ANIMAL VISION:Take a look at the world through a different set of eyes

DOG For a long time, it was thought dogs saw the world in black and white, distinguishing between objects only based on differences in light and contrast. It is now known that they have two-colour vision, seeing the world in shades of yellow and blue. Dogs have good night vision and the back of their eye contains a reflective layer known as tapetum, which helps to maximise light detection in the dark. However, the central part of their retina is only 20 per cent cone cells (compared to 100 per cent in humans), so although they see better in low light, their daytime vision is much less detailed than our own.

BIRD Birds have arguably the most advanced vision in the animal kingdom. Rather than seeing in three colours, most birds can see four, extending their visual range into the ultraviolet part of the spectrum. Each of their cone cells also contains a drop of oil, which acts as a filter, further increasing their visual acuity. Why birds evolved the ability to see ultraviolet light is unclear. Some have UV-reflective feathers, others use their keen eyesight to spot UV-bright moths, butterflies, and fruits, and birds of prey use their UV vision to track rodents, picking up on their bright urine trails among the dense vegetation in the fields.

INSECT Insects are so small that if they had eyes like ours, the tiny lens would be unable to bend and focus the light. Instead, they have compound eyes, built from many smaller units known as ommatidia. Each has its own lens, a crystalline cone, pigments and light-sensitive cells, and together they create a mosaic, similar to the pixels on a television screen. The more ommatidia an insect has, the higher the resolution of the image. Some insects, like dragonfl ies, have around 50,000 of these units, giving them a clear view of the world around them, allowing them to rapidly detect movement in their environment.

SNAKE Pythons, boas and pit vipers have eyes surprisingly similar to our own, but they are able to see something we can’t. Using specialist pit organs near their noses, these snakes can ‘see’ heat. The pits have a pinhole opening and at the bottom is a membrane similar to the retina, with a tightly packed network of heatsensitive neurones – between 500 and 1,500 cells per square millimetre. The signals from the eyes and the pits converge on the same point in the brain, allowing the snakes to produce a combined visual and thermal image, or to switch between the two, like putting on a pair of night-vision goggles. 

RAT Rats are much more responsive to changes in brightness than colour, and they were originally thought to be colour-blind – 99 per cent of the light-sensitive cells in their eyes are rods. However, it is now known that they are able to detect some colour. Most (88 per cent) of their cone cells are sensitive to green light, but the remainder allow them to detect light in the blue-ultraviolet end of the spectrum. This ability allows the rodents to see territorial urine marks left by other animals. Because they rely on rod cells to see, their visual acuity is low and their vision is much blurrier than our own.

HORSE Horses have their eyes on the sides of their head, so they have a much wider fi eld of view that we do. However, they cannot see directly in front of themselves at close range and have a triangular blind spot that extends about 1.2 metres (4 feet) in front of their faces. At longer distances, the horse can use both eyes together for binocular vision, but they are also able to use each eye separately. With one eye looking forward and one looking back, they can keep a careful watch for potential danger. Like most other mammals, they cannot see red, so their world is a combination of shades of yellow, blue, green and grey

Each pictures shows how each animal would see