While in the car, your daughter says, "What's that?" Up ahead, beside the road, a pair of tiny luminescent circles shine at you. It's dark, so the circles are all you can see. And in the twinkling of an eye, they disappear. By the time you have driven to the spot, there is nothing there.
With the arrival of fall and with darkness encroaching so much earlier, this is a common occurrence throughout the north on a drive home. Nocturnal animals make their appearances earlier in the evening and their shining eyes remind us that they are out there. Cats and dogs are just some of the animals with eye shine. Many species have eyes that shine in bright light. Humans, though, are not one of them.
So, where does eye shine come from? And why don't we have it? The answer lies in the evolution of the eye. The simplest form of the answer is that our eyes did not evolve to optimize our nighttime vision. But that deserves a bit more of an explanation.
Even though the eye is relatively small, it is perhaps one of the more complex parts of our bodies. Certainly, eyes are one of the more intriguing as they are said to be windows to our soul and they are the first thing that men look at in a woman.
The eyeball is a small, almost spherical sack that contains a viscous liquid. The sack has a small hole in it, called the pupil, that allows the passage of light inside. The size of the pupil is controlled by the surrounding coloured tissue called the iris. Interestingly enough, this basic anatomical structure has evolved many different times in many different species over the past four and a half billion years and while each version has a pupil, they are not all shaped the same. Cats, for example, have a vertical slit. Horses, on the other hand, have a horizontal one. For humans, our pupil is circular.
The pupil and iris, along with the front of the eye, are covered with the cornea. This actually also serves as a lens and helps focus all of the surroundings on the retina at the back of the eye. The lens inside the eye provides the fine focus that allows us to shift our vision. But the cornea is the first point of light capture.
Needless to say, the cornea must be transparent as must be the lens and the viscous fluid inside the eye in order for light to reach the retina. It is here that light is captured and converted to nerve impulses to be processed by the brain.
The retina has two distinct types of cells present - the rods and the cones. They have slightly different roles to play in vision. The rods are extremely sensitive to light and provide the shading for what we see. When comparing two identical blue balls, it is the rods that will tell you if one is not as well lit as the other. It is also the rods that measure the shading of the ball and indicate shadows.
The cones are good at seeing colour and thereby separating objects from one another. Refining that sense of colour allows us to have very high visual acuity. Indeed, the human eye has evolved to have extremely good visual acuity but at the cost of being able to see in shades of gray and at not being very good at seeing in the dark.
In nocturnal animals, such as cats, their eyes are dominated by rods. This allows them to see in much dimmer light than you and I. A cat is capable of seeing clearly in about one sixth of the light that we need. This is why a dark house, late at night, is no obstacle for a cat. It is their time to play.
But it's not just the density of rods that allow cats to see better. Their eyes have one more trick. Behind the retina is a layer called the tapetum lucidum, also known as the bright-carpet, which is responsible for reflecting any photons of light that get through the retina to the back of the eye back into the retina. Basically, it gives each rod two chances to capture a photon - one on the way in and another on the way out. Doubling the chance of capturing a photon decreases the amount of light necessary to see by half. It is this reflected light - from the tapetum lucidum - that we see when we catch an animal in our headlights.
Different species have different eye shine IDs. For example, the tapetum lucidum is red in a rabbit's eye but typically silver white or yellow for cats and deer. Labrador retrievers have a green tapetal reflection although the colour for a dog is breed dependent. Some dog and cat breeds are colour dilute, lacking tapetal pigmentation, and therefore their eye shows up as red due to the presence of blood vessels.
Eye shine is one way to identify animals. It certainly helps us see our furry friends in the dark and maybe more to the point, it helps them see us.
