David, of Sawston Village College, Cambridgeshire, UK, asks :-

Do dogs see the same colours we do?

Claudia Ugarte, of Massey University's School of Veterinarian Science, responded.

The short answer is "no" as far as we know from behavioural experiments.

The eye is lined inside by a tissue called the "retina". The function of the retina is to receive and conduct visual stimuli to the brain. It does receive the visual stimuli through two types of receptors, rods and cones. These receptors have very different functions and have evolved according to the time of the day the animal in question would be most active.

Rods are more sensitive to light (need less light and are better at night) but cannot see colour (at night colour is not very important, movement and depth of vision are). Cones on the contrary are not as sensitive (need more light) but can see finer detail and colour (good during the day).

So the balance between how many rods and how many cones are in the retina determines what is seen better and when the sense of vision will be most useful. From this you can make already in your mind that animals that live at night (like the hunting wild dogs from which the domestic dog had its origin) have more rods. The dog has a retina with lot of rods and only a central area that contains 20% cones. Dogs also have only two t ypes of cones (as oppose to 3 in humans which have very good diurnal vision) and are considered to be colour blind in the red-green spectrum. However they do see yellows, bluesand greys.

This particular colour spectrum seen by dogs is part of a group of characteristics of predatory nocturnal or semi-nocturnal animals. The other characteristics are eyes on the front of the face and not on the sides (i.e. horse), poor accommodation for seeing close and far, inability to see detail but very sensitive to motion (like dinosaurs). Dogs are considered to have a certain degree of myopia (they do not see as well as a human far away) and not see very well very close to their eyes either.