Why are cat's eyes split vertically?

We put Peter's question to zoologist Max Gray...

Max - So this is all to do with the fact that cats are very active at night and they need good night vision as well as having day vision - so they're not fully nocturnal and not fully diurnal. They're active at both times of day. But they've got very sensitive eyes so they can see at night but, as a result, during they day they can get very over-sensitive so they need to be able to control the amount of light that gets into their eyes really, really precisely - much more precisely than we do. And so, by having a vertical slit as a pupil they can narrow that down to a vertical slit and that controls the light reasonably well but they may need more control than that. By having a vertical slit they can then use their eyelids to close that slit further down to a point. So they have two ways of controlling the amount of light getting into their eyes.

Chris - Ingenious. What about animals like cows and horses - do they not have the slit? Certainly rabbits have slit running front to back, not up and down, don't they?

Max - Often it's not perfectly front to back. It's kind of at an angle and their eyes will move around as they move their heads because they're prey animals.

Chris - They're more interested in what's happening on the horizon?

Max - Yes. And when they're down eating it helps them see around them when they're vulnerable.

Chris - And I suppose if you're a predating animal like a cat, then you're going to be fixating on something and pouncing so you need to have very good depth perception and having a very small pupil is going to give you that, isn't it?

Max - Yes. That's mostly why you have eyes on predators at the front like ours and looking forwards because that's how you hunt. Whereas animals like rabbits and sheep will have eyes on the side of their heads so they can have a better field of vision for things that might pounce on them.

Chris - And we have a round one because that's the best compromise between distance, day, night?

Max - Yes. It's also easiest as well. Interestingly, if you look at cat species that are active during the day, lions are a good example. They don't have slit pupils, they have round, circular and you can go away and google this. If you google lions eyes, you look and they have round pupils like humans.