Want to know whether an animal is more likely to be a predator or someone else’s prey? Look at their eyes, according to a new study published in Friday’s edition of Science Advances that found a link between the shape of their pupil and their ecological niche.
As part of the study, experts at the University of California-Berkeley and Durham University in the UK looked at more than 200 different species of land animals. They found that creatures that have pupils that are vertically shaped slits are more likely to be ambush predators that are active during both the daytime and during nighttime hours.
They also found that animals with horizontally-elongated pupils are very likely to be plant-eating prey species that have eyes on the sides of their heads, and those with circular pupils tended to be “active foragers,” or animals that chase down their prey instead of sitting and waiting.
“Vertically orientated pupils are usually found in ambush predators – animals which essentially lie in waiting for their prey and then pounce,” co-author Gordon Love, a professor of physics at Durham University, explained to redOrbit via email. “They need to be able to judge distances to their prey accurately, and the vertical pupil helps this.”
“Horizontally oriented pupils are normally found in grazing herbivores with their eyes on the side of their head, which helps them see all around to detect predators,” he added.
Why? Love said that it’s because “horizontally elongated pupils optimizes the light into their eyes from all around in the horizontal plane.”
“For herbivores,” he added, “It is important that the elongation axis is parallel to the ground.” Grazing prey animals such as deer, sheep and horses have horizontal pupils because these help expand their effective field of view, computer simulations conducted by the authors discovered. Pupils that are stretched horizontally allows them to receive more light from the front, back and sides, while also limiting the amount of light from the sun so they can better see the ground.
Why are there no diagonal slits?
Love and lead investigator Martin Banks, a professor of optometry at UC Berkeley, built upon previous research published by former UC Berkeley optometry professor Gordon Walls during the 1940s. In this study, Walls explained that slit-shaped pupils enabled different musculatures, as well as a greater range in the total amount of light entering the eye.
For instance, the vertical slits of domesticated cats undergo a 135-fold change in area between constriction and geckos undergo a 300-fold change. In comparison, the circular pupils of humans undergo just a 15-fold change, the authors explained. Species active during both the day and the night require slit pupils to see in darker conditions without being blinded by sunlight.
However, this hypothesis does not explain why slits can’t be diagonal. The reason ambush predators need to have vertical pupil elongation, Love explained, is “because it has to be orthogonal to the spacing of their eyes. Their eyes are always spaced horizontally in a forward facing direction. In short, we don’t have animals who have eyes which are displaced diagonally – and hence the same is true for the orientation of the pupils.”
The authors noted that their work focused exclusively on terrestrial species, and that they plan to analyze the eyes of aquatic, aerial, and arboreal life in future studies. Love told redOrbit that they are currently studying the eyes of mice and fish. Their eyes have lenses with varying optical power, and the authors are exploring if this characteristic is linked to elongated pupils.
(Image credit: Thinkstock)
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