Specimen of the Month: Turtle Eyes (More Specifically, Pupils)

The art of taxidermy has come a long way since forms were made of sawdust and rags. Modern instruments, supplies, and an improved understanding of anatomy now allow us to create near-perfect reproductions of live animals. But despite our efforts, a taxidermy mount is never really perfect, is it?

This specimen of a common snapping turtle is a great example of skilled taxidermy at first glance. All of the turtle's parts are generally intact and the position appears natural. The glass eyes used aren't those of a snapping turtle, though. 

This specimen was prepared with some interesting eyes. Compare them to that of a live snapping turtle.

The beautiful iris and rounded pupil of a snapping turtle. Image: Turtles for Tomorrow

Supposing that the taxidermist referenced eyes on a live snapping turtle, or at least a photo of one, they would have noticed that a turtle actually has round rather than vertical slit pupils. This may seem counterintuitive when other reptiles, like some snakes and alligators, have the advantage of the latter.

Glass eyes used in taxidermy come in a variety of shapes, colors, and sizes which
help us to make more realistic mounts. 

So how do pupil shapes evolve if some reptiles have round pupils, but those of foxes, alligators, and domestic cats are all slit vertically? The theories aren't free from some anomalies, but pupil shape tends to point to particular predator/prey behavior patterns. 

Most ambush predators have vertical slit pupils, which help them to judge distance while they lie in wait. Many herbivores, who must always keep an eye out for predators, benefit from the panoramic view that a horizontal pupil provides. And then there are the various mammals, birds, predators, and prey, who all have round pupils. The only common pattern that seems to come out of their shared characteristics is that they may be more active at day, and to a lesser degree are taller animals. While this doesn't explain the snapping turtle, our understanding of such adaptions is getting sharper.

Perhaps I too would have glossed over the fact that the snapping turtle's pupils are round, had I not read a note about it in the specimen record. It makes me wonder how many animals I picture incorrectly because I haven't thoroughly thought out their adaptions. Think about a red fox and a wolf--what do their pupils look like? Does it fit with the theories?