If you’ve ever snapped a photo of your cat with a flash or caught a deer staring back at you from the headlights, you’ve witnessed one of nature’s most ingenious adaptations. Those glowing eyes aren’t generating light—they’re reflecting it, thanks to a remarkable structure called the tapetum lucidum.

What Is the Tapetum Lucidum?

Latin for “bright tapestry,” the tapetum lucidum is a reflective layer of tissue behind the retina in the eyes of many nocturnal and crepuscular animals. Think of it as a biological mirror that gives incoming light a second chance to reach the photoreceptor cells in the retina.

When light enters the eye, some is absorbed on its first pass. The rest hits the tapetum lucidum and bounces back through the retina for a second opportunity to be detected. This effectively amplifies available light, dramatically enhancing vision in low-light conditions.

In fiction, this adaptation makes for a fascinating trait. In A REFLECTION OF SHADOWS, Lady Colleen Stewart possesses a human version of this ability, giving her enhanced night vision that aids in her rooftop escapades across London. Her black Cat-sìth companion also has a tapetum lucidum, reflecting the shared evolutionary connection between her lineage and cats—a trait that helps both of them navigate darkness with ease.

Who Has It?

The tapetum lucidum appears in a surprisingly diverse range of creatures:

Carnivores: Cats, dogs, and other predators rely on it to hunt at dawn, dusk, or night.
Ungulates: Deer, horses, and cattle use it to detect predators in dim light.
Marine animals: Many fish and some sharks use it to navigate murky depths.
Others: Even some spiders, alligators, and lemurs have it.

Interestingly, most primates—including humans—lack a tapetum lucidum. Our ancestors evolved as diurnal creatures who relied on color vision, which performs best in bright daylight. But for Colleen and her Scottish lineage, evolutionary quirks allow her to gain abilities akin to her feline companions—climbing, leaping, and moving through the shadows in ways that leave others in the dark.

The Science of Eyeshine

That eerie glow you see in animal eyes at night—known as eyeshine—varies in color depending on the species and the tapetum’s composition. Materials like zinc, riboflavin, or collagen crystals reflect light differently.

Cats: Typically greenish-gold
Dogs: Yellow-green, blue, or white
Alligators: Distinctive red

In Colleen’s world, her own golden eyes echo the same principle—reflecting light in darkness, giving her an edge when sneaking through London streets or spotting danger before anyone else.

The Trade-Off

Like most evolutionary adaptations, the tapetum lucidum comes with a cost. While it increases light sensitivity—some studies suggest up to 50% more light reaching the photoreceptors—it can slightly reduce visual sharpness. The reflected light scatters, creating a subtle blurring effect.

For animals that need to detect movement or shapes at night, the trade-off is worth it. A slightly blurrier image you can actually see is far more valuable than a perfectly sharp one that’s cloaked in darkness.

For Colleen, this trade-off is trivial compared to the freedom it affords her: she can chase down a suspect, leap between rooftops, or tail a target in shadows where others see nothing. A slightly blurrier image in near-darkness is far more valuable than being blind to danger entirely.

An Evolutionary Marvel

The tapetum lucidum has evolved independently in multiple animal groups—a phenomenon known as convergent evolution. This underscores just how valuable enhanced night vision is for survival—whether in nature or fiction.

Next time you spot glowing eyes in the dark, pause for a moment to appreciate this elegant natural engineering.

And if you pick up A REFLECTION OF SHADOWS, you’ll join a heroine in possession of this night-vision, racing across rooftops and slipping through shadows with feline grace.

Nature’s Night Vision: The Tapetum Lucidum | A Reflection of Shadows