Grazing prey animals have horizontally elongated pupils that expand their field of vision. Their eyes rotate so that the pupils stay aligned with the ground regardless of whether their head is upright or pitched down. Gordon Love/Durham University.
Janet Fang 08 Aug 2015, 18:42

Pupils regulate the amount of light that hits an animal’s retina, and they come in a variety of shapes. Ours are round dots. Cats have vertical slits, sheep have horizontal slits. Researchers trying to understand why different pupil shapes evolved say it has to do with ecological niches: Are they the hunter or the hunted? The work is published in Science Advances this week. 

"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," UC Berkeley’s Martin Banks says in a statement. When dilated, their pupils (and those of geckos, for example) undergo up to a 300-fold change in area. Ours, by contrast, experience a 15-fold change. But, "why don't we see diagonal slits?" he adds. "This study is the first attempt to explain why orientation matters." 

Banks and colleagues categorized 214 land animals based on how they forage, when they’re active, and their pupil shape when constricted. Predators were further divided into active foragers who chase down prey and ambush predators who sit and wait before pouncing. The team then used computer models to study the effects of various shapes and orientations. 

Animals with pupils stretched horizontally tend to be plant-eating prey species with eyes on either side of their head. This creates a wider, more expanded field of view. Since they’re aligned with the ground, these pupils let in more light from the front, back, and sides, while limiting glaring sunlight from the above. Even when grazers like goats and antelope lower their heads to eat, their eyes rotate – by 50 degrees or more – to maintain that horizontal alignment. 

"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 explains. "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."

Ambush predators with front-facing eyes are the ones with vertically elongated pupils, which help them to control light input better and gauge distances more accurately. Predators rely on two cues: binocular disparity (which creates depth of field) and blur (when objects at different distances are out of focus). Vertical-slit pupils boost both. 

Among predators, vertically elongated pupils are further divided. There’s the vertical slit of house cats, and then there’s the (still vertical) subcircular pupil of big cats like lynxes and lions. Vertical pupils may maximize the ability of small hunters close to the ground to judge distances of prey: Over 82% of predators with vertical pupils studied had shoulder heights of less than 42 centimeters.

And finally, circular pupils are linked to active foragers, and these may be related to height. Our eyes, for example, are too far away from the ground to benefit from the light-control of vertical slits and the panoramic view of horizontal slits.

Image: Gordon Love/Durham University (top), M.S. Banks/Science Advances (middle), Telekhovskyi/Shutterstock (bottom).

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