Pond Turtles Have Eyes Like Humans

A research team studying the eyes of a specific type of pond turtle have learned that the reptiles' eyes function in ways similar to both high order and low order vertebrates.

Unlike humans and many other mammals like cats and monkeys, all of which have forward-facing eyes, turtles -- like most low order vertebrates -- have their eyes positioned on the sides of their head.

Having a lateral eye position gives the turtle a certain set of advantages, but the pond turtle's eye structure affects specific eye muscles' direction of pull as well as the turtle's eye position when its peripheral vision is blocked when tucking its head inside its shell.

Of the six muscles that move each eye, the ones that move the lateral eyes are different than the muscles used to control forward-facing eyes. Previous research on pond turtle eyes has shown that one of the turtle's eye muscles -- the superior oblique -- was active when the turtle moved its head from side-to-side in the same way that the same muscle is active in animals with forward-facing eyes.

The latest research, published in The Journal of Comparative Neurology, by a team that included Saint Louis University professor of pharmacological and physiological science Michael Ariel, tested a theory that the pond turtle has more characteristics of a front-eyed animal.

Ariel suggested the pond turtle's eyes were similar to a mammal with forward-facing eyes in three ways: physiologically, anatomically and behaviorally.

To test the theory, Ariel, who has studied pond turtles for 25 years, began looking at the eye movement response to nerve stimulation, examining how muscles were attached to the eyes and head and examining the turtle's eye positions.

The researchers found that a turtle pulls its eyes in different directions when its head is out of its shell compared to when its head is retracted deep within its shell.

"Because the pond turtle can pull its head entirely into its shell, resulting in an obstructed field of vision, it appears that this turtle has developed a way to compensate and direct its eyes forward to best examine its environment," the researchers wrote in a statement. "Moreover, the superior oblique muscle may play a role in this behavior as its direction of pull is more like that of a front-eyed animal than that of animals with eyes on the side of their heads."

Ariel said the pond turtle is unique among all animals because they block their peripheral vision by pulling their heads into their shell.

"Not all turtles can do this. A sea turtle, for example, cannot pull its head into its shell. We expected that pond turtles would be like other turtles and other lateral eye animals," he said. "That wasn't the case. Surprising, their eye movements can also be like that of humans".