Rather than producing their own light, the glowing appearance of animal eyes in the dark is a result of a specialized biological adaptation that enhances their vision in low-light conditions. This visual effect is known as eyeshine.
The Phenomenon of Eye Shine
When a light source, such as a flashlight or car headlights, illuminates a coyote’s eyes in the dark, the light is reflected back, creating the appearance of shining eyes. This reflection is often visible as a reddish-orange to yellowish-green hue, though the exact color can vary. The phenomenon is most noticeable in low-light environments, where even faint light sources can trigger the reflective effect.
This reflective capability provides a significant advantage for nocturnal and crepuscular animals, allowing them to navigate and hunt effectively when light is scarce. The eyeshine you observe is simply light that has entered the eye and been redirected outward.
The Science Behind It: Tapetum Lucidum
The biological mechanism behind eyeshine is a specialized layer of tissue located behind the retina, known as the tapetum lucidum. This Latin term translates to “bright tapestry” or “shining carpet,” accurately describing its reflective function. When light enters the eye and passes through the retina, any photons not immediately absorbed by the photoreceptor cells then strike the tapetum lucidum.
The tapetum lucidum acts like a retroreflector, bouncing the light back through the retina, giving the photoreceptor cells a second opportunity to absorb the light. This “double-dipping” of light significantly increases the amount of light available to the photoreceptors, thereby enhancing visual sensitivity in dim conditions. While this process improves night vision, it can result in a slightly blurrier image compared to vision in brighter light. The composition of the tapetum lucidum, which can include crystals or fibers, determines its reflective properties and the color of the eyeshine.
Beyond Coyotes: Other Animals with Eye Shine
The tapetum lucidum is not unique to coyotes; it is a common adaptation found in many vertebrates and some other animals that are active in low light. This includes a wide array of species, such as domestic cats and dogs, deer, raccoons, and even some fish and spiders. Many nocturnal predators, like mountain lions, possess this reflective layer, aiding their hunting abilities in darkness. Prey animals, such as deer, horses, and cattle, also exhibit eyeshine, which can help them detect predators at night.
The color of eyeshine can vary significantly between species, appearing as white, blue, green, yellow, pink, or red. For example, while coyotes often display greenish-gold or yellowish-green eyeshine, deer and elk eyes can appear white, and some bears may show yellow-to-red or even green eyeshine. These variations are influenced by the specific mineral content and structure of the tapetum lucidum, as well as the angle of observation and the light source. Humans, along with some other animals like squirrels and pigs, lack a tapetum lucidum and therefore do not exhibit eyeshine.