Top row, from left: cuttlefish, lion, goat.
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In contrast, species with vertical slit pupils are more likely to ambush prey rather than chase it, and be active both day and night.
The report also looked at what happens to the orientation of the horizontal pupil when the animal lowers its head to graze.
But vertical slits only came into their own at ground level.
Biologists, however, caution that these generalizations should not be misconstrued to mean that pupil shape accurately determines the feeding habits of a particular species.
Martin Banks, professor of optometry at Berkeley, and Gordon Love, director of the Centre for Advanced Instrumentation at Durham, say that prey animals have two key visual requirements: spotting and fleeing predators. Cephalopods like cuttlefish have truly weird, “W”-shaped pupils”. Thus, Banks said, the vertical pupil provides the best view for stereopsis. The data found that each of the shapes are due to a different way that the animal evolved throughout time, and are directly linked to the way those animals lived. Having eyes towards the side of their head helps them to see almost all around them.
For their recent study, Banks and his colleagues decided to keep things simple. “We restricted ourselves to just pupils that are elongated or not”, Banks explains.
To make matters easier and more accessible, the researchers studied a number of 214 land animals, from which a pattern emerged. Grazing prey animals such as deer, sheep and horses have horizontal pupils because these help expand their effective field of view, computer simulations conducted by the authors discovered.
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.
Tests showed that eyes with horizontal-slit pupils offered an expanded field of view.
“The photography part was new and fun and took more time than I care to remember”, he said. “It actually has a vertical pupil but it’s very hard to judge unless you work with them”. Of these, 44 had vertical pupils and 82% had shoulder heights less than 42 cms or 16.5 inches. They are the animals who lie in wait for the kill, patient for their prey to approach. But surely, when it drops its head to crop grass, the eyes would appear near vertical to the ground?
For example, the vertical slits of domestic cats and geckos undergo a 135- and 300-fold change in area between constricted and dilated states, while humans’ circular pupils undergo a mere 15-fold change.
But then the scientists began to wonder. 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. So this is something which goes against logic since it would normally be expected that the pupil will become perpendicular to the ground. Their eyes are always spaced horizontally in a forward facing direction. The challenge was to see if they could predict a relationship between an animal’s ecological niche and the shape formed by the pupil in its eye.
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In any case, not all scientists, especially vision specialists, share the result of the research, saying numerous creatures don’t fit into the descriptions. Novelists and movie-makers constantly have to imagine the pupil shape of fictional creatures like Lord Voldemort in Harry Potter, or the dinosaur Indominus Rex in Jurassic World.
