They say eyes are the windows to the soul, and while this may not be strictly true (and may be more than a little ambiguous), they can tell you rather a lot about how their owners lives. Many people find the gaze of a cat mesmerising, even slightly disconcerting, whereas the gaze of a goat is disconcerting only because you aren’t entirely sure what it’s looking at, or if it even knows itself. This effect likely has a lot to do with their eyes, specifically their pupils.
Cats are predators. In order to hunt effectively, they need to be good at a number of things. This first is depth perception. A cat which pounces on its prey and completely misjudges where it will land is not going to be a very efficient hunter (and will likely be very hungry). Its eyes, therefore, are positioned in the front of its skull, giving it bifocal vision and excellent depth perception. Its pupil shape is important because of the way in which it hunts. Cats are ambush predators, meaning they lie in wait or sneak up on prey before attacking from a short distance. They are also relatively short, so they cannot see over especially long distances. Finally, they hunt in a variety of light conditions, from very bright light to almost none. Cats’ pupils are vertical slits, which can expand to be almost round and contract to be extremely narrow. This shape enables them not only to optimise the amount of light which enters the eyes, but also helps in judging short distances extremely precisely.
Goats, on the other hand, are prey animals. Their survival depends on avoiding predators rather than catching food (because, fortunately, grass doesn’t move very fast). Their eyes are situated on either side of their head, which gives them an enormous range of vision, giving them more opportunity to see approaching predators. Their pupils are likewise designed to broaden their field of view. Goats, like many prey animals, have horizontal slit, or rectangular, pupils, which give them a panoramic view of their surroundings, particularly at ground level, where predators are likely to be. Interestingly, when goats and other ungulates bend down to graze (which they spend an awful lot of time doing), their eyeballs rotate to keep their pupils parallel to the ground!
So predator pupils are vertical slits, prey pupils are horizontal. Easy. But what about lions? Or tigers? Or bears (oh my!…sorry.)? These large predators have round pupils, not vertical slits.
This is mainly because of their height. Animals whose eyes are further off the ground, and whose vision is not obscured by as much vegetation, need to be able to see further distances. This usually means that their vision, particularly depth perception, is not quite as good as animals with vertical slit pupils, but this is something of a trade-off, largely made up for by the animal’s size, strength, intelligence or ability to hunt in a pack. Pinpoint accuracy is less important when you are 250kgs of muscle, teeth and claws. Or if you’re a wolf, and simply chase your prey until it’s exhausted, then tear it apart with your friends. Lovely.
So, slit-shaped pupils belong to short predators, round pupils belong to taller predators. Got it…..Ah, no. Not necessarily. This rule does not apply to everybody. Take snakes, for example. Some snakes have round pupils, others have vertical slits.
Most snakes hunt in a similar way, i.e. ambush their prey close to the ground, so distance vision is not a likely reason for this difference. Some say pupil shape is related to whether or not the snake is venomous. Supposedly, round pupils means the snake is not venomous, whereas slit pupils means it is. However, this is a myth. In fact, Australia’s inland taipan, arguably the most venomous snake in the world, has round pupils. The real reason for the difference is down to light. Like cats, some snakes hunt primarily at night or in low light conditions. This means they need large pupils to maximise the amount of light entering the eye, thus enabling them to see well in the dark. However, they do not want to be blinded in daylight, so their pupils need to be able to contract, reducing the amount of light entering the eye. This is why vertical pupils are the optimum shape. They enable the snake to make the most of whatever light conditions it is in, without compromising the depth perception or accuracy necessary for it to hunt efficiently. Snakes which hunt only during the day, however, do not need to be able to see as well at night, so have round pupils.
This light-controlling feature exists in some prey animals too. Some amphibians, such as frogs and toads, also possess slit-shaped pupils, as they are active during both night and day. However, as their survival depends on avoiding the plethora of animals which eat them, their pupils are horizontal, rather than vertical slits, maximising their field of view.
Geckos, likewise, make use of the slit-shaped pupil to control light, but in a rather interesting way. As hunting reptiles, their pupils sit vertically, but in bright light, contract to form a series of beads. It is believed that this feature enhances their depth perception in a variety of different light conditions and environments.
Speaking of different environments, fish and other denizens of the deep face entirely different conditions from land animals altogether. Light passing through water behaves differently from light passing through air, and becomes somewhat distorted. Some animals, such as flatfish and rays, have evolved crescent-shaped pupils, which help to mitigate the light-distorting effects of the water around them, without compromising the wide field of view they need to spot predators.
Some molluscs, such as cuttlefish, have taken this one step further. Their pupils are W-shaped, which enhances image contrast underwater and allows for better control of light hitting the retina. The W-shape balances bright light coming from above and darkness coming from below the animal, giving them good vision in both light conditions.
Finally on our list of pupil shapes, we have insects and arachnids, which don’t have any pupils at all. Done.
There are many more interesting and unique pupil shapes in the world which don’t necessarily adhere to normal rules, but here are a few rules-of-thumb as to why animals have different shaped pupils:
Horizontally elongated = prey animal
Vertical slits = short, ambush predator, active at night
Round = tall predator, spots prey from a distance, or only hunts in daylight
Crescent = underwater animal