Why animal eyes have pupils of different shapes

This widely cited study tested why pupil shape varies so systematically across animals, finding a strong link between vertical-slit pupils and a terrestrial, ambush-predator lifestyle, exemplified by the domestic cat. Using optical modeling and an ecological survey of many species, the authors showed that vertical slits suit short, active foragers and ambush hunters that need to judge distance to prey close to the ground, in part by exploiting blur and depth-of-field cues. The slit also permits an enormous range of pupil area between bright and dim conditions, helping nocturnal-capable predators function across light levels. Horizontal pupils, by contrast, tended to belong to grazing prey animals scanning the horizon, and round pupils to taller or more active predators. The work elegantly connected an everyday feature of the cat eye to predatory ecology and visual optics.