Do dogs and cats see in black and white or colour, and how do cats manage to see well enough to hunt at night?

 Answer:        The inner light-sensitive lining at the back of the eyeball is the retina and the cells there are of two main types called rods and cones. Rods are the receptors the eye uses for night-time viewing and sudden movement. Cones are for daytime vision and interpreting colour. Dogs and cats have both rods and cones in their retinas but both species, and more especially cats, have more rods than cones when compared with humans. This means that the cat’s night and motion vision is almost certainly superior to that of dogs, and both are superior to humans, essential for their hunting instincts. Other adaptation to low light levels include a light reflecting layer at the back of the eye (which gives the glowing ‘cat’s eye’ effect at night), and a relatively large eyeball with a pupil that can open very wide to capture the maximum amount of light. Strictly speaking cats and dogs are not nocturnal animals, who are active in the depth of the night (despite often showing an inclination to do so) but are crepuscular, which means that they are designated to do most of their hunting at dusk and dawn.

 At night colour is not perceived, only black and white and shades of grey. In daylight, dogs and cats can see colours but not to the same extent as humans. We think cats can distinguish purple, blue, green and yellow better than other colours, but do not perceive them as intensely and vibrantly as we do. Dogs, we think, are similar, being able to distinguish purple, blue and yellow.

 Cats appear to be slightly short-sighted, which would suggest that their vision is tailored more for closer objects, such as prey, that they can capture within running distance. Objects that are further away than several hundred yards rarely interest a cat.

Dogs, however, especially sight hounds like the Borzoi, are said to be able to see a hand waving up to a mile away. Some birds have even more incredible vision – the peregrine falcon, for instance, is said to have vision equivalent to being able to read a newspaper a mile away!

 Another interesting fact about sight is that, like humans, dogs and cats have binocular vision to enable them to judge distances well. Some flat-faced breeds, like Pekingese dogs and Persian cats, have a wider field of view than narrower, longer-nosed breeds such as Siamese cats and Collie dogs.

To the right is a colour-enhanced scanning electron micrograph of the retina in a guinea-pig eye. Totally invisible to the naked eye this image is captured far beyond the capability of a medical  microscope. On the left hand side of the picture is the photoreceptor layer which contains the light-sensitive rod cells that help convert light impulses into an image in the brain.