Do All Animals Have Circular Pupils?

Though circular pupils like humans have are commonly found among animals, not all animals have them. For instance, goats, some snakes, mongooses and hippopotami have rectangular pupils, and cats, vipers and foxes have vertical slit pupils. Frogs have some of the most unusually shaped pupils, with different species of frogs having triangular, heart-shaped and star-shaped pupils, but cuttlefish are perhaps the oddest of all, with W-shaped pupils.

More facts about animals' pupils:

  • Animals that hunt at night tend to have elliptically shaped pupils, which can open wider than circular pupils to take in the maximum amount of available light. They also can be contracted further than circular pupils to protect the eyes from very bright light.

  • Some snakes and lizards can almost completely close their pupils, protecting the back of the eye from all but a pinpoint of bright light.

  • Cats can change the amount of light hitting their retina 135-fold because of the shape of their pupils; humans can change it only tenfold.

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