The glowing blue color seen when light hits an animal’s eyes at night is not the actual color of the eye but a phenomenon called eyeshine. This eerie, bright reflection occurs because many nocturnal and deep-sea animals possess a biological mechanism designed to maximize vision in low-light conditions. Eyeshine is a result of light being bounced back out of the eye, giving the animal a temporary, mirror-like glow in the dark. The specific hue, whether blue, green, or yellow, is determined by the internal structure responsible for this reflection.
The Biological Structure Behind Eyeshine
The ability of an animal’s eyes to shine comes from a specialized layer of tissue called the Tapetum Lucidum. This layer acts as a retroreflector, positioned immediately behind the retina. The primary function of the Tapetum Lucidum is to greatly enhance visual sensitivity when light levels are extremely low.
The Tapetum Lucidum intercepts unabsorbed light rays and reflects them back through the retina for a second pass. This “second chance” for light absorption significantly increases the amount of light available to the photoreceptor cells, which can improve night vision by as much as 50 percent.
This reflective layer is structured differently across species, but it is typically composed of multiple layers of highly organized, reflective materials. For example, in many carnivores, the layer contains cells with crystalline materials like zinc-cysteine or riboflavin, while in herbivores such as cows and horses, it consists of a fibrous arrangement of collagen. The presence of the Tapetum Lucidum is a trade-off, as the light scattering that enhances night vision also slightly compromises visual acuity, making the animal’s vision somewhat blurrier.
Factors Determining the Reflected Color
The specific color of the eyeshine, which can range from blue and white to green, yellow, or red, is primarily determined by the unique composition of the Tapetum Lucidum in that particular species. This layer functions on principles similar to thin-film optics, where the reflective color is influenced by the type of cells, crystals, and pigments present.
For instance, the tapetum may be rich in zinc, riboflavin, or guanine crystals, each of which reflects light at a different wavelength, resulting in a distinct color. The color can also vary with age, as seen in young cats whose eyeshine is initially blue before shifting to yellow or green as they mature.
The perceived color is not absolute and can be influenced by external factors, including the angle at which the observer views the reflection and the color and intensity of the light source. A light source, such as a flashlight or a camera flash, shining directly into the eye at a specific angle will produce the most vibrant eyeshine, and a slight shift in the observer’s position can change the color that is seen.
Animals That Often Exhibit Blue Eyeshine
Blue eyeshine is often associated with species that have a tapetum layer composed of specific structures, such as a fibrous arrangement of collagen. This structure tends to scatter light towards the blue or white end of the visible spectrum.
Examples of animals that can exhibit a distinct blue eyeshine include horses and certain breeds of domestic dogs. In dogs, the color of the eyeshine can vary significantly by breed, and while many breeds have a yellow-green reflection, some, like Irish setters, can display a distinct blue or bluish reflection. Specific mammals like the pine marten and woodchuck have also been documented to produce an electric blue eyeshine.
Animals That Do Not Reflect Light
Not all animals possess the Tapetum Lucidum; those that are primarily active during the day, known as diurnal species, typically lack this reflective layer. This group includes humans and most haplorhine primates, which rely on high visual acuity in bright conditions rather than enhanced sensitivity in darkness. Instead of a reflective layer, the light that passes through the retina in these animals is absorbed by the dark melanin pigment in the choroid layer behind it.
When a light reflection is observed in a human eye, such as the “red-eye” effect in flash photography, it is a completely different phenomenon. This red glow is caused by the light from the flash reflecting off the retina’s dense network of blood vessels, which are rich in hemoglobin. Other animals that do not possess a tapetum include pigs, squirrels, and most birds.