Vid Vision scientists believe that the shape of pupils in animal eyes can often reveal whether the beast is a hunter or the hunter’s prey. For instance, mongooses exhibit forward-facing eyes but horizontal pupils, cuttlefish have W-shaped pupils, whereas geckos display circular pupils of exceedingly great size when dilated which diminish to several discrete pinholes when constricted.
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Researchers also learned that domesticated cats can narrow their pupils by 135-fold, while humans can only reduce pupils by 15 times. In contrast, those with horizontally elongated pupils are extremely likely to be plant-eating prey species with eyes on the sides of their heads.
“There are so many exceptions to the rules the authors think to have discovered, that there must be much more to pupil shape than being predator or prey, big or small”, said Ronald H.H. Kröger, a biologist from Lunds University in Germany.
The research, published in the Science Advances journal on Friday, built on a piece of work entitled The Vertebrate Eye and its Adaptive Radiation penned by UC Berkeley prof Gordon Walls in 1942.
Among the 65 frontal-eyed, ambush predators in this study, 44 had vertical pupils, and 36 of them had shoulder heights that were less than 42 centimeters (16.5 inches). So the team wanted to further explore this idea by testing why some slits are sometimes vertical and sometimes horizontal.
For prey animals, horizontal slit pupils had the added advantage of limiting the amount of dazzling light from the sun, making it easier to see the ground.
Scientists have now done the first comprehensive study of these three kinds of pupils.
“When they pitch their head down, their eyes rotate in the head to maintain parallelism with the ground”, says Banks. And circular pupils are linked to “active foragers” – animals that chase down their prey rather than creeping up and ambushing them. This find allows a new look into the animal kingdom and, as our eyes are for daytime activity, they also suggest new information about early humankind.
He explained that horizontal-shaped eyes help animals to view their surroundings panoramically to flee quickly, “Once they do detect a predator, they need to see where they are running”. Having a horizontal rectangle for a pupil can also provide the creatures a superior ability to see “out of the corner” of their eye. “So they’re either vertical, horizontal or round”. Professor Banks went to Oakland Zoo in California to observe at first hand, and Professor Love took his camera to the Yorkshire Dales to record changing pupil shapes in the field.
Utilizing PC models, the scientists used a sheep eye to support their speculations concerning why particular eye shapes could benefit the animals.
On the other side of the Atlantic, study co-author Gordon Love, a professor of physics at Durham University, found this same pattern when observing sheep and horses at nearby farms.
The findings could help researchers gain more insight into how complex organs like the eye work. For example, was it predator or prey, and active during the day or night?
It is important for ambush predators to have clear vision when they are hunting during the night, the researchers noted.
Among the key issues here are: stereoscopic vision, motion parallax and blurring. Thus, Banks said, the vertical pupil provides the best view for stereopsis. As for predators, they need vertical slits.
By using the difference between close focus, the animals can judge their distance from a target and pounce on it, said William Sprague, a postdoctoral researcher.
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The authors explained this by calculating that depth-of-field cues based upon blur are more effective for estimating distances for short animals than tall ones.
