The team used computer models based on a sheep’s eye to support theories about why horizontal, vertical and circular pupil shapes benefit different animals.

When the model’s pupil was horizontal, more light could be captured from the left and right of the eye, not surprisingly, and less light from below and above the eye. This would allow grazing animals to better detect predators approaching from different directions, the researchers said.

“People had been saying that the horizontal pupil helps expand the horizontal view of the ground, they just hadn’t shown that,” said Martin S. Banks, a visual scientist from Berkeley and lead author on the paper. “Our contribution was to build a model and show that that happened.”

But there was an obvious caveat to this conclusion: What would happen if the sheep bent its head to the ground to eat? Logic would suggest that the horizontal pupil would become perpendicular to the ground.