Eyeshine is the natural phenomenon of eyes “glowing” at night, resulting from light reflecting off an animal’s eye. This reflected light is not generated by the animal but is the result of an evolutionary adaptation that improves low-light vision. When a light source, such as headlights, hits the eye of a nocturnal animal, the light is sent directly back to the observer. This reflected glow can appear in various hues, including green, red, yellow, or the bright white that often catches the attention of drivers and hikers. The specific color of the reflection is a distinct clue to the animal’s identity.
The Biological Mechanism Behind Eyeshine
The reflective quality of eyeshine is caused by a specialized biological structure known as the tapetum lucidum, a Latin term meaning “bright tapestry.” This layer of tissue sits immediately behind the retina, the light-sensitive layer at the back of the eye. The primary function of the tapetum lucidum is to maximize the visual sensitivity of the eye in dim light conditions.
When light enters the eye, it passes through the retina, where some is absorbed by photoreceptor cells. Any unabsorbed light then hits the tapetum lucidum, which acts like a biological mirror. The light is retroreflected back through the retina, giving the photoreceptors a second chance to absorb it. This double-pass mechanism can enhance visual sensitivity by up to 50 percent, providing an advantage for animals active at night. The structure is composed of organized, highly refractive crystals or fibers, whose chemical makeup determines the specific color of the reflected light observers see as eyeshine.
Why Eyes Appear White: Specific Animals and Conditions
Bright white or silver eyeshine is most commonly associated with large prey animals like deer and elk, whose reflection is often described as brilliant white or pale bluish-white. This distinct reflection occurs because their tapetum lucidum is a tapetum fibrosum, composed of organized extracellular collagen fibers. These fibers create a highly reflective surface that tends to scatter light broadly. This broad scattering results in the bright, non-pigmented white or silver hue.
Other animals, including coyotes, certain dog breeds, and opossums, may also exhibit a white or silvery-white reflection. The appearance of a white glow can also be a result of viewing conditions rather than the animal’s biology alone. A bright light source, such as car headlights, can sometimes “bleach out” a faintly colored eyeshine, making a light yellow or blue reflection appear white to the observer. The angle at which the light strikes the eye and the observer’s position can also alter the perceived color, occasionally turning a typically green reflection into a silver-white one.
Color Variations for Identification
While white eyeshine is common in ungulates like deer and elk, other colors help identify different nocturnal species. Many common predators, such as domestic cats, raccoons, and foxes, display a green-to-yellow eyeshine. This greenish-gold color is often due to a tapetum cellulosum containing reflective crystals made of substances like riboflavin and zinc. The reflection helps them track prey effectively in low light.
In contrast, animals like rabbits and moose may show red or pink eyeshine. This red reflection is a result of blood vessels in the choroid layer behind the retina being illuminated. Since humans and other diurnal primates lack a tapetum lucidum, the red reflection from our eyes is light illuminating the vascular tissue. The color of eyeshine is a useful tool for identification, but it should be combined with other factors, such as the height of the eyes above the ground and the shape of the pupil, for accurate species determination.