At first glance, most eyes look the same. There's a small opening through which light passes. That light goes through the transparent liquid behind the lens and strikes the retina, a thin film of light-sensitive nerve cells that line the back of the eye. But there's actually a great deal more to vision than that.
Some animals, like humans, cats, and owls, have their eyes facing forward, while in some species the eyes face sideways, like cows and zebras. That distinction, while superficially simple, betrays complex underlying musculature.
That's because most animals have what's called a vestibulo-ocular reflex or VOR. That's the reflex that allows you to maintain your focus on a part of your visual field even while your head moves. If you rotate your head to the right, your eyes rotate left to compensate. (Not all animals have a VOR. Some birds, like chickens and pigeons, lack the reflex, which is why they bob their heads while they walk. It's a different solution to the same problem.) Without the ability to stabilize the world, we'd all get seasick very quickly.
While animals with forward-facing and sideways-facing eyes both have VORs, it works slightly differently. That is, the different muscles that control the eye do their work different ways. In forward-facing mammals, for example, the superior oblique muscle rotates the eye to the side, away from the nose. In mammals with eyes on the sides of their head, the same muscle instead does the opposite: it rotates the eyes inwards, towards the nose. These processes, called abduction and adduction, respectively, are two of the mechanisms that comprise the VOR.
In 2006, Saint Louis University School of Medicine researchers Michael Jones and Michael Ariel discovered that red-eared sliders (Trachemys scripta elegans), a type of pond turtle, have eye muscles that work as if they were front-facing mammals despite having lateral eyes. That is, when the researchers stimulated the the superior oblique muscle, the eyes rotated outwards, rather than inwards. That surprise led Jones and Ariel to explore this puzzling finding by looking into the physiology, anatomy, and behavior of this peculiar turtle's eyes. That study, led by J. R. Dearworth Jr. of Lafayette College, was published earlier this year in the Journal of Comparative Neurology.
One thing that sets these turtles apart from other types of turtles is that their visual fields change when they retract their heads into their carapaces. The turtles' shells restrict their peripheral vision and limit their head mobility. As a result, when their heads are retracted, their eyes are more like those of forward-facing mammals, and when extended, their eyes are more like those of side-eyed mammals. That's a unique challenge for the VOR, because it has to allow the turtles to maintain a stable field of vision both when their heads are extended and when they are retracted into their protective armor.
In 2006, Jones and Ariel discovered that the red-eared sliders' eyes behaved as if they were forward facing. In the new study, they also looked at the anatomy and physiology of the eyes. It turned out that their ocular anatomy and physiology were also unique for turtles. Say I gave you the eye of a red-eared slider, but you had guess what animal it came from. Based on its anatomy, physiology, and behavior alone, you'd assume that the eye was from an animal with forward-facing eyes, like a human. And you'd only be half right, since the turtles spend a lot of time with sideways-facing eyes.
Evolution was faced with a unique problem: an animal whose eyes usually face sideways, but sometimes face forwards. Each type of animal - forward-facing or sideways facing - evolved it's own method for achieving the VOR; here's a critter that is both types at once. In order to maintain the VOR, the red-eared slider evolved eyes that work differently from every other species with sideways facing eyes.
What that suggests is that the muscles that control the red-eared slider's eye evolved according to the constraints imposed by their unique ability to completely retract their heads into their shells, something no other turtle can do. The result was a peculiar turtle with human-like eyes.
Dearworth J.R., Ashworth A.L., Kaye J.M., Bednarz D.T., Blaum J.F., Vacca J.M., McNeish J.E., Higgins K.A., Michael C.L. & Skrobola M.G. & (2013). Role of the trochlear nerve in eye abduction and frontal vision of the red-eared slider turtle, Journal of Comparative Neurology, 521 (15) 3464-3477. DOI: 10.1002/cne.23361
Header image via Dr. Michael Ariel, used with permission. Second image modified from Dearworth et al. (2013).