When light hits an animal’s eyes in the dark, the resulting glow, known as eyeshine, is light reflected back to the observer, much like a mirror. This enhanced reflection is an evolutionary strategy that allows certain species to maximize their vision in low-light environments. While eyeshine can appear in various hues, from blue to green, an intense orange glow signals specific underlying anatomy. This vibrant reflection prompts the question of what biological differences cause this particular color to appear.
The Mechanism of Eyeglow
The ability of an animal’s eye to glow is due to a specialized layer of tissue called the tapetum lucidum, which means “bright tapestry” in Latin. This layer is situated directly behind the retina. When light enters the eye, it passes through the retina, and any photons not immediately absorbed by the photoreceptor cells strike this reflective surface.
The tapetum lucidum acts as a retroreflector, bouncing the light back through the retina for a second pass. This mechanism effectively doubles the chance that available light will stimulate the photoreceptors. This “second chance” for light absorption enhances visual sensitivity, giving nocturnal and crepuscular animals an advantage when hunting or navigating at night. Animals that lack this structure, like humans, have a dark layer that absorbs light after it passes the retina, which is why our eyes do not produce eyeshine.
Why Some Eyes Appear Orange
The specific color of the eyeshine is determined by the composition and structure of the tapetum lucidum. This reflective layer is often iridescent, meaning its color changes based on the angle of observation and the materials it contains. The orange or yellow-orange hue is linked to the presence of specific chemical compounds within the reflective cells.
In many species, the reflective material includes crystals, such as zinc-cysteine or the vitamin riboflavin. Riboflavin naturally possesses a yellowish fluorescence, and its concentration strongly influences the resulting eyeshine color toward the yellow-to-orange spectrum. The precise arrangement and density of these reflective crystals or fibers also contribute to thin-film optics, which selectively reflects longer wavelengths of light perceived as yellow, amber, or orange.
Identifying Animals By Orange Eyeshine
Orange or amber eyeshine is common among mid-to-large-sized nocturnal predators and omnivores. In North America, the eyeshine of the mountain lion and the black bear often appears as a deep amber or orange. A black bear’s eyeshine is typically large and round, set relatively low to the ground, reflecting the animal’s size and posture.
Canines, such as the coyote and certain fox species, can display an orange or reddish-orange eyeshine, though green is also common. The orange reflection in a domestic cat’s eye, which varies with age and breed, is a classic example of this coloration. Raccoons are known for a bright yellow or amber eyeshine, appearing as two small, close-set dots often seen at an intermediate height.
Identifying an animal requires considering the height of the reflection. A pair of orange reflections less than a foot off the ground suggests a smaller animal, while a reflection two to three feet high could indicate a larger predator. The shape of the pupil can also offer clues, as felines tend to have vertically elongated pupils that may appear as a thin, bright line of orange.
The American alligator is a non-mammalian example, displaying a reddish-orange eyeshine due to a different type of reflective layer. When spotting orange eyeshine near a body of water, especially at the water line, an alligator is a strong possibility. Wolf spiders, which are common across North America, reflect a vivid orange-red when light is shined on them at ground level.
Environmental Factors That Change Eye Color Perception
While biology dictates the true color of the eyeshine, the environment influences what the human eye perceives. The reflective layer is iridescent, meaning the angle at which the light hits the eye and the angle of observation can shift the perceived color. A reflection that is truly orange may appear more yellow or red depending on the observer’s distance and position relative to the animal and the light source.
The type of light source used alters the spectral energy reflected back. A warm-toned light, such as an older incandescent flashlight, has more red and yellow wavelengths to reflect, which may intensify an orange glow. A bright, cool-toned LED light may cause the same eyeshine to appear more yellow or even greenish. Atmospheric conditions like fog or haze can diffuse the light, washing out the intensity and altering the perceived color.