Noncircular pupils explained

Several years ago, I reviewed the book Evolution’s Witness: How Eyes Evolved, by Ivan Schwab. The book is downright encyclopedic, and I could not praise it highly enough. But in my review I wondered about elongated pupils, such as those of a cat, which are barely discussed the book. I remember reading somewhere that the elongated pupil could be stopped down farther than a circular pupil, but that explanation does not account for the problem that horizontal structures will be more clearly resolved than vertical structures (presuming that the pupil is elongated vertically and the eye is nearly diffraction limited).

A team from Berkeley and Durham University now proposes a better explanation. Without going into detail, they find that predators that ambush their prey, like cats, typically have vertically elongated pupils. From the abstract:

Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. This is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours.

One way to put it: All the blur due to defocus is in the vertical direction, so horizontal contours are blurred when defocused, whereas vertical contours are not, because the blur is parallel to the contour; see their Figure 2(A). I do not want to go into detail, but they demonstrate that ambush predators, like the cat, that prowl close to the ground benefit from having good stereo vision for vertical contours. Prey animals, like the goat, often have horizontal pupils, which supposedly facilitate wide-angle views. Curiously, their pupils remain horizontal regardless of the orientation of their heads.

This paper goes a long way toward explaining why different animals have differently oriented pupils. You may see a video and a short article here and an NPR report here.

The paper does not explain how, when I was an elongating pupil in fourth grade, my teacher, an ambush predator if ever there was one, managed to see through 360°.