Why do cats have vertical pupils? - Emma Bryce
Peering into the eyes
of different animals,
you’ll see some extraordinarily
shaped pupils.
But why?
It turns out that pupil shape
is a powerful indicator
of what role an animal plays
in its ecosystem.
Pupils mark the hollow opening
in the iris,
the eye’s band of pigmented muscle.
They’re the portholes through which
light enters the eye,
where it then strikes the retina
and activates light-sensitive cells,
setting the process of vision in motion.
Pupils are black because most of the
light that enters them is absorbed.
Their size changes in response
to brightness,
as well as certain drugs
and emotional and mental states—
but their basic form varies greatly
among species.
House cats, for one, are twilight hunters
with vertically elongated pupils.
In the dark, these structures
expand dramatically,
taking in the available light.
When it’s bright, they shrink into slits.
In fact, cat pupils are so flexible that
their maximum area is 135 times greater
than their minimum area—
whereas our pupils only shrink
and expand 15-fold.
And because of how the slit pupil
takes in light,
it creates sharp, vertical contours.
When the cat’s brain processes
the visuals from each eye,
the small but sharp differences
between them
help the cat judge
the precise distance of its target.
In fact, many other ambush hunters also
have vertically elongated pupils—
but mainly those whose eyes are
located closer to the ground.
This may be because these pupils are
especially useful in perceiving objects
at the relative short distances
these animals tend to hunt.
The world looks very different from behind
the horizontally elongated pupils
of many grazing and browsing animals,
like goats.
These pupils, situated
on either side of the head,
let horizontal bands of light in that
give the goat a near-360-degree view
and provide it with sharp,
horizontal images.
This helps goats detect any
disruption to the horizon—
alerting them to potential predators—
while still enabling them to see ahead
and detect obstacles
as they make their escape.
In fact, goats always keep their pupils
aligned with the horizon,
rotating their eyeballs in their sockets
as they move their heads up and down.
Meanwhile, nocturnal geckos have pupils
that shrink into slits
studded with pinholes
in higher light conditions.
Each pinhole projects a separate, sharp
image onto the geckos retina.
Scientists think that comparing
these different inputs
might help the gecko judge distance
without having to move.
And while they might have fooled you,
mantises and other insects and crustaceans
have “pseudopupils.”
These aren’t optical structures;
they’re optical illusions
experienced by the observer.
Mantises have compound eyes composed
of thousands of light-sensing units.
When some are aimed at you,
they appear black
because they’re absorbing most wavelengths
of incoming light—
but there’s no actual opening.
So, why do we have round pupils?
Elongated pupils help sharpen certain
dimensions of an animal's vision.
But scientists think that, for animals
like us with circular pupils,
this is a lower priority.
Instead of seeing some elements
of a scene in extreme focus,
we see a larger picture
in relative detail,
which enables more general skills
of observation.
This may be especially helpful
for foragers looking for food,
hunters eyeballing and chasing
their prey,
and social animals recognizing
other faces.
As we peer at different pupils,
patterns emerge.
And yet there are exceptions.
For example, Pallas’s cats and mongooses
are both small ambush predators,
but the Pallas’s cat has round pupils
and mongooses have goat-like pupils.
And we’ve only explored
a few pupil shapes.
Other animals have crescent-
or heart-shaped pupils.
And the cuttlefish has perhaps
some of the most bizarre:
their pupils are circular in the dark,
but W-shaped in the light.
So, what’s going on here?
Well, wouldn’t we all like to know?
