Animals with vertical pupils are more likely to be predators, while those with horizontal pupils are more likely to be herbivorous prey, according to UC Berkeley researchers.
In a study published in the journal Science Advances last week, researchers at UC Berkeley and Durham University documented a strong correlation between the orientation of the pupil and the ecological role of the animal as predator or prey. For the study, the researchers statistically analyzed 214 terrestrial species, including snakes, mongooses, horses and rhinoceroses.
The study is not the first to notice the relationship between the orientation of elongated pupils and ecological niche, according to Gordon Love, a professor of physics at Durham University and co-author of the study.
“However, we are the first to look at the correlation between a whole range of species with vertical and horizontal elongated pupils and whether they are ambush predators, active predators or foragers,” he said in an email. “More importantly, we have provided an explanation as to why pupils are oriented in a particular way.”
Animals with vertically slit pupils, such as cats and foxes, can see vertical edges such as trees or fences sharply, even if they are not focusing on them. But horizontal items are blurry unless focused on, according to William Sprague, a postdoctoral researcher in vision science at UC Berkeley who co-authored the study.
He contrasted the large “depth of field” — the large distance between the nearest and farthest point eyes focus on — required for short ground predators with that of a bird flying high above the ground that needs a smaller depth of field. Vertical pupils allow shorter predators to see well despite this large depth of field, he said.
Martin Banks, a professor of optometry at UC Berkeley and co-author of the study, previously performed depth perception studies in humans. He was interested in how humans perceive depth, and that got him thinking about how depth perception would differ in animals with slitted pupils.
According to Banks, the correlation between the pupil and the ecological niche had only been anecdotally documented in scientific literature before this study. As early as 1942, the literature mentioned that prey animals tend to have horizontal pupils that give a panoramic view of the ground, but this new study used computer models to confirm how these eyes work, Banks said.
The researchers also observed what happens when herbivores with horizontal pupils tilted their heads downwards to graze. Love, who lives in the Yorkshire Dales in Northern England, observed “cyclovergence,” or rotation of eyes to keep horizontal pupils level with the ground, among the many sheep that roam the area. Banks made similar observations at the Oakland Zoo while observing sheep and goats.
“We are probably not the first to notice this, but it is certainly not a well-known fact, and I have enjoyed bringing this to people’s attention,” Love said.
Banks said future research may create models of fish, not included in the study, that often have unusually shaped, sometimes “w”-shaped, pupils and dips in their retinas.
According to Banks, the total process of working on gathering and analyzing data, writing the paper and getting the paper published took between two and two and a half years to complete.
“I have enjoyed this work because the question posed is simple — anyone can understand it,” Love said. “The answer is also apparently simple, but there is a lot of complexity and subtlety in the solution, which isn’t obvious at first sight.”