It's easy to think of pupils as simple holes that widen in the dark and shrink in the light. But if you look across the animal kingdom, you'll see a variety of pupil shapes: vertical slits in cats and snakes, horizontal rectangles in goats and horses, and W-shaped crescents in cuttlefish, for instance. The shape of an animal's pupil can reveal a lot about how that animal sees and what it needs to survive.
In a perfect optical system, the shape of the pupil shouldn't matter much. "In an ideal world, the way optics is generally taught, the pupil's sort of irrelevant because all the light should be coming to an exact point anyway," said Jenny Read, a visual neuroscientist at Newcastle University in the U.K.
