The best time to distinguish male frogs from female frogs is during mating season. That's because the bulk of the unmistakable differences dictating gender are most apparent during reproductive activity. In most species, both genders have similar appearances with few clues as to male or female. If the males of a particular species are colored bright or dull, then so are the females.
For many frog species, the females are significantly larger than the males. This is to accommodate the amphibian mating process called amplexus. The male climbs on top of the female and she supports his weight as she discharges eggs and he spreads sperm on top of them. He is smaller so that his weight does not crush her. Her body is larger also to provide ample storage and transport room for the eggs.
Male and female frogs have differing genitals, but even this fact isn't particularly accessible in determining if an individual frog is male or female. This is because frog genitalia for both genders is housed within the frogs body. A male frog has two interior testicles and spermatic canal -- not a penis -- that produce the sperm. A female frog has ovaries and oviducts, as well as a uterus that stores the eggs until release, but is not involved in gestation.
Other Physical Differences
A small round disc called the tympanum covers the ears of both male and female frogs. On males of most frog species, the circumference of this small disc is larger than the eye of the frog. On females of most frog species, the disc's circumference is equal to the size of the frog's eye. During breeding season, the males develop gripper pads on their thumbs to aid them in amplexus. The females do not grow these pads. Males also develop a dark throat color during mating season that sets them apart from females, according to the Minnesota Department of Natural Resources. Otherwise, male and female frogs of each species tend to have the same coloring.
Male Only Behaviors
While eating habits, defensive maneuvers, use of camouflage and shedding of skin is the same for male and female frogs, there are a couple of behaviors that only male frogs engage in. The first is singing for a mate during the breeding season. The second is the "hugging" or "humping" aspect of amplexus. Male frogs don't restrict performing this behavior to only on female frogs. They will attach themselves to logs, rocks, trees, shoes and backpacks and hump until the mood passes.
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