Vision is the main sense in humans and also the one that many other animals rely heavily upon. Even though the eye in all vertebrates has a similar structural plan, vast differences exist in the way that we all see the world; it is surprising that even between similar species there are marked variations in their vision, so our outlook is in contrast to that of other primates. Here we take a look at how our sight compares to that of various animals.
Not all animals can differentiate between colours, but the ability to do so confers an advantage, as it allows us to distinguish one thing against its background; an example might be to detect ripe fruits or to find a mate. Whether an animal can see in colour and the range of shades that it can detect is down to the pigments it possesses in its cones; these are the type of photoreceptor found in the retina, which are able to absorb light of different wavelengths. The greater the number of pigments a species has, the wider its colour vision. Humans and Old World Monkeys possess three types of pigment, providing us with wider colour vision than cats, dogs and New World monkeys, which only possess two pigments; though they still have superior colour vision to bulls, which contrary to popular belief only see in various shades of grey. Animals with poorer colour vision compensate with other aspects of their sight – cats see better than us at night for instance – or have other senses that are heightened – such as their sense of smell in dogs. However, our ability to see in colour is not the best in the animal kingdom. Various birds possess four or more pigments in their cones and pigeons are believed to be one of the best animals at detecting very slight differences between varying shades. It also isn’t necessarily the most complex creatures that are the leaders when it comes to colour vision; mantis shrimps for instance have cones with twelve colour pigments.
Seeing beyond colour
Although humans and many other animals can’t see outside of the colour spectrum of wavelengths, there are a number who can do so. For example, bees and butterflies can see within the range of ultra-violet light and it is thought that being able to do so is advantageous, as flowers have special UV patterns that attract them in for pollination. Various raptors can also see UV light, which they use to help them in finding both prey and a mate, as well as for migratory purposes. Reindeers are unique amongst mammals, as it has recently been found that they too are sensitive to UV light. Living in snowy conditions where 90% of UV light is reflected, being able to detect this aids them in their search for food – lichens absorb UV – and protection from predators – fur and urine also take up UV – allowing these to appear black against the snow. However, by not being able to see UV light humans and their counterparts protect their retina from damage and seeing within a narrower spectrum allows sharper vision.
The vision of vertebrates can operate over a range of light intensities, but some animals can see better in the dark than others. It is our rods – the other type of photoreceptor – which allow us to see in dim light, but these do not allow us to see in colour or sharply; though it has been found that geckos and some frogs might be able to distinguish some colours in the dark. Owls and other nocturnal animals have better night vision as might be expected.
How well an animal can see around it is largely determined by the position of its eyes. Humans and owls have forward facing eyes, so are required to turn their heads if they wish to see the world to the sides, but by looking ahead are better able to judge distances when hunting. This is in contrast to an animal such as a horse or pigeon that has eyes at the side of its head, so can see in all directions; this is an evolutionary advantage allowing them to keep a look out for predators, but does mean they can’t see directly ahead of them.
Other forms of vision
Sight isn’t the only way that animals are able to form an image; this is well demonstrated by bats and certain snakes. Bats use echolocation to generate an image of their surroundings; they emit ultrasonic sounds to produce echoes then compare the sound sent out with the echo to build up a picture. Despite having poor vision, this allows them to detect their prey even when it is dark; they do though use their vision to navigate when the distances are beyond the range over which echolocation can work. Pit vipers, boas and pythons use thermal vision, which allows them to produce an infrared map, used primarily to target their prey; this is so accurate that even a rattlesnake with no vision can successfully detect and strike another animal.
While all animals have developed sight in one sense or another to help them find food, a mate, shelter and to escape danger, the diversity of vision that exists is immense, with species evolving eyesight that best suits their needs. Humans might be thought of as at the top of the animal kingdom and although their sight beats that of some animals in certain respects, by no means do they necessarily have the most superior vision.
Claire is a nurse and primarily writes about eyesight but her love for animals led to her researching this piece.