When it comes to whether an animal is predator or prey, the eyes have it -- or at lease when it comes to pupil shape, researchers say.

A creature's ecological niche -- and its likely place in the predator/prey food chain -- is reflected in pupil shape, scientists at the University of California, Berkeley, have found.

Pupils with a vertical orientation -- think of that house cat stalking a mouse -- are more likely to be found in ambush predators who will hunt prey in both day and night, they say.

Horizontally elongated pupils, on the other hand, are found in prey species with eyes widely spaced on the sides of their heads, the better to spot an approaching predator, they explain.

"We found a striking correlation between pupil shape and ecological niche," says study lead author Martin Banks, who along Berkeley colleagues and researchers in the United Kingdom analyzed eye configuration in 214 species of land-dwelling animals.

Pupil shape is not exclusively limited to vertical or horizontal, they point out; predators active during the day, including humans, have evolved circular pupils.

And there where some exceptions to predicted pupil shape, the researchers report.

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

The shape of an animal's pupil seems to have evolved to optimize its field of vision for its particular needs whether as predator or prey, he suggests.

In predator species active in both night and day, such as house cats, a vertical slit pupil yields an opening range that open widely for seeing in dim light and can yet close to a very tight slit so they're not blinded by daytime sunlight, the researchers explain.

In prey species, such as grazing animals like sheep or cows, horizontal pupils provide an expanded field of view, they point out.

"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 says.

The eyes of grazing animals can rotate by 50 degrees or more in each eye, 10 times the range of human eyes, the researchers note.

"We are learning all the time just how remarkable the eye and vision are," says study co-author Gordon Love of Durham University in the United Kingdom. "This work is another piece in the jigsaw puzzle of understanding how eyes work."

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