If you've ever gazed into a house cat's eyes and wondered why they don't look like yours or even like those of big cats like tigers, you were onto something.
The shape of an animal's pupils is linked to its lifestyle, a new study published in the journal Science Advances suggests. Scientists analyzed the pupil shapes and ecological roles of 214 species of land animals, including a variety of cats. Their results show that species with vertical slit-shaped pupils tend to be ambush predators, whereas those with circular pupils tend to be "active foragers," which chase down 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," lead author Martin Banks of the University of California, Berkeley, said in a statement.
This is not the only piece of the feline pupil puzzle that the researchers uncovered, however.
"A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground," co-author William Sprague, a postdoctoral researcher in Banks' lab, said in a statement. "So domestic cats have vertical slits, but bigger cats, like tigers and lions, don't. Their pupils are round, like humans and dogs."
Out of the 65 ambush predators with forward-facing eyes included in the story, 44 had vertical pupils. Even more strikingly, 82 percent of them were rather small, measuring less than 42 centimeters — or 16.5 inches — from foot to shoulder.
Yet, none this explains why cats have vertical slit pupils, whereas horses have horizontal slit pupils. The researchers found that horizontal slit pupils were most common among grazing prey animals like deer and sheep.
To figure out how the orientation of an animal's pupil's affects its vision, the researchers used computer models. Stretching out pupils horizontally, the researchers found, allows an animal to align its pupils with the ground. This, in turn, lets more light in from the back, front and sides.
"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," Banks said in a statement. "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."
Now that they've tackled terrestrial species, the researchers hope to turn their attention to the pupils of animals that spend most of their time up in the sky, in trees or underwater in upcoming studies.