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Why do some animal eyes have horizontal pupils?

Scientists have discovered that the shape of an animal’s pupils is related to whether it is hunter or hunted.

An analysis by researchers from University of California, Berkeley, of 214 species of land animals shows that species with pupils that are vertical slits are more likely to be ambush predators that are active both day and night.

On the other hand, 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”, or 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, who led the study.

“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.”

As for horizontally elongated pupils, which with few exceptions correspond to grazing prey animals such as sheep, deer and horses – the researchers found they expanded the effective 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 dazzling light from the sun above so the animal can see the ground better, the researchers 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.”

“A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground,” said William Sprague, a postdoctoral researcher in Banks’ lab. “So domestic cats have vertical slits, but bigger cats, like tigers and lions, don’t. Their pupils are round, like humans and dogs.”

Bill Condie

Bill Condie

Bill Condie is a science journalist based in Adelaide, Australia.

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