Close-up of a goat's amber eye with a horizontal bar-shaped pupil, a green field softly blurred behind
AI-generated image, not a photograph of a real person or place.
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ANIMAL EYES

Hunter or hunted?
Look at the pupil.

214 species compared

  • Slits for ambushers
  • Bars for grazers
  • Circles for chasers
  • Each shape solves a different problem

Pupil shape is a strong clue to how an animal lives.

Researchers at UC Berkeley and Durham University compared 214 species of land animals. Vertical slits went with ambush predators, horizontal bars with grazing prey, and round pupils with animals that chase their food.

Updated

Pick an animal

THE SHORT ANSWER

Shape follows
lifestyle.

The study found a striking link between pupil shape and ecological niche:
how an animal feeds and when it is active.

GRAZING PREY

Wide bars for wide views

Of the 42 plant-eating prey animals in the study, 36 had horizontal pupils. Computer models showed the bars widen the view along the ground, where predators usually come from.

See the numbers

AMBUSH PREDATORS

Slits for judging distance

Vertical slits keep upright edges sharp and blur horizontal ones, which helps judge the distance of a pounce without moving the head and giving the game away.

Pick an animal

HEAD DOWN

Eyes that stay level

When goats, sheep and horses lower their heads to graze, their eyes rotate so the pupils stay parallel to the ground, by 50 degrees or more in each eye.

How it works

A QUICK PUPIL CHECK

Whose eye
is it?

Pick an animal to see its pupil shape and what the study suggests that shape is for. The bar shows how elongated the pupil is.

Pupil check

Choose an animal
Domestic catVertical slit

An ambush predator active day and night. Its slit pupil can change in area about 135-fold, giving the range to see in dim light without being dazzled at midday.

SheepHorizontal bar

Grazing prey with eyes on the sides of the head. The bar-shaped pupil gives a panoramic view of the ground to spot predators and see where to run.

LynxSlightly tall oval

The study classes the lynx pupil as subcircular: elongated vertically, but not a full slit.

Lion or tigerRound

Big cats break the slit pattern. The researchers found slit pupils mainly in predators close to the ground, while taller ones like lions and tigers have round pupils.

HumanRound

Our circular pupil changes in area only about 15-fold, far less than a cat's slit.

Source: Banks MS et al., Why do animal eyes have pupils of different shapes? Science Advances 1(7):e1500391 (2015).

Based on Banks and colleagues, Science Advances (2015), and UC Berkeley's summary of the study.

TWO NUMBERS WORTH KNOWING

Slits stretch,
circles don't.

One figure for prey animals,
one for how much a slit pupil can change.

36 of 42

Herbivorous prey with horizontal pupils

And 26 of 27 terrestrial prey animals examined had eyes set far apart on the sides of the head.

PREY · SCIENCE ADVANCES 2015

135× vs 15×

Change in pupil area, cat versus human

A gecko's vertical slit changes about 300-fold. Slits close using extra muscles that squeeze the opening from the sides.

PUPIL RANGE · SCIENCE ADVANCES 2015

Among 65 front-eyed ambush predators in the study, 44 had vertical pupils, and 82 percent of those stood less than 42 cm at the shoulder.

THREE THINGS TO NOTICE

Watch eyes,
not just animals.

A sheep grazing with its head down in a green field, its eye visible at the side of its head
AI-generated image, not a photograph of a real person or place.

You can see the patterns from the study for yourself on a farm walk, at a zoo or at home.

  1. Compare cats big and small

    A pet cat's pupils narrow to slits in bright light; a lion's or tiger's stay round, as the study found for taller predators.

  2. Watch a grazing animal

    When a sheep, goat or horse drops its head to eat, look at the pupil: the researchers saw the eyes rotate to keep it level with the ground.

  3. Look for the missing diagonal

    The study found no land animals in its data with diagonally elongated pupils. Slits and bars line up vertically or horizontally.

HOW THE SHAPES WORK

Sharp one way,
blurred the other.

Close-up of a domestic cat's green eye with a narrow vertical slit pupil in bright light
AI-generated image, not a photograph of a real person or place.

An elongated pupil gives different depth of field in each direction. A vertical slit keeps vertical edges sharp at many distances while blurring horizontal ones, so an ambush predator can use both stereo vision and blur to judge distance.

A horizontal pupil does the reverse, keeping the ground ahead and behind in sharp focus for a panoramic view. It also limits dazzling light from the sky above, helping prey animals watch the ground and run over rough terrain.

Questions

Questions people ask

Why do goats have rectangular pupils?

A study of 214 land species found horizontal pupils are very likely in plant-eating prey with eyes on the sides of the head. Models showed the shape widens the view along the ground and cuts glare from above.

Why do cats have slit pupils?

Domestic cats are ambush predators active by day and night. Slits can change in area about 135-fold, and they help judge distances before a pounce.

Why don't lions have slit pupils?

The study found vertical slits mainly in ambush predators close to the ground. Taller predators such as lions and tigers have round pupils, like humans and dogs.

Do sheep's eyes really rotate?

Yes. The researchers observed that when grazing animals lower their heads, their eyes rotate to keep the pupils horizontal, by 50 degrees or more in each eye, about 10 times the range of human eyes.

Are there animals with diagonal pupils?

The study found no terrestrial species, among those it had data for, with diagonally elongated pupils.

Sources

Where the facts come from

  1. Banks MS et al. (2015) Why do animal eyes have pupils of different shapes? Science Advances (PMC full text)
  2. UC Berkeley News: Pupil shape linked to animals' place in ecological web (2015)
  3. Smithsonian Magazine: How do animals find food? The answer's in their eyes
  4. VNonline: Pupil shape helps animals survive (2015)

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