Why animals have different shaped pupils

06 August 2015
Posted by James Farr.

Why different land-based animals have different pupil shapes has been revealed by scientists in the US.

By looking at animals with vertically-stretched pupils, Martin Banks and his team at the University of California, Berkeley, found that they are very often ambush predators - animals that hide and attempt to pounce on their prey without warning.

Animals with horizontally-elongated pupils, on the other hand, are more likely to be prey species.

Writing in the journal Science Advances, Banks was able to put forward a number of benefits for each eye type.

Having very wide pupils provides animals with a broader field of vision in a horizontal plane, so prey species can more easily keep an eye out for predators by scanning larger swathes of their surroundings. This is also why their eyes are placed on the sides of their heads.

Eyes in this position can, however, cause problems when running. This is because the animals are forced to look out of the forward-facing corners of their eyes, which would cause blurring. To avoid this, the pupils are narrowed vertically. This reduces the amount of light entering the eye, minimising the blur and helping to make the process of seeing while you are running much more straightforward.

Ambush predators, on the other hand, have very different requirements. As they pounce, they need to accurately judge the distance to their prey, to increase the chance of each leap being successful. To maximise their ability to do this, they must make their pupils as tall and narrow as possible, producing vertical slits similar to those seen in cats.

However, a problem seemed to arise when considering the feeding habits of many prey animals. Sheep, for example, spend a lot of time with their heads tilted downwards to reach the grass. But to maintain the benefits of having horizontally-stretched pupils, the pupils must remain parallel to the ground.

They achieve this by rotating their eyeballs relative to their heads as they bend to eat, keeping their pupils parallel to the ground. Even more unusual is the fact that each eye must rotate in opposite directions, the left one going clockwise but the right one going anti-clockwise.

But what about humans? We have round pupils with no vertical or horizontal stretching. This is an evolutionary feature that corresponds to our love for patterns. If we were to have slit pupils in either orientation, this would cause a great deal of unwanted blurring and would drastically hinder reading capabilities, for example.

As Banks himself says, "humans just reached a different balance point in their goal to become good pattern-recognisers."

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