Most of us have probably caught our pet staring at something and wondered what it is they see – and do they see it the same way we do?
While it's hard to say for sure exactly what another creature's vision is like, we can make a good guess based on the structure and cells of their eyes, as well as the way they respond to certain behavioural tests.
Drawing on scientific evidence, we've created a series of graphics to give you an idea of how four different animals see – although owing to the amazing visual abilities of some creatures, some are easier to represent than others!
What's in an eye?
Much of what we can tell about an animal's vision comes from the eye itself – especially two important types of cell called rods and cones. Rods work well in the dark and are responsible for night vision, while cones – which function better in bright conditions – help us perceive colours.
Different types of cones respond to different colour wavelengths, so we can get an idea of the colours animals can and can't see well by looking at how many of each type of cone they have.
Humans have three types of cone corresponding to red, green and blue, but – and it's only fairly recently we've discovered this – not only can most animals see colour too, but many can see into the ultraviolet spectrum, allowing them to detect colours invisible to us.
What else do we know?
There are other ways of telling what animals might see. The position of the eyes on their head suggests their visual field – the scope of their vision, and whether they focus with both eyes (binocular vision) or each eye independently (monocular vision – most birds have this!)
Scientists are also interested in the concept of Flicker Fusion Threshold. In simple terms, this is the "frames per second" we see – for instance, human FFT is generally reckoned to be 60Hz. If a light turns on and off at a rate of 60 times per second, we'll see a continuous light rather than a flicker – which is, in fact, exactly what's happening every time we watch TV or look at a computer monitor.
Behavioural tests have shown that different animals have very different FFTs. Most insects have incredibly high visual refresh rates, for instance: they would see a movie as a series of slow-moving images rather than a moving picture.
What do dogs see?
Dogs' eyes have two types of cone cells that detect yellow and blue, so they can distinguish between these hues – although they probably see red, yellow and green as one colour, and blue and purple as one too. Some studies suggest that dogs can see into the ultraviolet spectrum too.
Their eyesight isn't as sharp as ours: it's believed to be about six times blurrier, although dogs have much better night vision. This is because their eyes have more rod cells, as well as a reflective structure called a tapetum that acts as a mirror to maximise light – this is also why dogs' eyes sometimes seem to glow in the dark!
What do cats see?
Cats have excellent night vision and have great control over their pupils, which lets them adjust to different lighting conditions. However, they're particularly short-sighted as their eyes can't change the shape of their lenses the way ours can. Its thought they have trouble focusing on objects further than about 18 feet away.
We're still not quite sure about their colour vision – some tests suggest they can see three colours, like we can, while others indicate only two, like dogs. They seem to have few of the cone cells that detect red, so may have difficulty seeing this colour.
What do bees see?
Bees see very differently to us. Like many insects, they see a rich and detailed ultraviolet world, so many flowers that look ordinary to us could be a riot of colours and markings to a bee. They have the "fastest" colour vision in the animal kingdom: they can easily distinguish individual flowers in a large field.
Bees also have compound eyes, so each eye has thousands of lenses rather than just one, each one providing a different picture. Their flicker fusion threshold is about 300Hz, five times faster than humans – which means they essentially see in "bullet time" (and can easily dodge being swatted!)
What do mantis shrimp see?
Mantis shrimp have the most bizarre and fascinating visual system known to science. It's debated whether they even truly "see" at all – some biologists claim their eyes work almost like a radar, sweeping the area in front of them to provide incredibly rich information, which appears to be processed in the eyes themselves rather than the brain.
The middle of their eyes have six rows of onmatidia, or facets, each of which has a different function, such as detecting certain wavelengths of light. Their eyes contain up to 16 different photoreceptors, which puzzles scientists as it's known that only three or four are necessary for fine colour discrimination – this would suggest they can distinguish billions upon billions of fine gradations of colour, including in the infrared and ultraviolet spectrums.
Why mantis shrimp need or have such incredibly sensitive vision is a mystery – it's been suggested they may use it to communicate secretly, using sections of the electromagnetic spectrum invisible to most creatures. It's believed that studying their unique optical system could help us improve high-tech components in DVD readers and cameras.