).  The considerably slower sirenians can also propel themselves with their fluke, but they can also walk on the bottom with their forelimbs.  The earless seals (Phocidae) swim by moving their hind-flippers and lower body from side to side, while their fore-flippers are mainly used for steering.  They are clumsy on land, and move on land by lunging, bouncing and wiggling while their fore-flippers keep them balanced  when confronted with predators, they retreat to the water as freshwater phocids have no aquatic predators. 
Some aquatic mammals have retained four weight-bearing limbs (e.g. hippopotamuses, beavers, otters, muskrats) and can walk on land like fully terrestrial mammals. The long and thin legs of a moose limit exposure to and friction from water in contrast to hippopotamuses who keep most of their body submerged and have short and thick legs. The semiaquatic pygmy hippopotamus can walk quickly on a muddy underwater surface thanks to robust muscles and because all toes are weight-bearing. Some aquatic mammals with flippers (e.g. seals) are amphibious and regularly leave the water, sometimes for extended periods, and maneuver on land by undulating their bodies to move on land, similar to the up-and-down body motion used underwater by fully aquatic mammals (e.g. dolphins and manatees). 
Beavers, muskrats, otters, and capybara have fur, one of the defining mammalian features, that is long, oily, and waterproof in order to trap air to provide insulation.  In contrast, other aquatic mammals, such as dolphins, manatees, seals, and hippopotamuses, have lost their fur in favor of a thick and dense epidermis, and a thickened fat layer (blubber) in response to hydrodynamic requirements. 
Wading and bottom-feeding animals (e.g. moose and manatee) need to be heavier than water in order to keep contact with the floor or to stay submerged, surface-living animals (e.g. otters) need the opposite, and free-swimming animals living in open waters (e.g. dolphins) need to be neutrally buoyant in order to be able to swim up and down the water column. Typically, thick and dense bone is found in bottom feeders and low bone density is associated with mammals living in deep water. 
The shape and function of the eyes in aquatic animals are dependent on water depth and light exposure: limited light exposure results in a retina similar to that of nocturnal terrestrial mammals. Additionally, cetaceans have two areas of high ganglion cell concentration ("best-vision areas"), where other aquatic mammals (e.g. seals, manatees, otters) only have one. 
Among non-placental mammals, which cannot give birth to fully developed young,  some adjustments have been made for an aquatic lifestyle. The yapok has a backwards-facing pouch which seals off completely when the animal is underwater, while the platypus deposits its young on a burrow on land.
Keystone species Edit
Beaver ponds have a profound effect on the surrounding ecosystem. Their first and foremost ecological function is as a reservoir for times of drought, and prevent drying of riverbeds. In the event of a flood, beaver ponds slow down water-flow which reduces erosion on the surrounding soil.  Beaver dams hold sediment, which reduces turbidity and thereby improving overall water quality downstream. This supplies other animals with cleaner drinking water and prevents degradation of spawning grounds for fish.   However, the slower water speed and lack of shade from trees (that have since been cut down to construct the dam), results in the overall temperature increasing.  They also house predatory zooplankton which help break down detritus and control algae populations. 
Beavers are herbivores, and prefer the wood of quaking aspen, cottonwood, willow, alder, birch, maple and cherry trees. They also eat sedges, pondweed, and water lilies.  Beavers do not hibernate, but rather they store sticks and logs in a pile in their ponds, eating the underbark. The dams they build flood areas of surrounding forest, giving the beaver safe access to an important food supply, which is the leaves, buds, and inner bark of growing trees. They prefer aspen and poplar, but will also take birch, maple, willow, alder, black cherry, red oak, beech, ash, hornbeam and occasionally pine and spruce.  They will also eat cattails, water lilies and other aquatic vegetation, especially in the early spring. 
Indian rhinoceros are grazers. Their diets consist almost entirely of grasses, but they also eat leaves, branches of shrubs and trees, fruits, and submerged and floating aquatic plants. They feed in the mornings and evenings. They use their prehensile lips to grasp grass stems, bend the stem down, bite off the top, and then eat the grass. They tackle very tall grasses or saplings by walking over the plant, with legs on both sides and using the weight of their bodies to push the end of the plant down to the level of the mouth. 
Manatees make seasonal movements synchronized with the flood regime of the Amazon Basin. They are found in flooded forests and meadows during the flood season when food is abundant, and move to deep lakes during the dry season.  The Amazonian manatee has the smallest degree of rostral deflection (25° to 41°) among sirenians, an adaptation to feed closer to the water surface. 
A moose's diet often depends on its location, but they seem to prefer the new growths from deciduous trees with a high sugar content, such as white birch, trembling aspen and striped maple, among many others.  They also eat many aquatic plants such as lilies and water milfoil.  To reach high branches, a moose may bend small saplings down, using its prehensile lip, mouth or body. For larger trees a moose may stand erect and walk upright on its hind legs, allowing it to reach plants 14.0 feet (4.26 m) off the ground.   Moose are excellent swimmers and are known to wade into water to eat aquatic plants. Moose are thus attracted to marshes and river banks during warmer months as both provide suitable vegetation to eat and water to bathe in. Moose have been known to dive underwater to reach plants on lake bottoms, and the complex snout may assist the moose in this type of feeding. Moose are the only deer that are capable of feeding underwater. 
Hippopotamuses leave the water at dusk and travel inland, sometimes up to 10 km (6 mi),  to graze on short grasses, their main source of food. They spend four to five hours grazing and can consume 68 kg (150 lb) of grass each night.  Like almost any herbivore, they consume other plants if presented with them, but their diet consists almost entirely of grass, with only minimal consumption of aquatic plants.  The pygmy hippopotamus emerges from the water at dusk to feed. It relies on game trails to travel through dense forest vegetation. It marks trails by vigorously waving its tail while defecating to further spread its feces. The pygmy hippo spends about six hours a day foraging for food, and they do not eat aquatic vegetation to a significant extent and rarely eat grass because it is uncommon in the thick forests they inhabit. The bulk of a pygmy hippo's diet consists of ferns, broad-leaved plants and fruits that have fallen to the forest floor. The wide variety of plants pygmy hippos have been observed eating suggests that they will eat any plants available. This diet is of higher quality than that of the common hippopotamus.  
The Amazon river dolphin has the most diverse diet among cetaceans, consisting of at least 53 species of fish. They mainly feed on croakers, cichlids, tetras, and piranhas, but they may also target freshwater crabs and river turtles.  South Asian river dolphins mainly eat fish (such as carp, catfish, and freshwater sharks) and invertebrates, mainly prawns. 
Generally, all aquatic desmans, shrews, and voles make quick dives and catch small fish and invertebrates. The giant otter shrew, for example, makes quick dives that last for seconds and grabs small crabs (usually no bigger than 2.8 inches (7 cm) across).  The Lutrine opossum is the most carnivorous opossum, usually consuming small birds, rodents, and invertebrates.  Water voles mainly eat grass and plants near the water and at times, they will also consume fruits, bulbs, twigs, buds, and roots. However, a population of water voles living in Wiltshire and Lincolnshire, England have started eating frogs' legs and discarding the bodies. 
Fur robes were blankets of sewn-together, native-tanned, beaver pelts. The pelts were called castor gras in French and "coat beaver" in English, and were soon recognized by the newly developed felt-hat making industry as particularly useful for felting. Some historians, seeking to explain the term castor gras, have assumed that coat beaver was rich in human oils from having been worn so long (much of the top-hair was worn away through usage, exposing the valuable under-wool), and that this is what made it attractive to the hatters. This seems unlikely, since grease interferes with the felting of wool, rather than enhancing it.  By the 1580s, beaver "wool" was the major starting material of the French felt-hatters.  Hat makers began to use it in England soon after, particularly after Huguenot refugees brought their skills and tastes with them from France. 
Sport hunting of the Indian rhinoceros became common in the late 1800s and early 1900s.  Indian rhinos were hunted relentlessly and persistently. Reports from the middle of the 19th century claim that some British military officers in Assam individually shot more than 200 rhinos. By 1908, the population in Kaziranga had decreased to around 12 individuals.  In the early 1900s, the species had declined to near extinction.  Poaching for rhinoceros horn became the single most important reason for the decline of the Indian rhino after conservation measures were put in place from the beginning of the 20th century, when legal hunting ended. From 1980 to 1993, 692 rhinos were poached in India. In India's Laokhowa Wildlife Sanctuary, 41 rhinos were killed in 1983, virtually the entire population of the sanctuary.  By the mid-1990s, poaching had rendered the species extinct there.  In 1950, Chitwan’s forest and grasslands extended over more than 2,600 km 2 (1,000 sq mi) and were home to about 800 rhinos. When poor farmers from the mid-hills moved to the Chitwan Valley in search of arable land, the area was subsequently opened for settlement, and poaching of wildlife became rampant. The Chitwan population has repeatedly been jeopardized by poaching in 2002 alone, poachers killed 37 animals to saw off and sell their valuable horns. 
Otters have been hunted for their pelts since at least the 1700s. There has been a long history of otter pelts being worn around the world. In China it was standard for the royalty to wear robes made from them. People that were financially high in status also wore them.  Otters have also been hunted using dogs, specifically the otterhound.  In modern times, TRAFFIC, a joint program of the World Wildlife Fund (WWF) and International Union for Conservation of Nature (IUCN), reported that otters are at serious risk in Southeast Asia and have disappeared from parts of their former range. This decline in populations is due to hunting to supply the demand for skins. 
Habitat degradation Edit
One problem at Lake Baikal is the introduction of pollutants into the ecosystem. Pesticides such as DDT and hexachlorocyclohexane, as well as industrial waste, mainly from the Baykalsk Pulp and Paper Mill, are thought to have been the cause of several disease epidemics among Baikal seal populations.  The chemicals are speculated to concentrate up the food chain and weaken the Baikal seal's immune system, making them susceptible to diseases such as canine distemper and the plague, which was the cause of a serious Baikal seal epidemic that resulted in the deaths of thousands of animals in 1997 and 1999. Baikal seal pups have higher levels of DDT and PCB than known in any other population of European or Arctic earless seal. 
In the 1940s, beavers were brought from Canada to the island of Tierra Del Fuego in southern Chile and Argentina, for commercial fur production. However, the project failed and the beavers, ten pairs, were released into the wild. Having no natural predators in their new environment, they quickly spread throughout the island, and to other islands in the region, reaching a number of 100,000 individuals within just 50 years. They are now considered a serious invasive species in the region, due to their massive destruction of forest trees, and efforts are being made for their eradication. 
In some European countries, such as Belgium, France, and the Netherlands, the muskrat is considered an invasive pest, as its burrowing damages the dikes and levees on which these low-lying countries depend for protection from flooding. In those countries, it is trapped, poisoned, and hunted to attempt to keep the population down. Muskrats also eat corn and other farm and garden crops growing near water bodies. 
Urban and agricultural development, increased damming, and increased use of hydroelectric power in rivers in countries such as Côte d'Ivoire and Ghana are threats to the African manatee's habitat and life, and thick congestion of boats in waterways may cause them to have a deadly run-in with the vessels. However, even natural occurrences, such as droughts and tidal changes, often strand manatees in an unsuitable habitat. Some are killed accidentally by fishing trawls and nets intended for catching sharks.  The Amazonian manatee is at risk from pollution, accidental drowning in commercial fishing nets, and the degradation of vegetation by soil erosion resulting from deforestation.  Additionally, the indiscriminate release of mercury in mining activities threatens the entire aquatic ecosystem of the Amazon Basin. 
As China developed economically, pressure on the Chinese river dolphin grew significantly.  Industrial and residential waste flowed into the Yangtze. The riverbed was dredged and reinforced with concrete in many locations. Ship traffic multiplied, boats grew in size, and fishermen employed wider and more lethal nets. Noise pollution caused the nearly blind animal to collide with propellers. Stocks of the dolphin's prey declined drastically in the late 20th century, with some fish populations declining to one thousandth of their pre-industrial levels.  In the 1950s, the population was estimated at 6,000 animals,  but declined rapidly over the subsequent five decades. Only a few hundred were left by 1970. Then the number dropped down to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. On December 13, 2006, the baiji was declared functionally extinct, after a 45-day search by leading experts in the field failed to find a single specimen. The last verified sighting was in 2004. 
As food Edit
Moose are hunted as a game species in many of the countries where they are found. While the flesh has protein levels similar to those of other comparable red meats (e.g. beef, deer and elk), it has a low fat content, and the fat that is present consists of a higher proportion of polyunsaturated fats rather than saturated fats. 
. like tender beef, with perhaps more flavour sometimes like veal”
Cadmium levels are high in moose liver and kidneys, with the result that consumption of these organs from moose more than one year old is prohibited in Finland.  Cadmium intake has been found to be elevated amongst all consumers of moose meat, though the meat was found to contribute only slightly to the daily cadmium intake. However the consumption of moose liver or kidneys significantly increased cadmium intake, with the study revealing that heavy consumers of moose organs have a relatively narrow safety margin below the levels which would probably cause adverse health effects. 
In the 17th century, based on a question raised by the Bishop of Quebec, the Roman Catholic Church ruled that the beaver was a fish (beaver flesh was a part of the indigenous peoples' diet, prior to the Europeans' arrival  ) for purposes of dietary law. Therefore, the general prohibition on the consumption of meat on Fridays did not apply to beaver meat.  This is similar to the Church's classification of other semiaquatic rodents, such as the capybara and muskrat.  
Evolving to dive deep
David Aldridge is a phytoplankton-loving marine biology PhD student at the National Oceanography Centre in Southampton, UK. Also the founder and editor of Words in mOcean, a website dedicated to publishing blog posts and features on marine science. We’ve asked David to guest post for us here at DSN. Enjoy!
We’ve all tried this before in a swimming pool or in the ocean, I’m sure: you hold your breath, dive under water, and try to swim as far as you can in an attempt to imitate that killer whale you saw at SeaWorld. After what feels like a life-time, but in all reality is a pitiful 30 seconds of lung-burning agony, you splutter to the surface gasping for oxygen like Newt Gingrich after 10 minutes on a treadmill. You ask yourself, “How on earth does Shamu do that?” The key to how marine mammals manage to hold their breath for so long — for well over an hour in the case of elephant seals and sperm whales — lies largely within their muscles, which are jam-packed full of myoglobin. Myoglobin is haemoglobin’s close relative and like most close relatives, it’s really good at showing up its inferior relation. It ferociously sucks up and holds onto oxygen, and is also less prone to being damaged by the acidic environment in muscles that follows oxygen-starvation it is just what a diving sea creature needs. We also have myoglobin in our muscles, but marine mammals can have more than 30 times the amount that we do. As a general rule, the more myoglobin in the muscles of a mammal, the longer it can “hold its breath”.
A 3D computer model of myoglobin, the oxygen-binding molecule that helps marine mammals to hold their breath for such a long time.
A group of scientists, led by Scott Mirceta from the University of Liverpool in the UK, have modelled the evolutionary history of myoglobin over the last 200 million years. They show that as marine mammals evolved, not only did the amount of myoglobin in their muscles increase, but that the myoglobin proteins became increasingly charged (less “sticky”), and therefore better able to repel each other, and do their job, whilst still carrying plenty of oxygen to the muscles. The scientists also managed to estimate how much myoglobin was found in the evolutionary ancestors of many marine mammals. The study provides many insights into the evolution of dolphins, whales, seals, and sea lions from terrestrial mammals — resembling you or me gasping for oxygen in a swimming pool — to the slick divers they are today.
The diving capability (represented by myoglobin surface charge) of modern marine mammals compared to their, now extinct, ancestors (source: Mirceta et al., 2013).
The ancestors of modern Paenungulates (a taxa including elephants, manatees and dugongs) were shown to be the first group of land mammals that took to the sea more than 64 million years ago. After making the water their home, increases in body size were a big evolutionary advantage allowing organisms to dive for longer (larger organisms can store more myoglobin and also have lower metabolisms). This is one reason why many marine mammals are so much bigger than their land-dwelling relatives.
We think of elephants as being huge, but marine mammals think they are puny and weak.
One finding which perhaps isn’t too surprising is that the ancestors of today’s whales, dolphins, seals and walruses had low levels of myoglobin when they first ventured into the sea. They would have been pretty damn useless at diving for any sustained period of time and almost certainly foraged for food, and went about their everyday business, in shallow waters. Some creatures, such as the manatees, never really changed they have low myoglobin concentrations in their muscles to this day and eek out their existence feeding on seagrass in shallow waters. But those organisms that began to pack their muscles with myoglobin were able to dive deeper and longer and, in the process, opened up a whole new world of feeding opportunities for themselves. They evolved from inelegant oxygen-gasping animals feeding near the surface, to highly refined deep-diving marvels of the ocean, capable of holding their breath for up to an hour, or more, and diving hundreds to thousands of metres beneath the surface. All they needed was millions of years of evolution. Unfortunately, you and I don’t have that sort of time. In the absence of a culture of “myoglobin-doping” (don’t laugh, cyclists are already talking about it!), it looks like we will have to make do with our pathetic 30 seconds of underwater suffering the next time we are in the water.
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Craig McClain is the Executive Director of the Lousiana University Marine Consortium. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Additionally, Craig is obsessed with the size of things. Sometimes this translated into actually scientific research. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (http://deepseanews.com/), a popular ocean-themed blog that has won numerous awards. His writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web.
3 thoughts on &ldquo DNA suggests whales descended from land mammals &rdquo
hi christopher. what do you think about this paper?:
I believed I addressed a similar topic in a previous discussion we had, but in case it wasn’t clear, I’ll try to elaborate on it here. This might sound a bit complicated, which may be even more difficult since English isn’t your native language, so please let me know if you need to be explain anything better!
So within the world of molecular phylogenetics research, the science of using DNA to reconstruct hereditary relationships among organisms, you basically have three types of results:
(1) some relationships amongst organisms are always or almost always recovered, no matter how much data you use. From my own experience, for instance, I believe every analysis I’ve ever performed results in tigers and domestic cats coming out more similar to each other than they are to other non-cat animals.
(2) some relationships are not apparent with small datasets (e.g., too few letters of DNA for comparison, not enough species for comparison) but become more and more robustly supported with larger datasets. One example of this might be the relationship of turtles to other reptiles. Initially they jumped all over the place (e.g., closer to lizards, closer to crocs, closer to crocs + birds, etc.) but with larger datasets, they seem to be consistently coming out as closest to birds + crocs. In other words, it appears to be a statistical power issue, namely that there isn’t enough information to infer the correct relationship.
(3) some relationships, regardless of dataset size, give you different results depending on the precise methodology or precise sets of genes you use. The base of placental mammals, which the paper you provided a link to refers, is one of these examples. Two major groups of mammals always come out together: Laurasiatheria (carnivores, hoofed mammals, whales, pangolins, insectivores, bats) + Euarchontoglires (primates, flying lemurs, tree shrews, rodents, rabbits and pikas). Together, this large group is called Boreoeutheria. The rest of placental mammals include Afrotheria (elephants, manatees, hyraxes, African insectivores, aardvark, elephant shrews) and Xenarthra (sloths, armadillos, anteaters). The relative positioning of Boreoeutheria, Afrotheria and Xenarthra switches depending on the analysis.
So examples in category 3 would seem to imply evidence against evolutionary theory. Different dataset, different analysis, and you get different relationships means that these results can’t be trusted, right?
Well, it’s a little more complicated than that. First off, in my experience, the vast majority of examples fall into categories 1 and 2 described above, meaning there seems to be a consistent signal in the DNA pointing to hereditary relationships between organisms. You never get walruses being more genetically similar to flies than other mammals, or lemon trees being more genetically similar to penguins than other plants.
But how do we explain examples in category 3? First off, nearly all examples in category 3 result in the inference of what we call short branches in phylogenetic trees. What this means is that there are very few differences in DNA that seem to unite one group of organisms with another. Assuming evolutionary theory accurately describes reality, what would lead to such few shared DNA substitutions? First off, if the lineages split very quickly, then there may not have been enough time for DNA substitutions to be fixed in their common ancestor, or any shared substitutions may have been erased with reversals in the DNA over time.
Alternatively, perhaps ample time passed, but the organisms evolved very slowly. The speed of evolution depends on a number of things including the population size, whether natural selection is acting on the mutation, and how quickly does the organism reproduce. We think whales, for instance, are notoriously slowly evolving, in part because they typically only have a single baby and it takes a long time for them to become sexually active.
Assuming the evolution has not happened very quickly or the amount of time that has passed was minimal, then interbreeding may have a major effect. Normally the way we think new species form, corresponding to those branches in phylogenies, is that a population of organisms splits into two, and the two populations stay separate and evolve into different species. If they stay isolated or at least don’t recognize each other as potential mates, then they will eventually be very different from each other. Now imagine two populations evolving separately from one another, and they are each accumulating different DNA substitutions, but for some reason an individual from population A mates with an individual of population B. That hybrid will have a mix of DNA from each population. If that hybrid then mates back with population A, and the population B DNA is neutral or provides an advantage, then it can spread throughout the population. This is a phenomenon called introgression, which you may have read in the news in the context of some humans possessing Neanderthal DNA.
So going back to the base of placental mammals: if there was a rapid splitting between one population into three (the boreoeutherian, xenarthran and afrotherian lineages) and/or introgression between one or more of the populations, then there would be minimal signal in the DNA to separate the different lineages. In fact, because of the very nature of these phenomena, you can get different results depending on the sets of genes you use.
thanks for the explanation christopher. i think i understand what you mean (more or less).
The name "pinniped" derives from the Latin words pinna "fin" and pes, pedis "foot".  The common name "seal" originates from the Old English word seolh, which is in turn derived from the Proto-Germanic *selkhaz. 
The German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit in 1811 he gave the name Pinnipedia to both a family and an order.  American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph, History of North American pinnipeds, a monograph of the walruses, sea-lions, sea-bears and seals of North America. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world.  In 1989, Annalisa Berta and colleagues proposed the unranked clade Pinnipedimorpha to contain the fossil genus Enaliarctos and modern seals as a sister group.  Pinnipeds belong to the order Carnivora and the suborder Caniformia (known as dog-like carnivorans).  Pinnipedia was historically considered its own suborder under Carnivora.  Of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea,  while the Phocidae belong to the superfamily Phocoidea. 
Otariids are also known as eared seals due to the presence of pinnae. These animals rely on their well-developed fore-flippers to propel themselves through the water. They can also turn their hind-flippers forward and "walk" on land.  The anterior end of an otariid's frontal bones extends between the nasal bones, and the supraorbital foramen is large and flat horizontally. The supraspinatous fossas are divided by a "secondary spine" and the bronchi are divided anteriorly.  Otariids consist of two types: sea lions and fur seals. Sea lions are distinguished by their rounder snouts and shorter, rougher pelage, while fur seals have more pointed snouts, longer fore-flippers and thicker fur coats that include an undercoat and guard hairs. The former also tend to be larger than the latter.  Five genera and seven species (one now extinct) of sea lion are known to exist, while two genera and nine species of fur seal exist. While sea lions and fur seals have historically been considered separate subfamilies (Otariinae and Arctocephalinae respectively), a 2001 genetic study found that the northern fur seal is more closely related to several sea lion species.  This is supported by a 2006 molecular study that also found that the Australian sea lion and New Zealand sea lion are more closely related to Arctocephalus than to other sea lions. 
Odobenidae consists of only one living member: the modern walrus. This animal is easily distinguished from other extant pinnipeds by its larger size (exceeded only by the elephant seals), nearly hairless skin and long upper canines, known as tusks. Like otariids, walruses are capable of turning their hind-flippers forward and can walk on land. When moving in water, the walrus relies on its hind-flippers for locomotion, while its fore-flippers are used for steering. In addition, the walrus lacks external ear flaps.  Walruses have pterygoid bones that are broad and thick, frontal bones that are V-shaped at the anterior end and calcaneuses with pronounced tuberosity in the middle. 
Phocids are known as true or "earless" seals. These animals lack external ear flaps and are incapable of turning their hind-flippers forward, which makes them more cumbersome on land. In water, true seals swim by moving their hind-flippers and lower body from side to side.  Phocids have thickened mastoids, enlarged entotympanic bones, everted pelvic bones and massive ankle bones. They also lack supraorbital processes on the frontal and have underdeveloped calcaneal tubers.  A 2006 molecular study supports the division of phocids into two monophyletic subfamilies: Monachinae, which consists of Mirounga, Monachini and Lobodontini and Phocinae, which includes Pusa, Phoca, Halichoerus, Histriophoca, Pagophilus, Erignathus and Cystophora. 
In a 2012 review of pinniped taxonomy, Berta and Morgan Churchill suggested that, based on morphological and genetic criteria, there are 33 extant species and 29 subspecies of pinnipeds, although five of the latter lack sufficient support to be conclusively considered subspecies. They recommend that the genus Arctocephalus be limited to Arctocephalus pusillus, and they resurrected the name Arctophoca for several species and subspecies formerly placed in Arctocephalus.  More than 50 fossil species have been described. 
Evolutionary history Edit
One popular hypothesis suggested that pinnipeds are diphyletic (descended from two ancestral lines), with walruses and otariids sharing a recent common ancestor with bears and phocids sharing one with Musteloidea. However, morphological and molecular evidence support a monophyletic origin.  Nevertheless, there is some dispute as to whether pinnipeds are more closely related to bears or musteloids, as some studies support the former theory    and others the latter.    Pinnipeds split from other caniforms 50 million years ago (mya) during the Eocene.  Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of Puijila in early Miocene deposits in Nunavut, Canada. Like a modern otter, Puijila had a long tail, short limbs and webbed feet instead of flippers. However, its limbs and shoulders were more robust and Puijila likely had been a quadrupedal swimmer—retaining a form of aquatic locomotion that gave rise to the major swimming types employed by modern pinnipeds. The researchers who found Puijila placed it in a clade with Potamotherium (traditionally considered a mustelid) and Enaliarctos. Of the three, Puijila was the least specialized for aquatic life. The discovery of Puijila in a lake deposit suggests that pinniped evolution went through a freshwater transitional phase. 
Enaliarctos, a fossil species of late Oligocene/early Miocene (24–22 Mya) California, closely resembled modern pinnipeds it was adapted to an aquatic life with a flexible spine, and limbs modified into flippers. Its teeth were adapted for shearing (like terrestrial carnivorans), and it may have stayed near shore more often than its extant relatives. Enaliarctos was capable of swimming with both the fore-flippers and hind-flippers, but it may have been more specialized as a fore-flipper swimmer.  One species, Enaliarctos emlongi, exhibited notable sexual dimorphism, suggesting that this physical characteristic may have been an important driver of pinniped evolution.  A closer relative of extant pinnipeds was Pteronarctos, which lived in Oregon 19–15 mya. As in modern seals, Pteroarctos had an orbital wall that was not limited by certain facial bones (like the jugal or lacrimal bone), but was mostly shaped by the maxilla. The extinct family Desmatophocidae lived 23–10 Mya in the North Atlantic and had elongated skulls, fairly large eyes, cheekbones connected by a mortised structure and rounded cheek teeth. They also were sexually dimorphic and may have been capable of propelling themselves with both the foreflippers and hindflippers. 
The ancestors of the Otarioidea and Phocoidea diverged 33 mya.  Phocids are known to have existed for at least 15 million years,  and molecular evidence supports a divergence of the Monachinae and Phocinae lineages 22 Mya.  The fossil monachine Monotherium and phocine Leptophoca were found in southeastern North America. The deep split between the lineages of Erignathus and Cystophora 17 Mya suggests that the phocines migrated eastward and northward from the North Atlantic. The genera Phoca and Pusa could have arisen when a phocine lineage traveled from the Paratethys Sea to the Arctic Basin and subsequently went eastward. The ancestor of the Baikal seal migrated into Lake Baikal from the Arctic (via the Siberian ice sheet) and became isolated there. The Caspian seal's ancestor became isolated as the Paratethys shrank, leaving the animal in a small remnant sea, the Caspian Sea.  The monochines diversified southward. Monachus emerged in the Mediterranean and migrated to the Caribbean and then the central North Pacific.  The two extant elephant seal species diverged close to 4 mya after the Panamanian isthmus was formed.  The lobodontine lineage emerged around 9 mya and colonized the southern ocean in response to glaciation. 
The lineages of Otariidae and Odobenidae split almost 28 Mya.  Otariids originated in the North Pacific. The earliest fossil Pithanotaria, found in California, is dated to 11 mya. The Callorhinus lineage split earlier at 16 mya. Zalophus, Eumetopias and Otaria diverged next, with the latter colonizing the coast of South America. Most of the other otariids diversified in the Southern Hemisphere. The earliest fossils of Odobenidae—Prototaria of Japan and Proneotherium of Oregon—date to 18–16 Mya. These primitive walruses had much shorter canines and lived on a fish diet rather than a specialized mollusk diet like the modern walrus. Odobenids further diversified in the middle and late Miocene. Several species had enlarged upper and lower canines. The genera Valenictus and Odobenus developed elongated tusks. The lineage of the modern walrus may have spread from the North Pacific to the Caribbean (via the Central American Seaway) 8–5 Mya and subsequently made it to the North Atlantic and returned to the North Pacific via the Arctic 1 mya. Alternatively, this lineage may have spread from the North Pacific to the Arctic and subsequently the North Atlantic during the Pleistocene. 
Pinnipeds have streamlined, spindle-shaped bodies with reduced or non-existent external ear flaps, rounded heads, flexible necks, limbs modified into flippers, and small tails.   Pinniped skulls have large eye orbits, short snouts and a constricted interorbital region.  They are unique among carnivorans in that their orbital walls are significantly shaped by the maxilla and are not limited by certain facial bones.  Compared to other carnivorans, their teeth tend to be fewer in number (especially incisors and back molars), are pointed and cone-shaped, and lack carnassials.  The walrus has unique upper canines that are elongated into tusks.  The mammary glands and genitals of pinnipeds can retract into the body. 
Pinnipeds range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. Overall, they tend to be larger than other carnivorans the southern elephant seal is the largest carnivoran.  Several species have male-biased sexual dimorphism that correlates with the degree of polygyny in a species: highly polygynous species like elephant seals are extremely sexually dimorphic, while less polygynous species have males and females that are closer in size. In lobodontine seals, females are slightly larger than males. Males of sexually dimorphic species also tend to have secondary sex characteristics, such as the prominent proboscis of elephant seals, the inflatable red nasal membrane of hooded seals and the thick necks and manes of otariids.   Despite a correlation between size dimorphism and the degree of polygyny, some evidence suggests that size differences between the sexes originated due to ecological differences and prior to the development of polygyny.  
Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered. Even some fully furred species (particularly sea lions) are less haired than most land mammals.  In species that live on ice, young pups have thicker coats than adults. The individual hairs on the coat, known collectively as lanugo, can trap heat from sunlight and keep the pup warm.  Pinnipeds are typically countershaded, and are darker colored dorsally and lighter colored ventrally, which serves to eliminate shadows caused by light shining over the ocean water. The pure white fur of harp seal pups conceals them in their Arctic environment.  Some species, such as ribbon seals, ringed seals and leopard seals, have patterns of contrasting light and dark coloration. All fully furred species molt phocids molt once a year, while otariids gradually molt all year.  Seals have a layer of subcutaneous fat known as blubber that is particularly thick in phocids and walruses.  Blubber serves both to keep the animals warm and to provide energy and nourishment when they are fasting. It can constitute as much as 50% of a pinniped's body weight. Pups are born with only a thin layer of blubber, but some species compensate for this with thick lanugos. 
Pinnipeds have a simple stomach that is similar in structure to terrestrial carnivores. Most species have neither a cecum nor a clear demarcation between the small and large intestines the large intestine is comparatively short and only slightly wider than the small intestine. Small intestine lengths range from 8 times (California sea lion) to 25 times (elephant seal) the body length. The length of the intestine may be an adaptation to frequent deep diving, as the increased volume of the digestive tract serves as an extended storage compartment for partially digested food during submersion. Pinnipeds do not have an appendix.  As in most marine mammals, the kidneys are divided into small lobes and can effectively absorb water and filter out excess salt. 
Pinnipeds have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.  Pinnipeds tend to be slower swimmers than cetaceans, typically cruising at 5–15 kn (9–28 km/h 6–17 mph) compared to around 20 kn (37 km/h 23 mph) for several species of dolphin. Seals are more agile and flexible,  and some otariids, such as the California sea lion, are capable of bending their necks backwards far enough to reach their hind-flippers, allowing them to make dorsal turns.  Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim. 
When swimming, otariids rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.  Fore-flipper movement is not continuous, and the animal glides between each stroke.  Compared to terrestrial carnivorans, the fore-limbs of otariids are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage  the hind-flippers serve as stabilizers.  Phocids and walruses swim by moving their hind-flippers and lower body from side to side,  while their fore-flippers are mainly used for steering.  Some species leap out of the water, which may allow then to travel faster. In addition, sea lions are known to "ride" waves, which probably helps them decrease their energy usage. 
Pinnipeds can move around on land, though not as well as terrestrial animals. Otariids and walruses are capable of turning their hind-flippers forward and under the body so they can "walk" on all fours.  The fore-flippers move in a transverse, rather than a sagittal fashion. Otariids rely on the movements of their heads and necks more than their hind-flippers during terrestrial locomotion.  By swinging their heads and necks, otariids create momentum while they are moving. Sea lions have been recorded climbing up flights of stairs. Phocids are less agile on land. They cannot pull their hind-flippers forward, and move on land by lunging, bouncing and wiggling while their fore-flippers keep them balanced. Some species use their fore-flippers to pull themselves forward. Terrestrial locomotion is easier for phocids on ice, as they can sled along. 
The eyes of pinnipeds are relatively large for their size and are positioned near the front of the head. One exception is the walrus, whose smaller eyes are located on the sides of its head.   This is because it feeds on immobile bottom dwelling mollusks and hence does not need acute vision.  A seal's eye is adapted for seeing both underwater and in air. The lens is mostly spherical, and much of the retina is equidistant from the lens center. The cornea has a flattened center where refraction is nearly equal in both water and air. Pinnipeds also have very muscular and vascularized irises. The well-developed dilator muscle gives the animals a great range in pupil dilation. When contracted, the pupil is typically pear-shaped, although the bearded seal's is more diagonal. In species that live in shallow water, such as harbor seals and California sea lions, dilation varies little, while the deep-diving elephant seals have much greater variation. 
On land, pinnipeds are near-sighted in dim light. This is reduced in bright light, as the retracted pupil reduces the lens and cornea's ability to bend light. They also have a well-developed tapetum lucidum, a reflecting layer that increases sensitivity by reflecting light back through the rods. This helps them see in low-light conditions.  Ice-living seals like the harp seal have corneas that can tolerate high levels of ultraviolet radiation typical of bright, snowy environments. As such, they do not suffer snow blindness.  Pinnipeds appear to have limited color vision, as they lack S-cones.  Flexible eye movement has been documented in seals.  The extraocular muscles of the walrus are well developed. This and its lack of orbital roof allow it to protrude its eyes and see in both frontal and dorsal directions.  Seals release large amounts of mucus to protect their eyes.  The corneal epithelium is keratinized and the sclera is thick enough to withstand the pressures of diving. As in many mammals and birds, pinnipeds possess nictitating membranes. 
The pinniped ear is adapted for hearing underwater, where it can hear sound frequencies at up to 70,000 Hz. In air, hearing is somewhat reduced in pinnipeds compared to many terrestrial mammals. While they are capable of hearing a wide range of frequencies (e.g. 500 to 32,000 Hz in the northern fur seal, compared to 20 to 20,000 Hz in humans), their airborne hearing sensitivity is weaker overall.  One study of three species—the harbor seal, California sea lion and northern elephant seal—found that the sea lion was best adapted for airborne hearing, the harbor seal was equally capable of hearing in air and water, and the elephant seal was better adapted for underwater hearing.  Although pinnipeds have a fairly good sense of smell on land,  it is useless underwater as their nostrils are closed. 
Pinnipeds have well-developed tactile senses. Their mystacial vibrissae have ten times the innervation of terrestrial mammals, allowing them to effectively detect vibrations in the water.  These vibrations are generated, for example, when a fish swims through water. Detecting vibrations is useful when the animals are foraging and may add to or even replace vision, particularly in darkness.  Harbor seals have been observed following varying paths of another seal that swam ahead several minutes before, similar to a dog following a scent trail,   and even to discriminate the species and the size of the fish responsible for the trail.  Blind ringed seals have even been observed successfully hunting on their own in Lake Saimaa, likely relying on their vibrissae to gain sensory information and catch prey. 
Unlike terrestrial mammals, such as rodents, pinnipeds do not move their vibrissae over an object when examining it but instead extend their moveable whiskers and keep them in the same position.  By holding their vibrissae steady, pinnipeds are able to maximize their detection ability.  The vibrissae of phocids are undulated and wavy while otariid and walrus vibrissae are smooth.  Research is ongoing to determine the function, if any, of these shapes on detection ability. The vibrissa's angle relative to the flow, not the shape, however, seems to be the most important factor.  The vibrissae of some otariids grow quite long—those of the Antarctic fur seal can reach 41 cm (16 in).  Walruses have the most vibrissae, at 600–700 individual hairs. These are important for detecting their prey on the muddy sea floor. In addition to foraging, vibrissae may also play a role in navigation spotted seals appear to use them to detect breathing holes in the ice. 
Diving adaptations Edit
Before diving, pinnipeds typically exhale to empty their lungs of half the air  and then close their nostrils and throat cartilages to protect the trachea.  Their unique lungs have airways that are highly reinforced with cartilaginous rings and smooth muscle, and alveoli that completely deflate during deeper dives.   While terrestrial mammals are generally unable to empty their lungs,  pinnipeds can reinflate their lungs even after complete respiratory collapse.  The middle ear contains sinuses that probably fill with blood during dives, preventing middle ear squeeze.  The heart of a seal is moderately flattened to allow the lungs to deflate. The trachea is flexible enough to collapse under pressure.  During deep dives, any remaining air in their bodies is stored in the bronchioles and trachea, which prevents them from experiencing decompression sickness, oxygen toxicity and nitrogen narcosis. In addition, seals can tolerate large amounts of lactic acid, which reduces skeletal muscle fatigue during intense physical activity. 
The main adaptations of the pinniped circulatory system for diving are the enlargement and increased complexity of veins to increase their capacity. Retia mirabilia form blocks of tissue on the inner wall of the thoracic cavity and the body periphery. These tissue masses, which contain extensive contorted spirals of arteries and thin-walled veins, act as blood reservoirs that increase oxygen stores for use during diving.  As with other diving mammals, pinnipeds have high amounts of hemoglobin and myoglobin stored in their blood and muscles. This allows them to stay submerged for long periods of time while still having enough oxygen. Deep-diving species such as elephant seals have blood volumes that represent up to 20% of their body weight. When diving, they reduce their heart rate and maintain blood flow only to the heart, brain and lungs. To keep their blood pressure stable, phocids have an elastic aorta that dissipates some of the energy of each heartbeat. 
Pinnipeds conserve heat with their large and compact body size, insulating blubber and fur, and high metabolism.  In addition, the blood vessels in their flippers are adapted for countercurrent exchange. Veins containing cool blood from the body extremities surround arteries, which contain warm blood received from the core of the body. Heat from the arterial blood is transferred to the blood vessels, which then recirculate blood back to the core.  The same adaptations that conserve heat while in water tend to inhibit heat loss when out of water. To counteract overheating, many species cool off by flipping sand onto their backs, adding a layer of cool, damp sand that enhances heat loss. The northern fur seal pants to help stay cool, while monk seals often dig holes in the sand to expose cooler layers to rest in. 
Pinnipeds spend many months at a time at sea, so they must sleep in the water. Scientists have recorded them sleeping for minutes at a time while slowly drifting downward in a belly-up orientation. Like other marine mammals, seals sleep in water with half of their brain awake so that they can detect and escape from predators.  When they are asleep on land, both sides of their brain go into sleep mode. 
Living pinnipeds mainly inhabit polar and subpolar regions, particularly the North Atlantic, the North Pacific and the Southern Ocean. They are entirely absent from Indomalayan waters.  Monk seals and some otariids live in tropical and subtropical waters. Seals usually require cool, nutrient-rich waters with temperatures lower than 20 °C (68 °F). Even those that live in warm or tropical climates live in areas that become cold and nutrient rich due to current patterns.   Only monk seals live in waters that are not typically cool or rich in nutrients.  The Caspian seal and Baikal seal are found in large landlocked bodies of water (the Caspian Sea and Lake Baikal respectively).
As a whole, pinnipeds can be found in a variety of aquatic habitats, including coastal water, open ocean, brackish water and even freshwater lakes and rivers. Most species inhabit coastal areas, though some travel offshore and feed in deep waters off oceanic islands. The Baikal seal is the only freshwater species, though some ringed seals live in freshwater lakes in Russia close to the Baltic sea. In addition, harbor seals may visit estuaries, lakes and rivers and sometimes stay as long as a year. Other species known to enter freshwater include California sea lions and South American sea lions.  Pinnipeds also use a number of terrestrial habitats and substrates, both continental and island. In temperate and tropical areas, they haul out on to sandy and pebble beaches, rocky shores, shoals, mud flats, tide pools and in sea caves. Some species also rest on man-made structures, like piers, jetties, buoys and oil platforms. Pinnipeds may move further inland and rest in sand dunes or vegetation, and may even climb cliffs.  Polar-living species haul out on to both fast ice and drift ice. 
Pinnipeds have an amphibious lifestyle they spend most of their lives in the water, but haul out to mate, raise young, molt, rest, thermoregulate or escape from aquatic predators. Several species are known to migrate vast distances, particularly in response to extreme environmental changes, like El Niño or changes in ice cover. Elephant seals stay at sea 8–10 months a year and migrate between breeding and molting sites. The northern elephant seal has one of the longest recorded migration distances for a mammal, at 18,000–21,000 km (11,000–13,000 mi). Phocids tend to migrate more than otariids.  Traveling seals may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water. 
Pinnipeds may dive during foraging or to avoid predators. When foraging, Weddell seals typically dive for less than 15 minutes to depths of around 400 m (1,300 ft) but can dive for as long as 73 minutes and to depths of up to 600 m (2,000 ft). Northern elephant seals commonly dive 350–650 m (1,150–2,130 ft) for as long as 20 minutes. They can also dive 1,500 m (4,900 ft) and for as long as 77 minutes.  The dives of otariids tend to be shorter and less deep. They typically last 5–7 minutes with average depths to 30–45 m (98–148 ft). However, the New Zealand sea lion has been recorded diving to a maximum of 460 m (1,510 ft) and a duration of 12 minutes.  Walruses do not often dive very deep, as they feed in shallow water. 
Pinnipeds have lifespans averaging 25–30 years. Females usually live longer, as males tend to fight and often die before reaching maturity.  The longest recorded lifespans include 43 years for a wild female ringed seal and 46 years for a wild female grey seal.  The age at which a pinniped sexually matures can vary from 2–12 years depending on the species. Females typically mature earlier than males. 
Foraging and predation Edit
All pinnipeds are carnivorous and predatory. As a whole, they mostly feed on fish and cephalopods, followed by crustaceans and bivalves, and then zooplankton and endothermic ("warm-blooded") prey like sea birds.  While most species are generalist and opportunistic feeders, a few are specialists. Examples include the crabeater seal, which primarily eats krill, the ringed seal, which eats mainly crustaceans, the Ross seal and southern elephant seal, which specialize on squid, and the bearded seal and walrus, which feed on clams and other bottom-dwelling invertebrates. 
Pinnipeds may hunt solitarily or cooperatively. The former behavior is typical when hunting non-schooling fish, slow-moving or immobile invertebrates or endothermic prey. Solitary foraging species usually exploit coastal waters, bays and rivers. An exception to this is the northern elephant seal, which feeds on fish at great depths in the open ocean. In addition, walruses feed solitarily but are often near other walruses in small or large groups that may surface and dive in unison. When large schools of fish or squid are available, pinnipeds such as certain otariids hunt cooperatively in large groups, locating and herding their prey. Some species, such as California and South American sea lions, may forage with cetaceans and sea birds. 
Seals typically consume their prey underwater where it is swallowed whole. Prey that is too large or awkward is taken to the surface to be torn apart.  The leopard seal, a prolific predator of penguins, is known to violently swing its prey back and forth until it is dead.  The elaborately cusped teeth of filter-feeding species, such as crabeater seals, allow them to remove water before they swallow their planktonic food.  The walrus is unique in that it consumes its prey by suction feeding, using its tongue to suck the meat of a bivalve out of the shell.  While pinnipeds mostly hunt in the water, South American sea lions are known to chase down penguins on land.  Some species may swallow stones or pebbles for reasons not understood.  Though they can drink seawater, pinnipeds get most of their fluid intake from the food they eat. 
Pinnipeds themselves are subject to predation. Most species are preyed on by the killer whale or orca. To subdue and kill seals, orcas continuously ram them with their heads, slap them with their tails and fling them in the air. They are typically hunted by groups of 10 or fewer whales, but they are occasionally hunted by larger groups or by lone individuals. Pups are more commonly taken by orcas, but adults can be targeted as well. Large sharks are another major predator of pinnipeds—usually the great white shark but also the tiger shark and mako shark. Sharks usually attack by ambushing them from below. The prey usually escapes, and seals are often seen with shark-inflicted wounds. Otariids typically have injuries in the hindquarters, while phocids usually have injuries on the forequarters.  Pinnipeds are also targeted by terrestrial and pagophilic predators. The polar bear is well adapted for hunting Arctic seals and walruses, particularly pups. Bears are known to use sit-and-wait tactics as well as active stalking and pursuit of prey on ice or water. Other terrestrial predators include cougars, brown hyenas and various species of canids, which mostly target the young. 
Pinnipeds lessen the chance of predation by gathering in groups.  Some species are capable of inflicting damaging wounds on their attackers with their sharp canines—an adult walrus is capable of killing polar bears.  When out at sea, northern elephant seals dive out of the reach of surface-hunting orcas and white sharks.  In the Antarctic, which lacks terrestrial predators, pinniped species spend more time on the ice than their Arctic counterparts.  Arctic seals use more breathing holes per individual, appear more restless when hauled out, and rarely defecate on the ice. Ringed seals build dens underneath fast ice for protection. 
Interspecific predation among pinnipeds does occur. The leopard seal is known to prey on numerous other species, especially the crabeater seal. Leopard seals typically target crabeater pups, which form an important part of their diet from November to January. Older crabeater seals commonly bear scars from failed leopard seal attacks a 1977 study found that 75% of a sample of 85 individual crabeaters had these scars.  Walruses, despite being specialized for feeding on bottom-dwelling invertebrates, occasionally prey on Arctic seals. They kill their prey with their long tusks and eat their blubber and skin. Steller sea lions have been recorded eating the pups of harbor seals, northern fur seals and California sea lions. New Zealand sea lions feed on pups of some fur seal species, and the South American sea lion may prey on South American fur seals. 
Reproductive behavior Edit
The mating system of pinnipeds varies from extreme polygyny to serial monogamy.  Of the 33 species, 20 breed on land, and the remaining 13 breed on ice.  Species that breed on land are usually polygynous, as females gather in large aggregations and males are able to mate with them as well as defend them from rivals. Polygynous species include elephant seals, grey seals and most otariids.  Land-breeding pinnipeds tend to mate on islands where there are fewer terrestrial predators. Few islands are favorable for breeding, and those that are tend to be crowded. Since the land they breed on is fixed, females return to the same sites for many years. The males arrive earlier in the season and wait for them. The males stay on land and try to mate with as many females as they can some of them will even fast. If a male leaves the beach to feed, he will likely lose mating opportunities and his dominance. 
Polygynous species also tend to be extremely sexual dimorphic in favor of males. This dimorphism manifests itself in larger chests and necks, longer canines and denser fur—all traits that help males in fights for females. Increased body weight in males increases the length of time they can fast due to the ample energy reserves stored in the blubber.  Larger males also likely enjoy access to feeding grounds that smaller ones are unable to access due to their lower thermoregulatory ability and decreased energy stores.  In some instances, only the largest males are able to reach the furthest deepest foraging grounds where they enjoy maximum energetic yields that are unavailable to smaller males and females. 
Other seals, like the walrus and most phocids, breed on ice with copulation usually taking place in the water (a few land-breeding species also mate in water).    Females of these species tend to aggregate less. In addition, since ice is less stable than solid land, breeding sites change location each year, and males are unable to predict where females will stay during the breeding season. Hence polygyny tends to be weaker in ice-breeding species. An exception to this is the walrus, where females form dense aggregations perhaps due to their patchy food sources. Pinnipeds that breed on fast ice tend to cluster together more than those that breed on drift ice.  Some of these species are serially monogamous, including the harp seal, crabeater seal and hooded seal.  Seals that breed on ice tend to have little or no sexual dimorphism. In lobodontine seals, females are slightly longer than males. Walruses and hooded seals are unique among ice-breeding species in that they have pronounced sexual dimorphism in favor of males.  
Adult male pinnipeds have several strategies to ensure reproductive success. Otariids establish territories containing resources that attract females, such as shade, tide pools or access to water. Territorial boundaries are usually marked by natural breaks in the substrate,  and some may be fully or partially underwater.   Males defend their territorial boundaries with threatening vocalizations and postures, but physical fights are usually avoided.  Individuals also return to the same territorial site each breeding season. In certain species, like the Steller sea lion and northern fur seal, a dominant male can maintain a territory for as long as 2–3 months. Females can usually move freely between territories and males are unable to coerce them, but in some species such as the northern fur seal, South American sea lion and Australian sea lion, males can successfully contain females in their territories and prevent them from leaving. In some phocid species, like the harbor seal, Weddell seal and bearded seal, the males have underwater territories called "maritories" near female haul-out areas.  These are also maintained by vocalizations.  The maritories of Weddell seal males can overlap with female breathing holes in the ice. 
Lek systems are known to exist among some populations of walruses.  These males cluster around females and try to attract them with elaborate courtship displays and vocalizations.   Lekking may also exist among California sea lions, South American fur seals, New Zealand sea lions and harbor seals.   In some species, including elephant seals and grey seals, males will try to lay claim to the desired females and defend them from rivals.  Elephant seal males establish dominance hierarchies with the highest ranking males—the alpha males—maintaining harems of as many as 30–100 females. These males commonly disrupt the copulations of their subordinates while they themselves can mount without inference. They will, however, break off mating to chase off a rival.  Grey seal males usually claim a location among a cluster of females whose members may change over time,  while males of some walrus populations try to monopolize access to female herds.  Male harp seals, crabeater seals and hooded seals follow and defend lactating females in their vicinity—usually one or two at a time,  and wait for them to reach estrus.  
Younger or subdominant male pinnipeds may attempt to achieve reproductive success in other ways. Subadult elephant seals will sneak into female clusters and try to blend in by pulling in their noses. They also harass and attempt to mate with females that head out to the water. In otariid species like the South American and Australian sea lions, non-territorial subadults form "gangs" and cause chaos within the breeding rookeries to increase their chances of mating with females.  Alternative mating strategies also exist in young male grey seals, which do have some success. 
Female pinnipeds do appear to have some choice in mates, particularly in lek-breeding species like the walrus, but also in elephant seals where the males try to dominate all the females that they want to mate with.  When a female elephant seal or grey seal is mounted by an unwanted male, she tries to squirm and get away, while croaking and slapping him with her tail. This commotion attracts other males to the scene, and the most dominant will end the copulation and attempt to mate with the female himself.   Dominant female elephant seals stay in the center of the colony where they are more likely to mate with a dominant male, while peripheral females are more likely to mate with subordinates.  Female Steller sea lions are known to solicit mating with their territorial males. 
Birth and parenting Edit
With the exception of the walrus, which has five- to six-year-long inter-birth intervals, female pinnipeds enter estrous shortly after they give birth.  All species go through delayed implantation, wherein the embryo remains in suspended development for weeks or months before it is implanted in the uterus. Delayed implantation postpones the birth of young until the female hauls-out on land or until conditions for birthing are favorable.   Gestation in seals (including delayed implantation) typically lasts a year.  For most species, birthing takes place in the spring and summer months.  Typically, single pups are born  twins are uncommon and have high mortality rates.  Pups of most species are born precocial. 
Unlike terrestrial mammals, pinniped milk has little to no lactose.  Mother pinnipeds have different strategies for maternal care and lactation. Phocids such as elephant seals, grey seals and hooded seals remain on land or ice and fast during their relatively short lactation period–four days for the hooded seal and five weeks for elephant seals. The milk of these species consist of up to 60% fat, allowing the young to grow fairly quickly. In particular, northern elephant seal pups gain 4 kg (9 lb) each day before they are weaned. Some pups may try to steal extra milk from other nursing mothers and gain weight more quickly than others. Alloparenting occurs in these fasting species  while most northern elephant seal mothers nurse their own pups and reject nursings from alien pups, some do accept alien pups with their own. 
For otariids and some phocids like the harbor seal, mothers fast and nurse their pups for a few days at a time. In between nursing bouts, the females leave their young onshore to forage at sea. These foraging trips may last anywhere between a day and two weeks, depending on the abundance of food and the distance of foraging sites. While their mothers are away, the pups will fast.  Lactation in otariids may last 6–11 months in the Galápagos fur seal it can last as long as 3 years. Pups of these species are weaned at lower weights than their phocid counterparts.  Walruses are unique in that mothers nurse their young at sea.  The female rests at the surface with its head held up, and the young suckle upside down.  Young pinnipeds typically learn to swim on their own and some species can even swim at birth. Other species may wait days or weeks before entering the water. Elephant seals do not swim until weeks after they are weaned. 
Male pinnipeds generally play little role in raising the young.  Male walruses may help inexperienced young as they learn to swim, and have even been recorded caring for orphans.  Male California sea lions have been observed to help shield swimming pups from predators.  Males can also pose threats to the safety of pups. In terrestrially breeding species, pups may get crushed by fighting males.  Subadult male South America sea lions sometimes abduct pups from their mothers and treat them like adult males treat females. This helps them gain experience in controlling females. Pups can get severely injured or killed during abductions. 
Pinnipeds can produce a number of vocalizations such as barks, grunts, rasps, rattles, growls, creaks, warbles, trills, chirps, chugs, clicks and whistles. While most vocals are audible to the human ear, a captive leopard seal was recorded making ultrasonic calls underwater. In addition, the vocals of northern elephant seals may produce infrasonic vibrations. Vocals are produced both in air and underwater. Otariids are more vocal on land, while phocids are more vocal in water. Antarctic seals are more vocal on land or ice than Arctic seals due to a lack of terrestrial and pagophilic predators like the polar bear.  Male vocals are usually of lower frequencies than those of the females. 
Vocalizations are particularly important during the breeding seasons. Dominant male elephant seals advertise their status and threaten rivals with "clap-threats" and loud drum-like calls  that may be modified by the proboscis.  Male otariids have strong barks, growls, roars and "whickers". Male walruses are known to produce distinctive gong-like calls when attempting to attract females. They can also create somewhat musical sounds with their inflated throats. 
The Weddell seal has perhaps the most elaborate vocal repertoire with separate sounds for airborne and underwater contexts.  Underwater vocals include trills, chirps, chugs and knocks. The calls appear to contain prefixes and suffixes that serve to emphasize a message.  The underwater vocals of Weddell seals can last 70 seconds, which is long for a marine mammal call. Some calls have around seven rhythm patterns and are comparable to birdsongs and whalesongs.  Similar calls have been recorded in other lobodontine seals  and in bearded seals. 
In some pinniped species, there appear to be geographic differences in vocalizations, known as dialects,  while certain species may even have individual variations in expression.  These differences are likely important for mothers and pups who need to remain in contact on crowded beaches.  Otariid females and their young use mother-pup attraction calls to help them reunite when the mother returns from foraging at sea.  The calls are described are "loud" and "bawling".  Female elephant seals make an unpulsed attraction call when responding to their young. When threatened by other adults or when pups try to suckle, females make a harsh, pulsed call.  Pups may also vocalize when playing, in distress or when prodding their mothers to allow them to suckle.  
Non-vocal communication is not as common in pinnipeds as in cetaceans. Nevertheless, when disturbed by intruders harbor seals and Baikal seals may slap their fore-flippers against their bodies as warnings. Teeth chattering, hisses and exhalations are also made as aggressive warnings. Visual displays also occur: Weddell seals will make an S-shaped posture when patrolling under the ice, and Ross seals will display the stripes on their chests and teeth when approached.  Male hooded seals use their inflatable nasal membranes to display to and attract females. 
In a match-to-sample task study, a single California sea lion was able to demonstrate an understanding of symmetry, transitivity and equivalence a second seal was unable to complete the tasks.  They demonstrate the ability to understand simple syntax and commands when taught an artificial sign language, though they only rarely used the signs semantically or logically.  In 2011, a captive California sea lion named Ronan was recorded bobbing its head in synchrony to musical rhythms. This "rhythmic entrainment" was previously seen only in humans, parrots and other birds possessing vocal mimicry.  In 1971, a captive harbor seal named Hoover was trained to imitate human words, phrases and laughter.  For sea lions used in entertainment, trainers toss a ball at the animal so it may accidentally balance it or hold the ball on its nose, thereby gaining an understanding of the behavior desired. It may require a year to train a sea lion to perform a trick for the public. Its long-term memory allows it to perform a trick after at least three months of non-performance. 
Cultural depictions Edit
Various human cultures have for millennia depicted pinnipeds. The anthropologist, A. Asbjørn Jøn, has analysed beliefs of the Celts of Orkney and Hebrides who believed in selkies—seals that could change into humans and walk on land.  Seals are also of great importance in the culture of the Inuit.  In Inuit mythology, the goddess Sedna rules over the sea and marine animals. She is depicted as a mermaid, occasionally with a seal's lower body. In one legend, seals, whales and other marine mammals were formed from her severed fingers.  One of the earliest Ancient Greek coins depicts the head of a seal, and the animals were mentioned by Homer and Aristotle. The Greeks associated them with both the sea and sun and were considered to be under the protection of the gods Poseidon and Apollo.  The Moche people of ancient Peru worshipped the sea and its animals, and often depicted sea lions in their art.  In modern culture, pinnipeds are thought of as cute, playful and comical figures. 
In captivity Edit
Pinnipeds can be found in facilities around the world, as their large size and playfulness make them popular attractions.  Seals have been kept in captivity since at least Ancient Rome and their trainability was noticed by Pliny the Elder. Zoologist Georges Cuvier noted during the 19th century that wild seals show considerable fondness for humans and stated that they are second only to some monkeys among wild animals in their easy tamability. Francis Galton noted in his landmark paper on domestication that seals were a spectacular example of an animal that would most likely never be domesticated despite their friendliness and desire for comfort due to the fact that they serve no practical use for humans. 
Some modern exhibits have rocky backgrounds with artificial haul-out sites and a pool, while others have pens with small rocky, elevated shelters where the animals can dive into their pools. More elaborate exhibits contain deep pools that can be viewed underwater with rock-mimicking cement as haul-out areas. The most common pinniped species kept in captivity is the California sea lion as it is both easy to train and adaptable. Other species popularly kept include the grey seal and harbor seal. Larger animals like walruses and Steller sea lions are much less common.  Some organizations, such as the Humane Society of the United States and World Animal Protection, object to keeping pinnipeds and other marine mammals in captivity. They state that the exhibits could not be large enough to house animals that have evolved to be migratory, and a pool could never replace the size and biodiversity of the ocean. They also state that the tricks performed for audiences are "exaggerated variations of their natural behaviors" and distract the people from the animal's unnatural environment. 
California sea lions are used in military applications by the U.S. Navy Marine Mammal Program, including detecting naval mines and enemy divers. In the Persian Gulf, the animals have been trained to swim behind divers approaching a U.S. naval ship and attach a clamp with a rope to the diver's leg. Navy officials say that the sea lions can do this in seconds, before the enemy realizes what happened.  Organizations like PETA believe that such operations put the animals in danger.  The Navy insists that the sea lions are removed once their mission is complete. 
Humans have hunted seals since the Stone Age. Originally, seals were hit with clubs during haul-out. Eventually, seal hunters used harpoons to spear the animals from boats out at sea, and hooks for killing pups on ice or land. They were also trapped in nets. The use of firearms in seal hunting during the modern era drastically increased the number of killings. Pinnipeds are typically hunted for their meat and blubber. The skins of fur seals and phocids are made into coats, and the tusks of walruses continue to be used for carvings or as ornaments.  There is a distinction between the subsistence hunting of seals by indigenous peoples of the Arctic and commercial hunting: subsistence hunters typically use seal products for themselves and depend on them for survival.  National and international authorities have given special treatment to aboriginal hunters since their methods of killing are seen as less destructive and wasteful. This distinction is being questioned as indigenous people are using more modern weaponry and mechanized transport to hunt with, and are selling seal products in the marketplace. Some anthropologists argue that the term "subsistence" should also apply to these cash-based exchanges as long as they take place within local production and consumption. More than 100,000 phocids (especially ringed seals) as well as around 10,000 walruses are harvested annually by native hunters. 
Commercial sealing was historically just as important an industry as whaling. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal.  The scale of seal harvesting decreased substantially after the 1960s,  after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.  Several species that were commercially exploited have rebounded in numbers for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its historic range, but has a population bottleneck.  Conversely, the Mediterranean monk seal was extirpated from much of its former range, which stretched from the Mediterranean to the Black Sea and northwest Africa, and only remains in the northeastern Mediterranean and some parts of northwest Africa. 
Several species of pinniped continue to be harvested. The Convention for the Conservation of Antarctic Seals allows limited hunting of crabeater seals, leopard seals and Weddell seals. However, Weddell seal hunting is prohibited between September and February if the animal is over one year of age, to ensure breeding stocks are healthy. Other species protected are southern elephant seals, Ross seals and Antarctic fur seals.  The Government of Canada permits the hunting of harp seals. This has been met with controversy and debate. Proponents of seal hunts insist that the animals are killed humanely and the white-coated pups are not taken, while opponents argue that it is irresponsible to kill harp seals as they are already threatened by declining habitat.  
The Caribbean monk seal has been killed and exploited by Europeans settlers and their descendants since 1494, starting with Christopher Columbus himself. The seals were easy targets for organized sealers, fishermen, turtle hunters and buccaneers because they evolved with little pressure from terrestrial predators and were thus "genetically tame". In the Bahamas, as many as 100 seals were slaughtered in one night. In the mid-nineteenth century, the species was thought to have gone extinct until a small colony was found near the Yucatán Peninsula in 1886. Seal killings continued, and the last reliable report of the animal alive was in 1952. The IUCN declared it extinct in 1996.  The Japanese sea lion was common around the Japanese islands, but overexploitation and competition from fisheries drastically decreased the population in the 1930s. The last recorded individual was a juvenile in 1974. 
As of 2021, the International Union for Conservation of Nature (IUCN) recognizes 36 pinniped species. With the Japanese sea lion and the Caribbean monk seal recently extinct, ten more are considered at risk, as they are ranked "Endangered" (Hawaiian monk seal, Mediterranean monk seal, Galápagos fur seal, Australian sea lion, New Zealand sea lion, Caspian seal, and Galápagos sea lion) or "Vulnerable" (northern fur seal, hooded seal, and walrus).  Species that live in polar habitats are vulnerable to the effects of recent and ongoing climate change, particularly declines in sea ice.  There has been some debate over the cause of the decline of Steller sea lions in Alaska since the 1970s. 
Some species have become so numerous that they conflict with local people. In the United States, pinnipeds are protected under the Marine Mammal Protection Act of 1972 (MMPA). Since that year, California sea lion populations have risen to 250,000. These animals began exploiting more man-made environments, like docks, for haul-out sites. Many docks are not designed to withstand the weight of several resting sea lions. Wildlife managers have used various methods to control the animals, and some city officials have redesigned docks so they can better withstand use by sea lions.   Sea lions also conflict with fisherman since both depend on the same fish stocks.  In 2007, MMPA was amended to permit the lethal removal of sea lions from salmon runs at Bonneville Dam.  The 2007 law seeks to relieve pressure on the crashing Pacific Northwest salmon populations.  Wildlife officials have unsuccessfully attempted to ward off the sea lions using bombs, rubber bullets and bean bags.  Efforts to chase sea lions away from the area have also proven ineffective.  Critics like the Humane Society object to the killing of the sea lions, claiming that hydroelectric dams pose a greater threat to the salmon.  Similar conflicts have existed in South Africa with brown fur seals. In the 1980s and 1990s, South African politicians and fisherman demanded that the fur seals be culled, believing that the animals competed with commercial fisheries. Scientific studies found that culling fur seals would actually have a negative effect on the fishing industry, and the culling option was dropped in 1993. 
Pinnipeds are also threatened by humans indirectly. They are unintentionally caught in fishing nets by commercial fisheries and accidentally swallow fishing hooks. Gillnetting and Seine netting is a significant cause of mortality in seals and other marine mammals. Species commonly entangled include California sea lions, Hawaiian monk seals, northern fur seals and brown fur seals.  Pinnipeds are also affected by marine pollution. High levels of organic chemicals accumulate in these animals since they are near the top of food chains and have large reserves of blubber. Lactating mothers can pass the toxins on to their young. These pollutants can cause gastrointestinal cancers, decreased reproductivity and greater vulnerability to infectious diseases.  Other man-made threats include habitat destruction by oil and gas exploitation, encroachment by boats,  and underwater noise. 
Mass Extinctions of Land-Dwelling Animals Occur in 27-Million-Year Cycle
Mass extinctions of land-dwelling animals—including amphibians, reptiles, mammals, and birds—follow a cycle of about 27 million years, coinciding with previously reported mass extinctions of ocean life, according to a new analysis published in the journal Historical Biology.
The study also finds that these mass extinctions align with major asteroid impacts and devastating volcanic outpourings of lava called flood-basalt eruptions—providing potential causes for why the extinctions occurred.
“It seems that large-body impacts and the pulses of internal Earth activity that create flood-basalt volcanism may be marching to the same 27-million-year drumbeat as the extinctions, perhaps paced by our orbit in the Galaxy,” said Michael Rampino, a professor in New York University’s Department of Biology and the study’s lead author.
Sixty-six million years ago, 70 percent of all species on land and in the seas, including the dinosaurs, suddenly went extinct, in the disastrous aftermath of the collision of a large asteroid or comet with the Earth. Subsequently, paleontologists discovered that such mass extinctions of marine life, in which up to 90 percent of species disappeared, were not random events, but seemed to come in a 26-million-year cycle.
In their Historical Biology study, Rampino and co-authors Ken Caldeira of the Carnegie Institution for Science and Yuhong Zhu of NYU’s Center for Data Science, examined the record of mass extinctions of land-dwelling animals and concluded that they coincided with the extinctions of ocean life. They also performed new statistical analyses of the extinctions of land species and demonstrated that those events followed a similar cycle of about 27.5 million years.
What could be causing the periodic mass extinctions on land and in the seas? Mass extinctions are not the only events occurring in cycles: the ages of impact craters—created by asteroids and comets crashing to the Earth’s surface—also follow a cycle aligning with the extinction cycle.
Astrophysicists hypothesize that periodic comet showers occur in the Solar System every 26 to 30 million years, producing cyclical impacts and resulting in periodic mass extinctions. The Sun and planets cycle through the crowded mid-plane of the Milky Way Galaxy about every 30 million years. During those times, comet showers are possible, leading to large impacts on the Earth. The impacts can create conditions that would stress and potentially kill off land and marine life, including widespread dark and cold, wildfires, acid rain, and ozone depletion.
“These new findings of coinciding, sudden mass extinctions on land and in the oceans, and of the common 26- to 27-million-year cycle, lend credence to the idea of periodic global catastrophic events as the triggers for the extinctions,” said Rampino. “In fact, three of the mass annihilations of species on land and in the sea are already known to have occurred at the same times as the three largest impacts of the last 250 million years, each capable of causing a global disaster and resulting mass extinctions.”
Lichen-covered flows from the Siberian flood basalts. Photo: Linda Elkins-Tanton
The researchers were surprised to find another possible explanation beyond asteroids for mass extinctions: flood-basalt eruptions, or giant volcanic eruptions that cover vast areas with lava. All eight of the coinciding mass die-offs on land and in the oceans matched times of flood-basalt eruptions. These eruptions also would have created severe conditions for life, including brief periods of intense cold, acid rain, and ozone destruction and increased radiation longer term, eruptions could lead to lethal greenhouse heating and more acid and less oxygen in the ocean.
“The global mass extinctions were apparently caused by the largest cataclysmic impacts and massive volcanism, perhaps sometimes working in concert,” added Rampino.
Swimming Proficiency Of Marine Mammals Ranks Them Among The World's Elite Animal Athletes
SANTA CRUZ, CA -- As any swimmer knows, moving through water is nothing like moving on land. When the ancestors of modern marine mammals first ventured into the ocean some 60 million years ago, they had to adapt to a medium 800 times denser and 60 times more viscous than air. The spectacular success of their descendants illustrates the remarkable power of natural selection.
According to a new study comparing the athletic abilities of different types of animals, modern marine mammals are so well adapted to aquatic life that they are as efficient in swimming as specialized land mammals are in running. Terrie Williams, an associate professor of biology at the University of California, Santa Cruz, found that elite animal athletes, from horses to killer whales, achieve an optimal efficiency for locomotion that is determined more by their basic mammalian physiology than by their mode of transportation.
"The bottom line is that terrestrial and marine mammals expend similar amounts of energy to live and move in their respective environments," said Williams, who has been studying the exercise physiology of terrestrial and marine mammals for over 20 years.
In an article published this month in the Philosophical Transactions of the Royal Society of London, Biology, Williams presented a comprehensive analysis of the energetic costs of locomotion in terrestrial, aquatic, and semiaquatic mammals. She also extended her comparisons to include flying mammals (bats), as well as fish and birds.
Her findings indicate that for specialized mammals, whether they run across the plains, swim through the oceans, or fly across the evening sky, the energetic cost of moving through the environment is about the same. Some may achieve higher speeds than others, but their efficiency -- the amount of energy required to move a set distance -- appears to be constrained by a physiological limit for the mammalian way of life.
Energy requirements do vary with an animal's size due to metabolic factors. A swimming grey whale, for example, expends less energy per pound than a bottlenose dolphin. But Williams was surprised to find that the relationship between body mass and energy use is the same for mammals that run, swim, and fly, whereas mammals, fish, and birds show distinct differences.
Williams analyzed physiological data from a wide variety of animals, drawing on her own research for some animals, such as dolphins, and on published data for others. She based her comparisons primarily on calculations of each animal's "total cost of transport." The cost of transport for an animal is like the miles-per-gallon rating of a car, she explained.
"If you think of a cheetah as a BMW, this paper shows that if you put that BMW motor on a streamlined boat, it would use the same amount of gas to move a mile in the water as it did on land," Williams said. "The trick is the chassis has to be adapted to the environment, and that means specializing."
Semiaquatic animals that try to have it both ways, like muskrats and mink, pay a high price for their versatility, sacrificing energetic efficiency for the ability to move back and forth between land and water. Williams found that the energetic cost of swimming for semiaquatic mammals is two-and-a-half to five times higher than for fully aquatic marine mammals.
These higher costs would also have applied to transitional species in the evolutionary lineages that led to modern marine mammals. The fossil record includes primitive cetaceans (ancestors of whales and dolphins) and archaic pinnipeds (seals and sea lions) with skeletal characteristics suggesting a semiaquatic lifestyle.
The ancient oceans must have offered significant advantages to these transitional mammals, because the energetic costs associated with the move from land to water would have been high, according to Williams. Other researchers have cited an abundance of food in the marine environment and reduced competition from other predators as factors that may have made it worthwhile for the ancestors of marine mammals to venture into the aquatic realm. In addition, it is likely that these animals limited the high energetic costs of swimming by jumping in and out of the water, the way modern mink and river otters do, Williams noted.
The sea otter is an unusual case because it has the body of a semiaquatic mammal, but spends most of its time in the water. Williams views sea otters as living on an "evolutionary edge," meeting their energetic requirements by such a slim margin that they are highly vulnerable to environmental perturbations, such as oil spills and coastal pollution.
"In order to make it in the marine environment, these mammals have to spend inordinate amounts of time grooming a fur coat," Williams said. "They don't have insulating blubber like other marine mammals, so to counterbalance the heat loss they have an extraordinarily high metabolism. They eat 25 percent of their body weight in food every day and have to eat every few hours. Overall, there is very little margin of safety for the sea otter."
Williams includes humans in the semiaquatic category, too, although only the best human swimmers (trained athletes) are able to swim as efficiently as a sea otter. In fact, Williams said, her curiosity about how mammals swim began when she worked as a lifeguard in college and was struck by the poor swimming abilities of humans.
"I began studying a wide variety of mammals to find out just what makes an efficient swimmer. By comparing semiaquatic mammals with highly adapted marine mammals, I could follow the physiological and morphological trends that lead to swimming proficiency," she said.
Williams's research included gathering physiological data from trained dolphins swimming alongside a moving boat. She has also used video cameras strapped to the backs of seals and dolphins to view their swimming dynamics while portable monitors recorded their heart rates.
When she compared different species of highly adapted marine mammals, Williams found that swimming style has relatively little effect on the cost of transport. Sea lions use their fore-flippers to propel themselves, while seals use their hind flippers and dolphins and whales use a distinctive undulatory motion, but they all achieve comparable energetic efficiencies. Other researchers have made similar observations regarding the performance of two-legged and four-legged runners.
Comparing marine mammals with fish, Williams found that the overall cost of transport is higher for swimming mammals than for fish, primarily because mammals, being warm-blooded, have to expend energy to maintain their body temperature. When it comes to the energetic cost of swimming by itself, disregarding the baseline costs of maintaining a mammalian physiology, dolphins and sea lions swim just as efficiently as salmon.
To round out the analysis, Williams also took a look at flying creatures. The four species of bats she examined showed transport costs comparable to those of other mammals. For birds, however, transport costs fall in between those for mammals and fish.
It is not clear why the cost of swimming in fish should be lower than the cost of flying in birds, while both are energetically cheaper than running in mammals, Williams noted. But the fact that mammalian specialists in all three modes of locomotion have essentially the same transport costs suggests a physiological explanation. For instance, the respiratory systems of fish, birds, and mammals may differ in the efficiency with which they are able to deliver oxygen to muscles.
"For years, researchers have been focusing on the differences between runners, flyers, and swimmers," Williams said. "This study is exciting because it highlights the similarities between mammalian athletes of all kinds."
Materials provided by University Of California, Santa Cruz. Note: Content may be edited for style and length.
The World Conservation Union (IUCN) promotes the conservation of species, assesses their conservation status worldwide, and publishes an annual list of threatened species. The 2003 IUCN Red List of Threatened Species lists 125 carnivores as threatened. Five are listed as Extinct, no longer living: the Falkland Island wolf, the Caribbean monk seal, the sea mink, the Barbados raccoon, and the Japanese sea lion. The black-footed ferret is classified as Extinct in the Wild. The five Critically Endangered species, facing an extremely high risk of extinction, are the red wolf, the Ethiopian wolf, the Iberian lynx, the Mediterranean monk seal, and the Malabar civet.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Did you know that whales and dolphins and other cetaceans don’t make saliva? That’s what I said. Well, you know, it makes sense, right, if you’re surrounded by water? But they’ve also lost genes involved in blood clotting. Hm. Imagine all the drastic changes that their wolf-like, land-dwelling ancestor had to go through to become the streamlined ocean animals that they are today.
A team of researchers was interested in figuring out how this evolution happened on a genetic level. They mapped out 85 different genes that were lost in this aquatic transition. And their results were published in the journal Science Advances.
To walk us through the genetic steps whales and dolphins had to go through to make it into the water, meet Mark Springer, one of the authors on that study. He’s also a professor of biology at University of California at Riverside. Welcome to Science Friday.
MARK SPRINGER: Thank you. It is a pleasure to be here.
IRA FLATOW: It’s our pleasure to have you. Usually when you’re studying animals from millions of years ago, you look for fossils. But in your study, you looked at molecular fossils. What does that mean?
MARK SPRINGER: So in the genome, we have many different genes. And as you mentioned, the gene that’s expressed in saliva, it’s one of the genes in cetaceans that’s no longer needed. But even though a gene is no longer needed, there are remnants of that gene that are still in the genome. It’s just a dead gene or a fossil gene, if you will.
And it has mutations that have been fixed in that gene and make it inactive. So it’s a broken gene. It can’t do its job. And we were interested in looking for different genes that are broken, that formerly were functional and would have coded for different proteins. And so there are now alignments that are available for many different mammals.
And the alignment that we worked with is an alignment of genome sequences for more than 60 different mammals, including four different cetaceans. And we screened all of the protein coding genes for genes that have these inactivating mutations. And we were looking for genes where the inactivating mutations are shared by all of the different cetaceans. And we ended up with this list of about 85.
IRA FLATOW: 85– these are genes that changed from when they were– let me go backwards from now. What are the closest living relatives, or maybe not living relatives, on land that are close to the cetaceans?
MARK SPRINGER: So for a long time, it was not clear what the closest relative of cetaceans was. But in the last 30 years, we have learned a tremendous amount based on genes and also based on the fossil record. And what came from a study of different genes is that the closest living relative of cetaceans is hippopotamuses. And we think that they diverged from each other about 54 or so million years ago.
And living cetaceans, there’s two main groups of them. There are the baleen whales. And then there are the toothed whales. And they have a common ancestor that they shared with each other, probably about 37 million years ago. So there’s this transitional period from when whales diverged from hippos, until we have the last common ancestor of the toothed whales and the baleen whales. And we were interested in the changes, the genes that were lost on that particular branch in the evolutionary history leading to whales.
IRA FLATOW: OK, so give us an idea of what the steps were. What happened to allow the these land animals to become ocean-dwelling animals?
MARK SPRINGER: So we can learn about the different steps based on the fossil record and also from the genes that we find in the genome. So there are changes that occur with the skin. So the skin is much thicker, and the hair has been lost. Almost all hair has been lost in cetaceans. And that probably just gets in the way and causes additional friction.
You were mentioning saliva. Well, when you’re looking at a lot of these cetaceans, they’re just swallowing food whole anyway. And so they’ve lost a lot of taste receptors. And pretty much every organ system– if you’re looking at the kidneys, if you’re looking at the lungs, if you’re looking at various sensory systems, the eyes, if you’re looking at olfaction or the sense of smell– all of these systems are reorganized.
And what’s great about cetaceans is that this is one of the most remarkable macroevolutionary transitions that in the history of vertebrates. And we have access, not only to a wonderful fossil record now over the last 30 years, but also these genomes. And when we sort of take the genomic fossils and then the fossils that we find in rocks and put it all together, we can kind of piece together and learn about some of these steps.
So if you look at cetaceans, one of the things that they do is dive. And sometimes they stay down a very long time. There’s a beaked whale that has the longest recorded dive of more than two hours. And it was at a depth of almost 3,000 meters. So diving, it’s a very difficult thing to do. Mammals have lungs. They’re not like fish with gills.
And so one of the challenges that cetaceans have is, when they are diving and they’re down for a long time, they only have a limited amount of oxygen. And so one of the things that they do is that they reduce the amount of blood flow to the extremities of the body. And that’s something that we call peripheral vasoconstriction. So the blood vessels leading to the periphery of the body are constricted.
Well, a consequence of that is, it’s more likely that blood clots will form. And so a couple of the genes that we found were inactivated. On this common ancestral branch leading to whales are genes that are involved with blood clot formation. So these genes don’t work anymore. And it makes it less likely that cetaceans will get blood clots during diving.
IRA FLATOW: That’s interesting. Whales have flippers that appear very different than human hands, but the bones and the structures are still in there, right? What about whales that allowed them to move in the water? Did they have to lose genes or gain genes or change the genes that allowed them to rejigger their hands or their limbs into flippers?
MARK SPRINGER: There are a lot of changes in the sequence in the expression of some of the different genes. So some of the hox genes that are involved with the patterning of limbs, the sequence of expression is different. So we’re not finding so much of the gene loss associated with the transition from a fully terrestrial animal with long limbs to a whale, where you have flippers and then you’ve essentially lost the hindlimbs entirely. That seems to be mostly a case of changing the expression and the timing of expression of different genes.
IRA FLATOW: You found the gene that there was a loss of a gene for melatonin.
IRA FLATOW: Why is that important?
MARK SPRINGER: Well, melatonin is commonly known as the sleep hormone. And sleep is very challenging for fully aquatic marine mammals. We don’t have gills. Mammals need to resurface regularly to breathe. So it’s hard to sleep if you need to resurface regularly to breathe.
And ocean waters can also be very cold. And if you fall asleep, you lose body heat. So in a thermally challenging environment, it’s useful to do not have to fall asleep. And so whales have this unique way of sleeping. And they put only one side of the brain to sleep at a time.
And so it’s something that we call unihemispheric sleep. And so they’re alternating between the right side of the brain and the left side of the brain that they’re putting to sleep. So this allows them to do the things that they need to do to stay warm and to breathe without kind of compromising their ability to sleep.
But how does an animal sleep with only one side of the brain at a time, unlike us, where we’re sleeping with both sides of the brain? Well, the melatonin, this sleep hormone, it’s produced at night much more than in the day. So it’s kind of turned on at night and then off during the day. And it’s associated with sleep. And melatonin, it’s produced from what’s sometimes called the happy chemical, which is serotonin.
And there are a couple of enzymatic steps to get there. And both of the genes that code for these enzymes are broken in cetaceans. So they don’t work. So cetaceans can’t make melatonin. And we think that this loss of these genes may have been important in facilitating this unihemispheric sleep that is characteristic of cetaceans. And so it was an important change in this transition to a fully aquatic existence that we see in cetaceans.
IRA FLATOW: What is there about this land to water transition that you find so fascinating? Why do you study it? What interests you about it?
MARK SPRINGER: Well, it’s sort of the reverse of what happened hundreds of millions of years ago. So if you go back maybe 380 to 360 million years ago, our vertebrate ancestors were coming out of the water. And so that was one of the most important events in the history of vertebrate evolution. That’s what gave rise to tetrapods. And now in the Cenozoic, we have a group, whales, that have returned to the water.
So it’s fascinating because it’s a big macroevolutionary transition. And it involves all of the different organ systems of the body. And everything needs to be refashioned. So it’s not just a matter of changing hair color or this or that. It’s a sort of whole scale rearrangement of the entire body plan. So that’s why I think it’s so interesting. And also, because we have this sort of complementary approach, where we can take fossils and we can also learn from genes and try and piece together everything that has happened.
IRA FLATOW: Thank you for taking the time to join with us, Mark. Mark Springer, one of the authors on a study published in Science Advances, he’s also a professor of biology at the University of California at Riverside.