More Than Meets the Eye: The Answer to Why Animals Have Different Eye Pupil Shapes
In a recent study with 214 different terrestrial species of varying pupil shapes and foraging mode (i.e. predator or prey), Professor Martin S. Banks and his colleagues, at the University of California, Berkeley and the United Kingdom’s Durham University, identified a striking and significant correlation between pupil shape and ecological niche. Through computer-generated eye models and multinomial logistic regression, the group showed that pupil shape was accurately predicted by how an animal gathered food, whether it foraged on plants or hunted other animals.
To understand why this correlation existed, Banks used point-spread functions (PSFs), which visually describe how an imaging system (for instance, the eye) responds to a single, localized source of light, to illustrate the 3-D diffraction patterns of light in both a vertically slit and a horizontally elongated pupil. What he found was that animals with vertically slit pupils were more likely to be ambush predators that waited until their prey was relatively close before attacking. Banks’s PSFs and eye models demonstrated that the vertically elongated pupil allowed for a sharper field of vision for vertical contours – vertical lines in physical space. This condition helped predators easily identify and estimate the locations of prey on flat, horizontal ground, and the restricted horizontal field of vision also appeared to aid predators in estimating distances along the ground where the prey were.
However, vertically slit pupils worked best only for short animals, whose eyes were fairly close to the ground. Additional statistical analyses showed that taller animals that actively chase their prey (for instance, a tiger or human, as opposed to a house cat), as well as birds, were more likely to have round, circular pupils instead of vertically slit pupils. This illustrated a fascinating correlation between eye height and the possibility of a predator having a circular instead of a vertically slit pupil.
On the other hand, species with horizontally elongated pupils tended to be prey, animals that often need a horizontally panoramic view of their surroundings to quickly detect and escape from potential predators. The horizontally elongated pupil was advantageous for these animals, because it made horizontal contours sharp, thereby optimizing a large horizontal range of vision and the ability to scan the environment for predators. The vertical restriction of this pupil also appeared to reduce the amount of overhead sunlight that entered the eye, hence allowing the eye to capture light in the more important directions along the ground, rather than from above.
Overall, Banks and his colleagues made a very convincing case of why animal eyes have differently shaped pupils. Their thorough statistical analyses, eye models, and PSFs, have accurately highlighted the significant correlation between pupil shape and ecological niche. Especially as the first comprehensive study of its kind, this research is a great beginning towards better understanding more about the eye and the role of pupil shape in vision. Nonetheless, there are still many studies that can be conducted to further enhance and add to this research. For example, Banks’s study focused primarily on terrestrial species – species on dry land, but it would be great to also consider examining the various pupil shapes of species that thrive in different environments: those that live in water, those that fly in the air, or those that reside in forests and underground. And of course, there are those few animals with pupils of the most bizarre shapes and forms: the gecko’s multiple pinhole-like openings in the eye, and the cuttlefish’s W-shaped pupil. Analyzing these special pupils in future studies will also likely provide a great wealth of new information to explain the mystery of why pupil shapes exist as they do.
Literature Cited
Banks, M. S., Sprague, W. W., Schmoll, J., Parnell, J. A. Q., & Love, G. D. (2015). Why do animal eyes have pupils of different shapes? Science Advances, 1(7) doi:10.1126/sciadv.1500391
