Clay Logan, of Balclutha Primary School, asks :-

What animals are colour blind?

Christine Thomson, an anatomist at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

Visual stimuli are detected by two types of photo-receptors in the retina at the back of the eye. Colour vision is detected by cone cells and requires bright light, whereas shapes, especially in dim light, are detected by rod cells. This means that at night you only see with rod cells and what you see tends to be in shades of grey, rather than in colour.

Different types of cones cells respond to different wavelengths (colours) of light. Humans can usually distinguish red, blue and green colours. However, humans that are colour blind usually cannot differentiate between red and green colours due to lack of the appropriate cone cell types.

Dogs don’t have a full range of cone cells and so see in shades of blues, greys and yellow. Cats cannot see red. However, dogs and cats have more rod cells and can see at night better than humans. The ability to tell ripe (red) fruit from unripe fruit is important in foraging animals such as primates, so they will have good red-green colour vision. Reptiles, fish and birds may see four different colour wavelengths. Conversely, animals that are used to spending time in dimly lit environments, such as diving marine mammals or nocturnal animals, tend to have single cone types and, therefore are colour blind. Many invertebrates have good colour vision and although they may not see red colour well, they can see ultraviolet wavelengths.

A general rule of thumb is that if one sex of a species is brightly coloured, then the species will be able to see all those colours. Similarly, animals that live in a brightly coloured environment (e.g. fish in a coral reef) probably perceive a wide range of colour.