Staring into the eyes of a wild animal might seem like an invitation to make you a meal, but new research suggests pupil shape is enough to reassure you or not. A team at UC Berkeley discovered that the ecological niche within which an animal sits – whether it hunts or is hunted, and when and how that hunt takes place – is a strong predictor of the pupil shape of that animal.
The study saw vision scientist Martin Banks, who is a professor of optometry at UC Berkeley, collaborate with Durham University in the UK, investigating whether earlier theories around slitted eyes and circular pupils were indeed connected with feeding habits.
Their discoveries certainly seem to lend weight to the previous thinking. For instance, species with circular pupils are likely to be so-called “active foragers,” which tend to chase down prey.
In contrast, species with vertical slit pupils are more likely to ambush prey rather than chase it, and be active both day and night.
Species with horizontally elongated pupils, meanwhile, are more likely to be plant-eating prey species. In such cases, the eyes are commonly on either side of their heads, rather than at the front.
Although suggestions as to why animals like cats had slitted pupils – believed to give decent vision in low-light conditions such as nighttime, yet still avoid being dazzled during the day – were already commonplace, the study set out to discover why the orientation of the pupil also had an impact.
Turns out, horizontal pupils benefit prey species as it expands their effective field of view. Horizontally-stretched pupils are better aligned with the ground, the team discovered, which means more useful light from the front, back, and sides, but at the same time less dazzling sunlight from above.
For predator animals with vertical pupils, though, that orientation maximizes both binocular disparity – figuring out distance for prey far away – and blur – for closer animals – though their total body size also plays a part.
Domestic cats have vertical pupils yet lions and tigers do not; the team say that’s likely because the advantage works better for prey closer to the ground, more likely to be of interest to a smaller cat.
Follow-up studies plan to look at eye shape in animals in aquatic, aerial, and arboreal settings.
SOURCE UC Berkeley