Why do animals have differently shaped pupils?

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Only the least observant could fail to notice that pupils come in striking different shapes. Not in humans, of course, where they are invariable circular, but in many other terrestrial animals where they are sometimes round, but also often elongated and slit-like. The orientation of these slits also varies; most predators, including your neighbourhood tabby, have vertical slits, while in prey animals they are more likely to be horizontal. This intrigued a team of optometrists and physicists from the University of California, Berkeley, and Durham University (UK), and so they set out to discover what drives these variations. And in a paper published in Science Advances1, they think they have the answer.

They first tried to understand what difference it makes if pupils are elongated rather than round. Several effects have been suggested in the past, but the authors identified one in particular as the most critical. It’s related to the ability of pupils to change size to accommodate different light levels; without this we would be blinded in bright sunlight and visionless in the dark. It turns out that the light throughput of an elongated or slit pupil can be adjusted more than a circular pupil, meaning that eyes with elongated pupils can handle a wider range of light levels. So we would expect that nocturnal and polyphasic animals – those that are active both at night and in the day – would be more likely to have elongated pupils. And it turns out that, on the whole, this is the case.

But this doesn’t explain the orientation of the pupil. Why might predators tend to have vertical slits while their prey’s are elongated? Here things get a little more complicated. The authors argue that it comes down to how animals judge distance. The textbook explanation is through stereo; seeing the same scene with two eyes allows depth to be estimated from horizontal shifts between the two images. The more an object appears to shift, the closer it is. The accuracy of this method depends on the distance between the eyes, which has an obvious upper limit. One way of simulating more widely-spaced eyes is to move the head from side-to-side. But for ambush predators, like our friendly cat, rocking her head while preparing to spring an ambush is probably not the best hunting strategy.

Luckily for her, she has an extra trick up her sleeve; it’s also possible to estimate the distance to an object by gauging how out of focus it appears. As her eyes accommodate to an object at a certain distance, objects which are far in front or behind that point will be blurred. Of course, this only works if there is detectable blur at the distances she is interested in. The degree to which with blur changes with distance is known as the depth-of-field, and this depends strongly on the pupil size.

So with all this in mind, what is the optimal depth-of-field strategy for a cat? To work out distances from stereo she would like vertical contours to be in focus, since it is easier to determine the horizontal shift between two vertical contours. For horizontal contours, which tend to be landscape features, a small depth of field makes it possible to estimate distance from blur. So she wants a large vertical depth of field and small horizontal depth of field. This is exactly what a vertical slit pupil provides.

So what about horizontal slits for prey? Their main concern is avoiding predators, and so they need a wide field of vision to give them the greatest chance of spotting one. This is the reason that most have eyes on the side of their head rather than the forward-facing eyes associated with predators. While our eyes, with circular pupils, have poor eccentric vision (we can’t see much out of the corner of our eyes), the paper shows that horizontal slit pupils improve eccentric vision horizontally (for scanning along the horizon) at the expense of vertical vision (seeing things coming from above or below).

The pedant might point out that many grazing animals spend the majority of their time with their head inclined. So are the advantages lost while they are eating? Apparently not, because many of these species have the ability to roll their eyes to counteract the effect of bending down (cyclovergence), demonstrating how important the horizontal pupil orientation must be.

A final question was whether these adaptions were simply the results of pressure on some common ancestor, and so inherited largely by chance, or whether evolution is continuing to drive changes in pupil shape. By reconstructing ancestral trees the authors were able to show that different pupil shapes have evolved multiple times independently. This means that pupil shape must of importance to the survival strategy of at least some animals, perhaps something to ponder when we are designing our own imaging and computer vision systems.


  1. Banks, Martin S., et al. “Why do animal eyes have pupils of different shapes?.” Science advances 1.7 (2015): e1500391

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