New research reports a correlation between an animal’s pupil shape and its ecological niche.
In the journal Science Advances, researchers at the UC Berkeley and at Durham University in the United Kingdom present results of their analysis of pupil shape in 214 land animal species.
Those with vertically elongated pupils, or vertical slits, were more likely to be ambush predators that are active day and night. In contrast, those with horizontally elongated pupils are extremely likely to be plant-eating prey species with eyes on the sides of their heads. Circular pupils were linked to active foragers, animals that chase down their prey.
“For species that are active both night and day, like domestic cats, slit pupils provide the dynamic range needed to help them see in dim light yet not get blinded by the midday sun,” said Martin Banks, a UC Berkeley professor of optometry and one of the study’s lead authors. “However, this hypothesis does not explain why slits are either vertical or horizontal. Why don’t we see diagonal slits? This study is the first attempt to explain why orientation matters.”
Using computer models, the researchers found that the horizontal pupils typically seen in grazing prey animals like sheep, deer and horses expanded their field of view. When stretched horizontally, the pupils are aligned with the ground, getting more light in from the front, back and sides. The orientation also helps limit the amount of light from the sun above so the animal can see the ground better, they said.
“The first key visual requirement for these animals is to detect approaching predators, which usually come from the ground, so they need to see panoramically on the ground with minimal blind spots,” said Banks. “The second critical requirement is that once they do detect a predator, they need to see where they are running. They have to see well enough out of the corner of their eye to run quickly and jump over things.”
For ambush predators with vertical-slit pupils, the authors emphasized the importance of accurately gauging the distance they need to pounce on their prey. They explained that vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. It is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours.
The authors also found that vertical pupils were linked to predators that were close to the group. For example, domestic cats have vertical slits, and tigers and lions have round pupils like humans and dogs.
They expect to examine associations of eye position and pupil shape in aquatic, aerial and arboreal life in future studies.