The shape of animals’ pupils reflects whether they are the hunters or the hunted according to new research, with horizontal slits perfect for animals like goats and gazelles, and vertical variants ideal for cats.
A major piece of research that looked into more than 200 species found that an animal’s role in the food chain is a strong predictor of pupil shape, with each animal evolved to maximise the key elements of sight that they rely on.
For ambush predators, for example, vertical slits serve two crucial purposes.
Firstly, they offer binocular disparity, which allows the hunter to establish depth perception over long distances, and break the camouflage of a hiding animal.
Secondly, when the predator closes in on a kill, they offer blur, meaning objects within reach are in focus, with all others out of focus.
Size matters for animal pupils
Domestic cats undergo a 135-fold change in area between constricted and dilated pupils, nine times more than humans.
Weirdly, it emerged that this is only true of smaller hunters, with larger cats like tigers and lions enjoying rounded pupils like dogs and humans.
For prey, however, it’s all about the horizon. Horizontal slits allow these animals to expand the relevant field of view.
Almost exclusively targeted from ground level, grazing animals need to see in as panoramic a fashion as possible.
So 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 dazzling light from the sun above so the animal can see the ground better, the researchers said.
A fine form of evolution
“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 Martin Banks, a UC Berkeley professor of optometry that led the study, now published in Science Advances.
“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.”
However, it’s not just the shape of the pupils that help prey, but also how they can manipulate it.
Rotation key to safety
For example, when a goat lowers its head to the ground to eat grass, logically its eyes should rotate along with the neck, meaning it would have vertical pupils when at its lowest point.
However, the researchers discovered that these animals can rotate their eyes to always maintain a pupil position symmetrical with the horizon.
To check this out, Banks spent hours at the zoo, surrounded by school kids on field trips, watching different animals.
That offered example after example of this amongst a range of grazing animals.
“We are learning all the time just how remarkable the eye and vision are,” said study co-author Gordon Love, a professor of physics at Durham University.
“This work is another piece in the jigsaw puzzle of understanding how eyes work.” This video, produced by Love, explains it even better: