Pupil shape linked to animal’s place in ecological web
The new research doesn’t only demystify the housecat, however; it also reveals that the freaky horizontal, rectangular pupils sported by goats and sheep likely help these prey animals scan the horizon for predators – and watch the terrain when sprinting from danger. Having a horizontal pupil enhances the amount of light they can receive in front of and behind them while reducing the amount of light from above and below.
For instance, the vertical slits of domesticated cats undergo a 135-fold change in area between constriction and geckos undergo a 300-fold change.
The researchers examined 214 land animal species and identified three distinct shapes of pupils: horizontal, vertical, and round. Love told redOrbit that they are now studying the eyes of mice and fish.
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.
Why are some pupils slit-shaped? 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”.
“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.
However, this hypothesis does not explain why slits can’t be diagonal. “This study is the first attempt to explain why orientation matters”.
To expand their field of vision, sheep and other grazing prey animals sport horizontally elongated pupils. Their eyes have lenses with varying optical power, and the authors are exploring if this characteristic is linked to elongated pupils. We found that eyes of goats, deer, horses, and sheep rotate as they bend their head down to eat, keeping the pupil aligned with the ground.
“A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground”, said researcher Dr William Sprague, from the University of California at Berkeley, US.
It seems that evolution has prepared all creatures to adjust to their best survival, thus providing them the proper means to either detect prey or spot incoming predators. “They have to see well enough out of the corner of their eye to run quickly and jump over things”, Banks adds.
But what happens to this orientation when the animal lowers its head to graze?
“To check this out, I spent hours at the Oakland Zoo, often surrounded by school kids on field trips, to observe the different animals”, said Banks.
In contrast, horizontally elongated pupils are almost always found in grazing animals, which have eyes on the sides of their head. Grazing animals’ eyes can rotate by 50 degrees or more in each eye, a range 10 times greater than human eyes, the researchers said. This is particularly useful for animals that are active both day and night, allowing for much better vision in low light conditions. For that reason, while shorter animals such as domestic cats have vertical-slit pupils, larger lions and tigers do not.
The researchers used computer models to study the effects of differently shaped pupils.
Our work has focused on the visual benefits of vertical and horizontal pupils in mammals and snakes.
To judge horizontal distances, though, cats and other slit-pupil predators likely use blur, Banks said. Banks and his colleagues found that, of 42 herbivorous prey animals in their database, 36 had horizontal pupils.
Among 65 frontal-eyed ambush predators in the study, 44 had vertical pupils and 82% of them had shoulder heights less than 16.5 inches.