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“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 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. To explain why horizontally elongated pupils, with few exceptions, corresponded to grazing prey animals such as sheep, deer, and horses, the researchers turned to computer models to study the effects of different pupil shapes and found that horizontal pupils expand the effective field of view. Why don’t we see diagonal slits? This study is the first attempt to explain why orientation matters.” Blind spots “However, this hypothesis does not explain why slits are either vertical or horizontal. “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,” says Martin Banks, professor of optometry. The current study builds upon the foundation set by the late Gordon Walls, a professor of optometry at the University of California, Berkeley, who published “The Vertebrate Eye and Its Adaptive Radiation” in 1942 that put forward the theory, generally accepted, that slit-shaped pupils allow for different musculature and a greater range in the amount of light entering the eye.įor example, the vertical slits of domestic cats and geckos undergo a 135- and 300-fold change in area between constricted and dilated states, while humans’ circular pupils undergo a mere 15-fold change.
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The findings, published in the journal Science Advances, present a new hypothesis as to why pupils are shaped and oriented the way they are, researchers say. Circular pupils are linked to “active foragers,” or animals that chase down their prey. Those with horizontally elongated pupils are extremely likely to be plant-eating prey species with eyes on the sides of their heads. Species with pupils that are vertical slits are more likely to be ambush predators that are active both day and night. A new analysis of 214 species of land animals, both prey and predators, offers an explanation for the difference. House cats have pupils that are vertical slits, but bigger cats, like tigers and lions, don’t. University University of California, Berkeley