The subject of how deer perceive their visual world has been the object of much interest in recent years. The anatomy of a deer’s eye follows the same basic scheme as those of other vertebrates, although there are some subtle differences – one relates to the UV filtering ability of the lens, which we shall come to shortly. Deer eyes have an oval (slot-shaped) pupil that is orientated laterally, such that the pupil runs parallel to the horizon; this may help the deer focus on the entire horizon at once, rather than relying on the spot-focus afforded by a circular pupil.
Behind the retina, in the choroid (or vascular) layer, is a layer of reflective cells common to all nocturnal mammals collectively called the tapetum lucidum (from the Latin meaning “bright tapestry”). The tapetum cells reflect light that would otherwise be lost when it passes through the retina back into the eye, thereby increasing the amount of available light. The tapetum is responsible for the “eye-shine” familiar to hunters and often unfortunate car drivers; in deer, the eye-shine is typically orange, although the effect is a form of iridescence, so the colour will vary according to the angle of the light.
The eyes of deer are situated on either side of the head, which gives the animal a wide field of view. In his fascinating 2006 book Deer of the Southwest, Arizona Fish and Game Department biologist Jim Heffelfinger writes: “A deer’s eyes are set on the side of the head, allowing them to monitor almost a complete circle (310o)”. To put that in perspective, humans have a visual field of about 180 degrees. The drawback to having the eyes situated on the side of the head is that you lose binocular vision, which means severely limited depth perception. For deer, it seems reasonable that being able to see what’s sneaking up from pretty much any angle would be of greater benefit than being able to accurately judge how far away the ‘sneaker’ is.
For many years deer were thought to have a retina containing only rod cells that provided pretty poorly defined, black and white, vision. Rod cells are used for low-light vision (far fewer photons are needed to stimulate a rod cell than a cone, which means that it works well in low light conditions) and provide low resolution monochromatic vision. Cone cells, by contrast, afford colour vision and clarity (fine detail) in good light conditions. So, if a deer had a retina composed entirely of rods, the animal would see a rather blurry greyscale picture of the world – a stationary object would be difficult for the animal to resolve.
Field observations by stalkers who were able to approach within close quarters of deer, provided they froze when looked at, seemed to back-up the idea of poor vision in these animals. In 1978, however, Donald Witzel at the US Department of Agriculture published the results of a retinographic survey on White-tailed deer (Odocoileus virginiarus) in the American Journal of Veterinary Research. Upon studying the retina of these deer, Witzel found both rod and cone cells, suggesting that deer may have better vision than they were originally given credit.
Unfortunately, the study of deer vision—indeed, deer senses in general—have been rather limited. The majority of studies have been conducted on Fallow (Dama dama) and White-tailed deer and it is arguably unwise to extrapolate these results to other species. Nonetheless, many (although, not all) subsequent studies on these two species have confirmed Witzel’s findings. In 1994, for example, Gerald Jacobs and colleagues at the University of Georgia in Athens published a paper in the Journal of Comparative Physiology detailing aspects of the retinal sensitivity of Fallow and White-tailed deer. Professor Jacobs and his co-workers found that these species had both rod and cone cells – the rod pigments had a maximum sensitivity (called a “lambda max”) at 497 nm, which is in the blue spectrum. The biologists found two types of cone cells or, more accurately, cone cells that had one of two different pigments in them: one had short wavelength sensitivity (450 – 460 nm, again in the blue) and the other had sensitivity at the middle wavelengths (537 nm for White-tailed and 542 nm for Fallow) in the green part of the spectrum.
So, it seems that these deer are able to detect colours in the blue-green part of the electromagnetic spectrum, similar to a human with deuteranopia (red-green colour blindness), and that the rod cells may help the deer discriminate between shades of these colours. These findings are not a surprise to most biologists – deer are predominantly active from dusk until dawn and an ability to discern blue light is a great aid to low light vision.
Perhaps more interesting than deer being able to discern hues of blue and green is their vision in the ultraviolet part of the spectrum. While dissection of the deer eye reveals a granula iridica (sometimes called corpus nigrans – a projection of the iris into the eye that acts as a sunshade to reduce glare in bright light), deer don’t appear to possess a UV filter. Adult humans (excluding those who are aphakic, having a missing or damaged lens) cannot see light in the ultraviolet spectrum at 10 – 400 nm; the human lens contains UV filters, most notably 3-hydroxykynurenine (3OHKyn), that prevent light of this wavelength entering the eye. Deer, by contrast, don’t have this yellow pigment, suggesting that they may be sensitive to ultraviolet light. Indeed, an entire industry has sprung up in America offering hunters washing powder that doesn’t leave particles on the clothing that would otherwise reflect UV and make the person positively glow in a deer’s eyes. I should mention that there is still some debate over this idea; Jacobs and his team failed to find any significant response of their deer retinas to UV light.
Most of the experiments to-date have involved analysis of the deer’s retina and this has the potential risk of overlooking rare cone cells. Perhaps more importantly, the mere presence of cone cells on the retina tells us nothing of the deer’s ability to apply any colour vision they may afford. In a bid to circumvent some of these problems, a team at Stockholm University’s Zoology Department took a different approach. The researchers, fronted by Bjorn Birgersson, assessed the colour vision of Fallow deer through a series of behavioural tests; their results were published in the journal Animal Behaviour during 2001. The team found that all four individuals were able to discriminate green from grey, irrespective of brightness. The conclusion was that Fallow deer can use limited dichromatic (two colour) vision to discriminate between objects, by generalizing over slightly different colours in the green spectrum. The scientists suggest that blue/green-shifted colour vision may be useful in discriminating between different plant species or different parts of plants that might be of variable nutritional (or toxic) value.
The results of the 2001 study seem to fall in line with circumstantial evidence of colour perception in deer. In his 1995 book, Roe Deer: Conservation of a native species, Richard Prior recounts some fascinating stories of the visual acuity of this species. Prior tells of one captive Roe that reacted to the different coloured coats worn by its keeper; it paid no attention to a blue coat, but fled “crying in fear” when the keeper wore a red coat. In addition, Prior notes how, over the years, many Roe keepers and stalkers have become convinced that this species is able to recognise familiar clothing. In the 2015 series of the BBC’s Autumnwatch, Chris Packham noted that deer have lower visual acuity than humans, owing to them having 10-times the number of rods per cone that we have. Something we can discern at a distance of 200m (660 ft.) a deer could only discern at 20m (66 ft.).
In the end, however good one considers deer vision to be, I think that Richard Prior sums the situation up succinctly in his Deer Watch book. In this fascinating guide to deer and deer stalking, Prior points out that deer flee through often dense woodland, so their eyesight can’t be that poor!