Pupil Shape Reveals Whether Animal is Hunter or Hunted
Eyes are the windows to the soul. But on a more mundane level, pupil shape is suggestive of something else. And that is whether the concerned animal is a predator or a hunted creature. More than 200 species got analyzed and the results were conclusive.
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The animal’s ecological niche was the biggest predictor of its pupil shape. Those animals having vertical slits for eyes were hunting creatures. They cornered their prey. And they were active all day and night. Those animals with horizontal slits were herbivorous creatures. And they had eyes on both sides of their skull.
The study, led by vision scientist Martin Banks, a UC Berkeley professor of optometry, in collaboration with the United Kingdom’s Durham University, was published in the journal Science Advances.
Round pupils belonged to foraging species that pursued their prey. The study got carried out by an optometrist. It shows how pupil shape is an indicator of the type of animal. An earlier scientist wrote a tome on the topic.
According to him, slit-shaped pupils made for different muscles. And more light entered the eye too. The slits of felines and geckos transform by 135- and 300-fold increases and decreases. As for that of us human beings, it was a mere 15-fold change. For nocturnal animals like cats, the slits provide enough light to see in the dark.
“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 are either vertical or horizontal. Why don’t we see diagonal slits? This study is the first attempt to explain why orientation matters.”
Such animals as sheep, deer and horses had horizontal pupils. These windows expanded the animal’s range of vision. They gather more light from all directions. Yet they prevented the harsh rays of the sun from causing any glare in the animal’s field of vision.
The herbivore horizontal pupil's made to alert the animal. Thus it can detect any approaching predator. The eyes also allow the prey to run as fast as possible away from the predator.
Yet the question remained that how will this strategy work. When the herbivores bent their heads down to graze on the grass, what will be the outcome. Well, all evidence points towards the rotation of their pupils to stay on a level with the ground. This is a wondrous find.
“The first key visual requirement for these animals is to detect approaching predators, which usually come from the ground, so they need to see panoramically on the ground with minimal blind spots,” said Banks.
“The second critical requirement is that once they do detect a predator, they need to see where they are running. They have to see well enough out of the corner of their eye to run quickly and jump over things.”
Grazing animals have a range of vision that is ten times more wider than us humans. As for predators, they need vertical slits. That is so that they can pounce on their prey with accuracy and precision.
Among the key issues here are: stereoscopic vision, motion parallax and blurring. Motion parallax was of special importance. And vertical slits maximize stereoscopic vision.
“A surprising thing we noticed from this study is that the slit pupils were linked to predators that were close to the ground,” said William Sprague, a postdoctoral researcher in Banks’ lab.
“So domestic cats have vertical slits, but bigger cats, like tigers and lions, don’t. Their pupils are round, like humans and dogs.”
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These studies have lent us an in-depth view. Now we know the vision dynamics of carnivores and herbivores.