When light illuminates the darkness, a common sight is the apparent glow from animal eyes. This phenomenon, often called “eye shine,” is not light being emitted by the animal but rather a reflection of an external light source, such as a flashlight or car headlights. It is a specialized adaptation that helps many animals navigate and hunt in low-light conditions.
The Phenomenon of Eye Shine
The biological mechanism responsible for eye shine is a reflective layer located behind the retina in the eyes of many nocturnal and crepuscular animals. This layer is known as the tapetum lucidum.
When light enters the eye, it first passes through the retina. Any light not absorbed on this initial pass then strikes the tapetum lucidum, which acts like a biological mirror. This layer reflects the unabsorbed light back through the retina, giving photoreceptor cells a “second chance” to absorb it, effectively increasing the light available to the eye. This adaptation significantly enhances visual sensitivity in dim light, allowing animals to see better in conditions where humans would struggle. The presence of a tapetum lucidum can increase an animal’s sensitivity to light by as much as 50 percent.
The Biology Behind Red Eye Glow
The color of the eye shine, including red, is determined by the specific composition and structure of the tapetum lucidum, as well as the presence of blood vessels behind the retina. Red eye shine is often attributed to a less reflective tapetum or the significant presence of blood vessels in the choroid layer, which lies behind the retina and reflects red light. The specific minerals, such as zinc or riboflavin, within an animal’s tapetum can also influence the reflected color.
Humans, for instance, lack a tapetum lucidum, and the “red-eye effect” seen in flash photography occurs because the camera’s flash illuminates the richly vascularized retina, reflecting its red color. Some animals, particularly those lacking a tapetum, may also exhibit red eyeshine due to the reflection of light off these blood vessels.
Common Animals with Red Eye Shine
Alligators and crocodiles frequently display a reddish-orange or iridescent pink eye shine. This is attributed to their tapetum lucidum and vascularization behind it, which reflects light in this hue. Certain bird species, such as some owls, can also have bright red eye shine.
Among mammals, animals like rabbits and pikas commonly show red eyeshine. Moose eyes tend to glow red, though deer and elk often exhibit white eyeshine. Some rodents, including rats, and even certain bears, can also display a red reflection. The intensity and specific shade of red can vary depending on factors such as the angle of observation and the individual animal’s age or unique eye chemistry.
Variations in Eye Shine Color
While red eye shine is common, eye shine can manifest in a spectrum of other colors. These variations are determined by the tapetum lucidum’s specific structure, composition, and pigments. For example, reflective materials like guanine crystals or collagen fibers influence the color of the reflected light.
Cats commonly exhibit green or yellow eye shine, though their eye shine can also appear orange or even red, especially in blue-eyed or albino cats. Dogs can also display green or yellow eye shine, but their reflections may also be blue, orange, or even violet, depending on the amount of zinc or riboflavin in their eye cells. Other animals, like some horses, can have blue eyeshine, and deer often show a white or silvery glow.