Sometimes, I learn the most interesting things from trying to answer the simplest questions.

A friend asked me last week why house cats had slit pupils but the great cats (lions and tigers) had round ones.

I honestly didn't know the answer, so I did some research to find out. It turns out Ronald Kroger from Lund University, Sweden, did some work on this subject.

He found that animals with slit pupils also had lenses in their eyes that had multiple focal lengths that were arranged in concentric circles. This made me think of the new multifocal contact lenses I'd seen advertised.

Having a slit pupil allows the cat to control the light entering the eye while still having access to all the different focal lengths of their lenses.

Kroger also found that animals with round pupils (including big cats) had lenses with a single focal length. Since the pupil can contract evenly around the lens, there is maximal light control.

Interestingly, this research showed that in all cases examined, the multifocal lenses accompanied slit pupils and single focus lenses had round pupils. It is unclear which adaptation occurred first, the pupil shape or the lens optics.

Some other research seems to point also to a relation between the time of day a particular animal hunts to the type of pupils they have. However, not all nocturnal hunters have slit pupils, so this theory may not pan out.

Like I said in the beginning, the simplest questions can have the most interesting answers.

Dr. Michael Dill is a veterinarian at Bienville Animal Medical Center in Ocean Springs, Miss.