Vertical-slit pupils maximize both cues.” “Binocular vision works better at judging differences when contours are vertical and objects are at a distance, while blur comes into play for horizontal contours and near-field targets. “The remaining two cues, binocular disparity and blur, work together with vertically elongated pupils and front-facing eyes,” the scientists said. They ruled out motion parallax as a factor since using that cue would require head movement that could reveal the predator’s position. They identified three cues generally used to gauge distance: stereopsis motion parallax, in which closer objects move farther and faster across our field of vision and blur, in which objects at different distances are out of focus. “Grazing animals’ eyes can rotate by 50 degrees or more in each eye, a range 10 times greater than human eyes.”įor ambush predators with vertical-slit pupils, the scientists noted the importance of accurately gauging the distance animals would need to pounce on their prey. Sure enough, when goats, antelope and other grazing prey animals put their head down to eat, their eyes rotated to maintain the pupils’ horizontal alignment with the ground,” Prof Banks said. “To check this out, I spent hours at the Oakland Zoo, often surrounded by school kids on field trips, to observe the different animals. They have to see well enough out of the corner of their eye to run quickly and jump over things,” Prof Banks explained.īut what happens to this orientation when the animal lowers its head to graze? If the pupil follows the pitch of the head, they would become more vertical. The second critical requirement is that once they do detect a predator, they need to see where they are running. “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. The orientation also helps limit the amount of dazzling light from the sun above so the animal can see the ground better. When stretched horizontally, the pupils are aligned with the ground, getting more light in from the front, back and sides. The scientists found that the horizontal pupils expanded the effective field of view. To explain why horizontally elongated pupils, with few exceptions, corresponded to grazing prey animals such as sheep, deer and horses, the study’s authors – Prof Martin Banks from the University of California, Berkeley, and co-authors – turned to computer models to study the effects of different pupil shapes. In contrast, those with horizontally elongated pupils are very likely to be herbivorous species with eyes on the sides of their heads.Ĭircular pupils were linked to active foragers, or animals that chase down their prey. Species with pupils that are vertical slits are more likely to be predators that are active both day and night. Image credit: Tambako the Jaguar / CC BY-ND 2.0. The study suggests that pupils could reveal whether one is hunter or hunted.
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