![]() ![]() “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,” said Banks. The orientation also helps limit the amount of dazzling light from the sun above so the animal can see the ground better, the researchers said. When stretched horizontally, the pupils are aligned with the ground, getting more light in from the front, back and sides. They 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 researchers turned to computer models to study the effects of different pupil shapes. Their eyes rotate so that the pupils stay aligned with the ground regardless of whether their head is upright or pitched down. Why don’t we see diagonal slits? This study is the first attempt to explain why orientation matters.” Grazing prey animals have horizontally elongated pupils that expand their field of vision. “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,” said Banks. The classic text on eye physiology 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. This current research builds upon the foundation set by the late Gordon Walls, a UC Berkeley professor of optometry who published The Vertebrate Eye and Its Adaptive Radiation in 1942. The findings were published today in the journal Science Advances. The study, led by vision scientist Martin Banks, a UC Berkeley professor of optometry, in collaboration with the United Kingdom’s Durham University, presents a new hypothesis as to why pupils are shaped and oriented the way they are. Circular pupils were linked to “active foragers,” or animals that chase down their prey. In contrast, 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. Why do some animals have horizontally elongated pupils? (Video produced by Gordon Love, Durham University, in collaboration with UC Berkeley)Īn analysis of 214 species of land animals shows that a creature’s ecological niche is a strong predictor of pupil shape. While the eyes may be a window into one’s soul, new research led by UC Berkeley scientists suggests that the pupils could also reveal whether one is hunter or hunted.
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