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Why do cats lick people? It is not an affectionate gesture: cat licking is the same as a flea’s tongue probing the skin. This might help explain why cats have evolved into such fearsome hunters.
It was always something of a mystery why cats are such successful hunters. Cats are fast and well adapted to hunting in the dark, but how can this best be achieved when cats have such bad eyesight and are generally colour-blind?
Cats have a reputation as being nocturnal hunters, but there have been many reports of them being active in the day, often hunting birds. The ability of cats to hunt prey in the daylight may be a recent adaptation, since there is no evidence that our closest living relative, the African golden cat, is an active hunter.
So, what could be behind the evolutionary success of the cats?
It was recently shown that there is a direct link between the evolution of cat colour vision and its enhanced abilities to hunt prey in the day.
The fact that cats do have poor vision is well-known. Cats have evolved to be highly adapted to hunting in dim light because they need excellent vision to detect small prey.
Colour vision is most useful in dim light. Cats have lost most of their cone cells, which are responsible for detecting colour in dim light. These cones are found in the retina of most mammals, but in cats they are concentrated at the rear of the retina.
The remaining cone cells, in the so-called ‘mid-periphery’, are much more sensitive to bright light than the cone cells at the centre of the retina. They can see better in bright light but cannot see much in dim light, and so it is better for hunting prey in the bright light.
Cats do not have to rely on their cone cells in dim light. They have a very high density of rod cells, and can detect colour even when their cone cells are in darkness.
Cat retina. A cat has more cone cells in the centre, and a higher density of rod cells in the mid-periphery than a human or other felids. The cat retina is thus better adapted to detect prey in bright light. Credit: Nature Neuroscience, doi.org/3b3j
There is some evidence that cats also have a higher density of rod cells in the mid-periphery of their retina than other felids, such as dogs.
This is interesting because cats are descended from dogs. The ancestors of cats, Felidae, diverged from the ancestors of dogs, Canidae, about 26-36 million years ago (Ma). This was at the start of the Cenozoic Era, which is the era during which mammals began to diversify.
The density of rod cells in cats is higher than in dogs and other felids.
How could this affect hunting?
It may be that the ancestors of cats hunted better in bright light than the ancestors of dogs, because they had a higher density of rod cells in the mid-periphery.
What is also interesting is that it seems the densities of rod cells in the cat retina are higher than the densities of rod cells in the eyes of other felids, such as lions and leopards. This suggests that there may have been more selection for a higher density of rod cells in the cats than in other felids, such as leopards and lions, over the past 30 million years.
This may have implications for understanding the origins of the colour vision of cats.
Most cats are colour-blind and have no cone cells in their retina. This means that they cannot see colour, although they can see light.
To make up for their loss of cone cells, cats have evolved an ability to see colour based on differences in light intensity. Cats can see light by its colour – for example, the colours of birds are determined by the wavelengths of the light that they reflect – but they cannot see the colours themselves.
It is thought that cats evolved the ability to see the colour of their prey using rod cells in the retina.
The reason that cats are able to see their prey by its colour is that cats have evolved an extremely high density of rod cells in the mid-periphery of their retina.
It appears that cat rods have evolved not only to help them to see colour, but to detect prey in bright light. This appears to have evolved from the need for cats to hunt in the bright light of the day.
Why don’t cats use their cone cells to see colour?
Cats do not use their cone cells to see colour because they evolved to hunt at night.
During the daytime, cats hunt in bright light. The density of their rod cells in the mid-periphery of the retina means that they are better able to see prey in bright light.
Cats evolved to hunt at night because there are no predators that hunt by day. It is the advantage of hunting at night that allows cats to evolve such high densities of rod cells in the mid-periphery of the retina.
It is important to remember that when talking about colour vision, we are talking about vision in dim light. Colour vision is a function of cone cells, and these are used by all mammals to detect colour in dim light.
There are many other functions of cones. For example, cones are used to detect the direction of a light source and the speed at which an object is moving in dim light.
There is no evidence that cats have lost these functions.
Cats are colour-blind and have no cone cells in their retina, but it is not because they cannot see colour.
The high density of their rod cells in the mid-periphery of their retina means that cats can see their prey in bright light, so they have no need to use their cone cells.
Cats may not be able to see colour in dim light. Cats have lost most of their cone cells in the retina.
Cats have evolved to hunt at night because there are no predators that hunt by day. It is the advantage of hunting