The picture above is taken from a Telegram channel and the text in Russian goes as follows.
Prey has got a horizontally shaped pupil to better scan the horizon and a predator's pupil has a vertical shape to better focus on a prey.
Is this true?
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Oddthinking
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Generally true. Some nuance applies, plus exceptions.
It is generally true that eyes of predators are different than eyes of prey. Not only are the pupils sometimes different in the way suggested in the claim, but eye placement and some other factors are different as well. The general theory accepted by biologists is that predators tend to fixate on their prey targets, so they evolve eyes that suit looking forward and intently fixating on single points. Prey, on the other hand, is suited well by having a broader periphery that helps them spot approaching predators, even if at the expense of the ability to converge the eyes and look straight forward.
Scientists have now done the first comprehensive study of these three kinds of pupils [horizontal slits, vertical slits, and round]. The shape of the animal's pupil, it turns out, is closely related to the animal's size and whether it's a predator or prey.
Eye Shapes Of The Animal World Hint At Differences In Our Lifestyles - NPR
The nuance and exceptions is that slits are by no means the only pupil shape. Round, as in humans, is common. There's also some crazy shapes like 'W', crescent, heart, and pinholes. The generally true part is that horizontal slits are typically only on prey while vertical slits are typically only on predators. I don't know of any exceptions to this specific point.
The researchers gathered information on 214 species. They noted the pupil shape and the location of the eyes on the head, plus the animal's lifestyle. ...
When they pulled everything together, a clear pattern emerged. In the journal Science Advances, the scientists report that there's a strong link between the shape of an animal's pupil and its way of life.
"If you have a vertical slit, you're very likely to be an ambush predator," says Banks. That's the kind of animal who lies in wait and then leaps out to kill. He says these predators need to accurately judge the distance to their prey, and the vertical slit has optical features that make it ideal for that.
But that rule only holds if the animal is short, so its eyes aren't too high off the ground, Sprague says.
"So for example foxes, in the dog lineage, have vertical pupils, but wolves have round pupils," he says.
And while a small pet cat has vertical slits, Sprague says, "the larger predators, like lions and tigers, have round pupils."
In general, round pupils seem to be common in taller hunters that actively chase down their prey, says Banks.
Meanwhile, he says, if you're the kind of animal that gets hunted, "you're very likely to have a horizontal pupil" and to have your eyes on the side of your head. That makes sense, he says, because it gives prey animals a panoramic view, so they can best scan all directions for danger.
ibid
There's a lot more going on with evolved eye structure, some of which you can learn about in the linked NPR article.
The NPR article is based off
- "Why do animal eyes have pupils of different shapes?", Banks, Sprague, Schmoll, Parnell, and Love, Science Advances 07 Aug 2015: Vol. 1, no. 7, e1500391 DOI: 10.1126/sciadv.1500391
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8I skimmed the paper. It puts forward a hypothesis of *why* it helps, optically. (I think it would take me an hour or two to understand it though.) – Oddthinking Aug 17 '19 at 00:23
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1Actually, it sounds like it's generally false: if you've got vertical-slit pupils, you're a predator, but being a predator doesn't mean you've got vertical-slit pupils. – Mark Aug 17 '19 at 21:15
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@Mark The generality I'm pointing out is the first outlook. Given a vertical slit pupil, it's very likely to be on a predator. But the nuance/exception is that by no means do *all* predators have vertical slits. – Aug 17 '19 at 21:59
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6Further generality is that pupil shapes *do* correlate strongly to predator/prey behaviors. – Aug 17 '19 at 22:01
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I have a problem with this explanation. Eyes move, just like an animal's head. Predators are carnivores, preys tend to be herbivores, so they spend a lot of time grazing (plants have less energy, more eating time needed). While eating, their heads are tilted 90 degrees from normal, and those horizontal slits become vertical. So the tendency is there, but this explanation makes me skeptical. – Nyos Aug 19 '19 at 01:38
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4@Nyos Did you read through in the link? Some prey eyes actually rotate with the head to maintain horizontal orientation. – Aug 19 '19 at 02:26
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8Note that most of these biases (eye placement, pupil, ...) tend to focus on **land-based** animals. These generally don't apply to marine or flying animals, who have a higher need for peripheral view (including the predators) due to navigating a 3D space as opposed to the predominantly 2D space land animals navigate. – Flater Aug 19 '19 at 11:31
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1@Flater A quick image search for "bird eyes" and "fish eyes" seems to indicate that almost all birds and fish have round pupils, and almost always on the sides. Owls seem to be the sole exception among birds having them in front, but most birds, even other predators, are on the sides. Some fish have more unusual placement, such as on top for bottom-feeders, but still mostly on the sides. – Darrel Hoffman Aug 19 '19 at 13:56
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@DarrelHoffman My observations as well. I'm pretty sure though that predator birds can fixate both eyes on single points in front of them. – Aug 19 '19 at 15:06
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6@DarrelHoffman Their eyes may be on the sides of their heads, but clearly raptors' eyes point forward. https://images.unsplash.com/photo-1534251369789-5067c8b8602a?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&w=1000&q=80 – nasch Aug 19 '19 at 15:16
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@Nyos Here's my layman's understanding. Many prey animals who eat on the ground feed in groups. Geese, deer, rabbits. prairie dogs, etc. When an animal has its head down eating, it's not going to be able to effectively watch out for predators regardless of how its eyes work. So they rely on others in the group to watch out while they're eating. Watch a bunch of geese some time. One or a few will have their heads up while the rest are grazing. – nasch Aug 19 '19 at 15:19
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@nasch Certainly, there's much more to survival and evolution than eyes. In discussions like these, it's easy to think in isolated ways, but eyes and all other structures didn't evolve in isolation. They all work together in individual bodies. I'm not sure, but I bet there's studies on effectiveness of structural variances as it relates to other variances. Such studies represent less isolated thinking. – Aug 19 '19 at 16:27
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3@fredsbend I don't disagree, but you may have missed my point. Nyos was saying the explanation of horizontal pupils is wrong, because when grazing the pupils would be vertical. My point is that grazing and looking out for predators are mutually exclusive activities for many animals. When your head is down in the grass, you're not going to be able to see any predators coming no matter what shape your pupils are. – nasch Aug 19 '19 at 18:43
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Some University of California scientists suspect the eye shape to be highly related with the animal's lifestyle.
When they pulled everything together, a clear pattern emerged. In the journal Science Advances, the scientists report that there's a strong link between the shape of an animal's pupil and its way of life.
"If you have a vertical slit, you're very likely to be an ambush predator," says Banks. That's the kind of animal who lies in wait and then leaps out to kill. He says these predators need to accurately judge the distance to their prey, and the vertical slit has optical features that make it ideal for that.
But that rule only holds if the animal is short, so its eyes aren't too high off the ground, Sprague says.
A vertical eye is seen on ambush predators as they need to judge distance, but only when their eyes aren't high off the ground. Examples, a house cat has vertical eye slits but a tiger has round eyes, a fox has vertical eye slits but a wolf has round eyes.
A round eye was mostly seen on taller animals that chase down their prey.
A horizontal eye slit is indeed mostly found on prey animals as it gives them a wider field of view. Interestingly, since many of these animals (horses, sheep, goats, etc) lower their head to graze their eyes also have the ability to rotate so that the slit stays horizontal.
