Why cats have vertical slit pupils

Your eyes are your 'windows to the world', and it is your pupils that control how much light comes through those 'windows'. But why do some animals - like cats - have their pupils stretched vertically, whereas others - such as sheep - have horizonal pupils. James Farr spoke to Martin Banks from the University of California, Berkely, who has been looking into how these different pupil shapes came about...

Martin - There's a class of animals that tend to have vertical slit pupils, and a class that tends to have horizontally elongated pupils. The former are very likely to be ambush predators, meaning that they hide and jump out at prey, and the latter, the ones with horizontal pupils, they're extremely likely to be prey animals that other animals predate upon. And our argument is that the orientation of the pupil in both cases is advantageous for these particular animals.

James - And why would that be the case? Why would, for example, the prey need these horizontal slits?

Martin - They are on the ground, and they need to see panoramically along the ground to detect prey that might be approaching from some unknown direction. And what we showed is by having a horizontally elongated pupil, it maximizes the light input along the horizontal plane. So, we think that's the right way to orient the pupil to help them see panoramically along the ground and detect predators. The other thing about these animals is their strategy for not being captured is also to run. And that's a really interesting problem because they run with their heads forward but their eyes are on the side of their head. And so, in effect, they have to see out the side of their eye to run effectively. And by restricting the pupil vertically making it short that helps decrease blur that would otherwise occur in the corner of the eye. So, we think that's the right way to orient the pupil for these animals.

James - What about the vertical slits? Why would an ambush predator, why would they not want to see a wide area? Is there something else they need to focus on?

Martin - They're not so worried about being predated upon, but what they typically do is hide and wait to lurch at an unsuspecting prey animal. So, their task is not to see panoramically so much as to gauge distance accurately. So, this is a really clever adaptation to open the pupil up vertically while narrowing it horizontally in order to maximize their distance estimation capability.

James - One potential problem is if an animal, say a sheep, still wants to make sure that it's not going to get attacked by a wolf from its side. But if it leans its head down to eat, is that not going to cause problems at all?

Martin -  Yeah. That's a great question. We worried about that. We thought that might be fatal to our idea. So, we went to a zoo and to a farm, and we recorded video of goats, sheep, deer, and horses. And we found that the eye rotates in the head so the pupil remains roughly parallel with the ground even as they pitch their head down or pull their head upright. That movement is opposite in the direction in the two eyes as the head pitches down, the left eye has to rotate clockwise relative to the head and the right eye has to rotate counterclockwise relative to the head. And that's a movement humans can make but we make tiny ones and these animals are making much bigger ones. It's a really pretty remarkable capability.

James - Humans don't fit into either category of having vertical slits or horizontal slits. Why is that?

Martin - Well. There are multiple demands on the eyes. They're important for pattern recognition, in humans we read text and do fine recognition of objects. Having a vertical slit pupil or a horizontal slit pupil creates an astigmatic-like effect that could hurt some pattern recognition capabilities. So, we think humans just reached a different balance point in their goal to be good pattern recognizers.