Less a function of fear than bewilderment; simply put, deer freeze in headlights because they can’t see.Deer Eyes vs. Human Eyes. The eyes of deer and people share a number of features, but they also have a few significant differences. Pupil: Unlike human pupils, which are round and when dilated do not span much of the eye, deer pupils are elliptical (like a cat’s) and can dilate to cover the entire width of the orb. This greater dilation allows a lot more light to reach the retina, improving vision particularly at dawn and dusk when deer are most active.
Lens:The lens of a deer’s eye is larger than a human’s, and this allows more light to penetrate to the retina.
In addition, because people are most active during the day, our lenses are slightly yellow to filter out damaging ultraviolet rays. By contrast, since deer are active at night and have little danger of eye damage from ultraviolet light, their lenses are clear, which enable deer to see a bit of the UV spectrum – ultimately giving it even better night vision.
Tapetum:Essentially a reflector at the back of the eye (behind the retina), the tapetum lucidum reflects light back again across the retina, drastically increasing its light exposure.
Although not found in humans, it is a common bit of eye anatomy in the animal world. This reflective layer is found in dogs, cats, raccoons, rodents, birds and even fish, and is the cause of eyeshine – or the reflective glowing often seen when a light hits an animal in the dark.
Retina:Both deer and humans have two types of photoreceptors on the retina – rods and cones.
The cones are activated by brighter light and are sensitive to color, as well as help with distance vision. People have three different types of cone cells, each sensitive to a different color light – red, blue and green. On the other hand, deer have only two types of cone cells, those that can see blue and yellow.
As a result, deer see red and orange poorly, and also do not see detail and at a distance as clearly as humans.
However, deer have a much greater proportion of rod cells. Rods enable both humans and deer to distinguish between light and dark and to see in dim light (and at night). Since deer have many more rods, they are better able to see in low light, and thus have far superior night vision.
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