Can cats see in the dark?
Cats are definitely established as a domestic animal. The Egyptians loved them so much they mummified them, and for a time during the reign of the Pharaohs, killing a cat meant the death penalty for the perpetrator.
Cats are definitely established as a domestic animal. The Egyptians loved them so much they mummified them, and for a time during the reign of the Pharaohs, killing a cat meant the death penalty for the perpetrator.
Thomas Huxley, the 19th Century British biologist, even claimed that the cat made the British Empire great. His round-about reasoning was circuitous - spinsters in villages had cats, the cats ate mice, the bees (apparently the favourite food of mice) then flourished, and so the red clover around the village was fertilised by the bees and grew well.
Then the cattle ate the clover and grew nice and meaty, and finally, loyal young British males ate the beef and grew up big and strong and manned the ships that made the British Empire great - or so reckoned Thomas Huxley.
Cats are popular. They even have their own mythology - which includes the ridiculous claim that they can see in the dark.
The truth is that cats cannot see in absolute darkness any more than we can. However, they are much better adapted than humans for seeing in low levels of light.
They use three clever evolutionary adaptations to allow them to do this.
First, as compared to a human eye, the cat eye can let in several times more light.
Second, the cat eye is very richly endowed with rods. They are two types of cells in the eye that turn the incoming light into electrical signals - cones and rods. Cones are less sensitive, and work better when there's lots of light around. Rods are more sensitive and work better in low light situations. In daytime, the rods just switch off. Cats have lots of rods.
Third, cats have an extra "mirror" layer at the back of the eye behind the retina, which means that the incoming light has two chances to hit the rods. This mirror layer is called the "tapetum lucidum". It's made from 15 types of cells, and is very reflective. It glows a silvery-greeny-golden colour in most cats, but can be ruby-red in Siamese cats.
The incoming light enters the front of the eye, passes through the transparent innards of the eye, and enters the retina. At this stage, the light will register if it hits a rod. In the human eye, if it misses, the light is absorbed in a black layer behind the retina, and is gone forever.
But in the cat eye, if the light has not hit a rod, it will reflect off the mirror layer and be bounced. The light now has a second chance to hit a rod, and to be put to work.
So when you shine a very bright light (such as a camera flash) into a human eye, you don't see the black layer, because it's black. But you do see a reddish colour, from the blood vessels that nourish the retina.
There are a lot of them, because the retina is a very oxygen-hungry organ. These blood vessels are in front of the retina, so they light up well. The reason we don't "see" these blood vessels normally is because our brain "electronically" snips them out of our vision. But the camera does see them, and hence the dreaded red-eye in so many flash photos.
And when you shine a bright light (such as a torch, or car headlights) into the eyes of a cat at night, you see a reflection of that light, thanks to the tiny "mirror" in the back of the cat's eye.
This made Percy Shaw a very wealthy man.
In 1933, while working as a road-repairer on the English roads on a foggy night, he saw this reflection. He immediately came up with the idea of making artificial "catseyes" to mark the centre of the road.
He patented his invention in 1934. It had a convex lens (like the lens in the human eye) in front of an aluminium mirror. They were set into a rubber pad, which in turn, was encased in a cast iron housing, which was then mounted in the road and oriented so as to reflect the car's headlights back to the driver. In 1935, he started making them in bulk in a factory.
In 1937, a British Ministry of Transport study found that his version of catseyes (or "reflective road studs") survived better than his competitors, and so they are still used today.
So Percy Shaw, and his bank account, were able to bask in the reflected glory of his catseyes.
Published 12 December 2005
© 2024 Karl S. Kruszelnicki Pty Ltd
