Goat pupils are horizontal rectangles rather than round circles.
That unusual shape helps a grazing prey animal monitor a very wide area.
Horizontal pupils expand the field of view
Goats have eyes positioned toward the sides of the head.
Combine that placement with a horizontal pupil and the animal gains broad panoramic vision.
It can watch a large portion of the horizon without turning its head.
Why does the horizon matter?
Predators usually approach across the ground.
A grazing animal needs to keep watching its surroundings while its head is lowered to eat.
A wide horizontal view helps detect movement from the sides.
The pupil also helps manage light
The horizontal shape can control how light enters the eye from different directions.
That helps preserve useful vision across the ground while limiting some bright overhead light.
What happens when a goat lowers its head?
Here is the really strange part.
The eyes rotate in the sockets enough to keep the pupils roughly aligned with the ground.
The goat bends down.
The pupil stays horizontal relative to the horizon.
Why do predators often have different pupils?
Some ambush predators have vertical slit pupils, which can support depth judgments at close range.
Prey animals have a different visual problem.
They need broad surveillance.
Eye design follows lifestyle.
Is this like cat eyes?
Cats have their own specialized eye anatomy, including the reflective layer behind the retina that explains why cats’ eyes glow in the dark.
Goats solve a different visual challenge with horizontal pupils.
A goat’s pupil looks strange to us because its eyes are built for a prey animal that needs to watch almost everything at once.
And that’s the short explanation.
Related explanations: Why do cats’ eyes glow in the dark?, Why do animals have tails?, and Why do zebras have stripes?