It’s a question that has long interested scientists working on vision and optics.
In a new study published in the journal Science Advances, colleagues from Durham, Berkeley and I explain why these pupil shapes have developed.
Goats, sheep, horses, domestic cats, and numerous other animals have pupils which vary from fully circular in faint light to narrow slits or rectangles in bright light. The established theory for this is that elongated pupils allow greater control of the amount of light entering the eye. For instance, a domestic cat can change its pupil area by a factor of 135 from fully dilated to fully constricted, whereas humans, with a round pupil, can only change area by a factor of 15. This is particularly useful for animals that are active both day and night, allowing for much better vision in low light conditions.
The cat on the right has got its night-vision goggles on. Mark Sebastian (L); Kurt Bauschardt (R), CC BY-SA
However, if the only reason for elongated pupils was to control the amount of light entering the eye, the orientation would not be important: horizontal, vertical, or diagonal would all offer the same advantages. Instead, the pupils are almost always horizontal or vertical, which suggests there must be other benefits which explain this orientation.
Pupils fit for every niche
Our work has focused on the visual benefits of vertical and horizontal pupils in mammals and snakes. One of the most interesting factors we found is that the orientation of the pupil can be linked to an animal’s ecological niche. This has been described before, but we went one step further to quantify the relationship.
We found animals with vertically elongated pupils are very likely to be ambush predators which hide until they strike their prey from relatively close distance. They also tend to have eyes on the front of their heads. Foxes and domestic cats are clear examples of this. The difference between foxes and wolves is down to the fact wolves are not ambush predators – instead they hunt in packs, chasing down their prey.
In contrast, horizontally elongated pupils are nearly always found in grazing animals, which have eyes on the sides of their head. They are also very likely to be prey animals such as sheep and goats.
We produced a computer model of eyes which simulates how images appear with different pupil shapes, in order to explain how orientation could benefit different animals. This modelling showed that the vertically elongated pupils in ambush predators enhances their ability to judge distance accurately without having to move their head, which could give away their presence to potential prey.
Sheep can usually see you coming. Sarah Nichols, CC BY-SA
Grazing animals have different problems to deal with. They need to check all around for prey and they need to flee rapidly in case of attack. Having eyes towards the side of their head helps them to see nearly all around them. Having a horizontal pupil enhances the amount of light they can receive in front of and behind them while reducing the amount of light from above and below. This allows them panoramic vision along the ground to help detect potential predators as early as possible. The horizontal pupil also enhances the image quality of horizontal planes and this enhanced view at ground level is also an advantage when running at speed to escape.
So, vertically elongated pupils help ambush predators capture their prey and horizontally elongated pupils help prey animals avoid their predators.
We realised our hypothesis predicted that shorter animals should have a greater benefit from vertical pupils than taller ones. So we rechecked the data on animals with frontal eyes and vertical pupils and found that 82% are what is considered “short” (which we defined as having a shoulder height of less than 42cm) compared with only 17% of animals with circular pupils.
We also realised that there is a potential problem with the theory for horizontal elongation. If horizontal pupils are such an advantage to grazing animals, what happens when they bend their head down to graze? Is the pupil no longer horizontally aligned with the ground?
We checked this by observing animals in both a zoo and on farms. We found that eyes of goats, deer, horses, and sheep rotate as they bend their head down to eat, keeping the pupil aligned with the ground. This remarkable eye movement, which is in opposite directions in the two eyes, is known as cyclovergence. Each eye in these animals rotates by 50 degrees, possibly more (we can only make the same movement by a few degrees).
As the horses head moves, its pupils remain aligned with the ground. Source: Gordon Love
There are still some unexplained pupils in nature. For example, mongooses have forward-facing eyes but horizontal pupils, geckos have huge circular pupils when dilated which reduce down to several discrete pinholes when constricted and cuttlefish have “W”-shaped pupils. Understanding all these variations is an interesting challenge for the future.
Gordon Love receives funding from the Engineering and Physical Sciences Research Council for some of this work.