While the diversity of eye shapes throughout the animal kingdom is well known, it seems like there’s a considerable difference between the eyes of grazing animals and those of predators, who have vertical slits rather than horizontal pupils.
A study analyzed the eyes of 214 different species and indicates that the shape may have a great deal to do with competition within the ecosystem. Those animals more likely to be hunted – such as goats and antelope have developed visual acuity that lets them scan the horizon for predators, whereas predators have eyes that allow them to target the movement of potential prey.
The new study was led by scientists from the University of California, Berkeley, along with Durham University of Great Britain, and has been published today in the journal Science Advances.
They used a computer generated model of a sheep’s eye to determine how its shape could benefit various species, taking note of the direction of light into the pupil. A horizontal pupil took in light from left and right – making it easier to spot predators lurking in the grass.
“People had been saying that the horizontal pupil helps expand the horizontal view of the ground, they just hadn’t shown that,” according to visual scientist Martin S. Banks, of Berkeley and who co-authored the new paper. “Our contribution was to build a model and show that that happened.”
While it seems obvious to note that herbivores such as deer and sheep tend to keep their heads to the ground as they eat, a number of photographs of goats taken at a local petting zoo revealed that their eyes are capable of turning as much as 50 degrees north when their heads are lowered – meaning that the pupils are never perpendicular to the ground – a new discovery in scientific literature. This is common in horses as well as antelopes and deer.
By contrast, the slit eyes seen typically in pit vipers are capable of long depth perception which is ideal for stalking prey.
Not everyone is quick to embrace this distinction, however. Animals like the chinchilla are not predators but have vertical slits for pupils, for example. “There are so many exceptions to the rules the authors think to have discovered, that there must be much more to pupil shape than being predator or prey, big or small,” says Ronald H.H. Kröger, a biologist of Lunds University in Germany who was not involved with the study.
Jenny Read, a vision scientist of Newcastle University in Britain, was more supportive, calling the new research “an incredibly neat example” of natural selection at work – how competition between predators and prey can influence eons of development.
“It’s very striking that with all the attention that animal eyes have attracted over the years that in the scientific literature no one has apparently noticed or commented on this,” she added.