Why do pupils come in different shapes for different species?

July 19, 2016, 7:12 a.m.

Ever look at a cat’s vertically slip pupils or a goat’s horizontally slit pupils or your own round pupils and wondered why it is that pupils come in so many shapes?

For instance, the pupil of the gecko pictured here looks like a series of pin-pricks connected by a vertical slit. Why all the variation? It's a question that has fascinated scientists for a long time.

A study published in 2015 by a team of researchers makes a connection between the shape of an animal’s pupil and its ecological niche. They took a look specifically at land animals — 214 species in all — and three shapes of pupil: vertical, horizontal or round. They found a pattern connecting the pupil shape and the animal's activity.

The researchers write in the journal Science Advances:

Species with vertically elongated pupils are very likely to be ambush predators and active day and night. Species with horizontally elongated pupils are very likely to be prey and to have laterally placed eyes. Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. This is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours. Horizontally elongated pupils create sharp images of horizontal contours ahead and behind, creating a horizontally panoramic view that facilitates detection of predators from various directions and forward locomotion across uneven terrain.

How about round pupils, like those found in humans? Those tend to belong to predators that are higher up off the ground and tend to chase their prey. For instance, foxes and wolves are both canids, but foxes have elongated pupils while wolves have round pupils. Foxes are short ambush predators while wolves are tall pack hunters. Another example is domestic cats and lions. House cats have elongated pupils while their taller lion cousins have round pupils.

For prey species with horizontally elongated pupils, the shape allows them to receive the most possible light from in front and behind, giving them a panoramic view of the world that helps them to detect predators. Grazing animals including sheep, goats, deer and horses have an additional adaptation that ensures this panoramic view even while eating. When they lower their heads to the ground to eat, their eyes rotate so that the pupil stays aligned with the horizon.

“This remarkable eye movement, which is in opposite directions in the two eyes, is known as cyclovergence,” writes Gordon Love, one of the researchers. “Each eye in these animals rotates by 50 degrees, possibly more (we can only make the same movement by a few degrees).”

This study provided some fascinating insights into three of the many pupil shapes out there. But what about those weird W-shaped pupils of cuttlefish?

A 2013 study notes that the shape helps to control the light entering the eye, which is typically uneven under water.

"Our data show that the W-shaped pupil projects a blurred 'W' directly onto the retina and that it effectively operates as vertical slits for the frontal and caudal parts of the visual field," write the researchers. "Computing a retinal illumination map shows that the W-shaped pupil is effective in balancing a vertically uneven light field: The constricted pupil reduces light from the dorsal part of the visual field significantly more than it reduces light from the horizontal band."

In other words, the pupil shape helps even out the different levels of light coming in from above, below, in front and behind so that the cuttlefish has a clearer view of the world.

The U-shaped, W-shaped and dumbbell-shaped pupils of different cephalopod species may also help to increase contrast and thus play a role in sensing color, which is helpful in dim or murky water.

There's much more to learn about the pupils and vision of different species, but one thing is for sure: each shape has a purpose, and reveals something important about the life of that animal.