Have you ever wondered why some animals such as goats have pupils shaped like horizontal slits while there are those that take on vertical shapes like the rattle snakes and cats?
The question has lead a research group to study 214 different species; the results were published Friday. The answer might be tied to the animal’s survival instinct. While animals with vertical and circular pupils are excellent at hunting prey, animals with horizontal pupils are better at spotting predators from a distance.
However, not all scientists, particularly vision experts, don’t share the conclusion of the findings. According them, there are many animals that don’t fit into the categories. The study, which was a collaborative effort between a team of scientists from the University of California, Berkeley, and Durham University in Britain, was published in the journal Science Advances.
Using computer models, the researchers used a sheep eye to bolster their theories concerning why specific eye shapes could benefit the animals. The computer model revealed that a horizontal eye could easily capture light from its left and right sides, and, obviously, lesser light from the bottom and top part of the eye. This would grant grazing animals the advantage of better seeing any predators that might be coming towards them in various directions.
“People had been saying that the horizontal pupil helps expand the horizontal view of the ground, they just hadn’t shown that,” said Martin S. Banks, a visual scientist from Berkeley and lead author on the paper. “Our contribution was to build a model and show that that happened.”
However, there’s a catch to that reasoning: when the goat bends its head to graze, the horizontal pupil would transform to take a perpendicular shape in relation to the ground. The researchers on their part, while taking pictures of goats at a pitting zoo, made an unexpected discovery. That is, a goat’s eyes rotate up to 50 degrees when their head is bent downward, maintaining the pupil’s position parallel to the ground.
The research team expanded their study to include horses, antelopes and other grazing animals and discovered that they have the ability to rotate their eyes as well. Using the same computer model, Dr. Banks and members of the team went on to identify whatever advantages vertical slit eyes may possess. They learned that it help predators ambush their unsuspecting prey by enhancing their depth perception while increasing focus on their prey.
One glitch in this observation is that large such as tigers and lions possess circular eyes and not vertical slits. The researchers have a plausible explanation: the height of the animals; they are taller, so they don’t need to have their eyes compensate as much compared to the smaller predators.
Critics cite that there are many other examples that would be able to counter the authors’ explanations. One good example they mentioned is the chinchilla, which eats grass but has vertical-slit eyes. “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.
