Why do some animals have round pupils while others, like cats, have elliptical pupils

A. It's possible that, as humans, we'll never completely understand the complexity of other animals' vision. However, it seems that -as elliptically shaped pupils belong mainly to nocturnal hunters - they may help with night-time vision.

A hunters eyes are a vital piece of equipment, and those whose eyes are optimised for hunting at night will need to protect their eyes during the daytime when there is more light. Reducing the size of the pupil is one way of doing this.

Q. Why are elliptical pupils better than circular ones for protecting the eyes

A. Pupils, of course, change shape to allow more or less light to enter the eye.

Elliptical pupils can be very quickly closed down to a much smaller aperture than circular ones, and need fewer muscles to do so. They can also be open much wider than circular pupils for when light levels are very low at night. Human's pupils are opened and closed by muscles around the edge; it isn't a particularly efficient system, but it doesn't need to be as humans restrict most of their activities to the daylight hours.

Q. Can elliptical pupils be completely closed

A. Yes, some snakes and geckos can close their pupils down completely -leaving only a miniscule pinhole aperture. So if the sleeping animal is disturbed during the day, the back of the eye is well protected. It also means that, regardless of distance, all objects are in focus.

Q. Doesn't an elliptical pupil distort vision

A. No, in fact the pinhole aperture is much better for focusing, especially for objects that are perpendicular to the opening -for example, small prey running across the ground -making them ideal for cats and snakes.

Do you want to quiz The AnswerBank on why some animal's organs are different form others

by Lisa Cardy