August 11, 2015
Scientists discover why cats have vertical pupils
Wolves and foxes are closely related and share many of the same
characteristics. But look at their eyes – where wolves have rounded pupils
like humans, foxes instead have a thin vertical line. But it isn’t just
canines -across the animal kingdom, pupils come in all shapes and sizes. So
why the differences?
It’s a question that has long interested scientists working on vision and
optics. In a new study published in the journal Science Advances, colleagues
from Durham, Berkeley and I explain why these pupil shapes have developed.
Goats, sheep, horses, domestic cats, and numerous other animals have pupils
which vary from fully circular in faint light to narrow slits or rectangles
in bright light. The established theory for this is that elongated pupils
allow greater control of the amount of light entering the eye. For instance,
a domestic cat can change its pupil area by a factor of 135 from fully
dilated to fully constricted, whereas humans, with a round pupil, can only
change area by a factor of 15. This is particularly useful for animals that
are active both day and night, allowing for much better vision in low light
The cat on the right has got its night-vision goggles on. (Mark Sebastian
(L); Kurt Bauschardt (R), CC BY-SA)
However, if the only reason for elongated pupils was to control the amount
of light entering the eye, the orientation would not be important:
horizontal, vertical, or diagonal would all offer the same advantages.
Instead, the pupils are almost always horizontal or vertical, which suggests
there must be other benefits which explain this orientation.
Pupils fit for every niche
Our work has focused on the visual benefits of vertical and horizontal
pupils in mammals and snakes. One of the most interesting factors we found
is that the orientation of the pupil can be linked to an animal’s ecological
niche. This has been described before, but we went one step further to
quantify the relationship.
We found animals with vertically elongated pupils are very likely to be
ambush predators which hide until they strike their prey from relatively
close distance. They also tend to have eyes on the front of their heads.
Foxes and domestic cats are clear examples of this. The difference between
foxes and wolves is down to the fact wolves are not ambush predators –
instead they hunt in packs, chasing down their prey.
In contrast, horizontally elongated pupils are nearly always found in
grazing animals, which have eyes on the sides of their head. They are also
very likely to be prey animals such as sheep and goats.
We produced a computer model of eyes which simulates how images appear with
different pupil shapes, in order to explain how orientation could benefit
different animals. This modelling showed that the vertically elongated
pupils in ambush predators enhances their ability to judge distance
accurately without having to move their head, which could give away their
presence to potential prey.
Grazing animals have different problems to deal with. They need to check all
around for prey and they need to flee rapidly in case of attack. Having eyes
towards the side of their head helps them to see nearly all around them.
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. This allows them panoramic vision along the ground to help detect
potential predators as early as possible. The horizontal pupil also enhances
the image quality of horizontal planes and this enhanced view at ground
level is also an advantage when running at speed to escape.
So, vertically elongated pupils help ambush predators capture their prey and
horizontally elongated pupils help prey animals avoid their predators.
We realised our hypothesis predicted that shorter animals should have a
greater benefit from vertical pupils than taller ones. So we rechecked the
data on animals with frontal eyes and vertical pupils and found that 82% are
what is considered “short” (which we defined as having a shoulder height of
less than 42cm) compared with only 17% of animals with circular pupils.
We also realised that there is a potential problem with the theory for
horizontal elongation. If horizontal pupils are such an advantage to grazing
animals, what happens when they bend their head down to graze? Is the pupil
no longer horizontally aligned with the ground?
We checked this by observing animals in both a zoo and on farms. We found
that eyes of goats, deer, horses, and sheep rotate as they bend their head
down to eat, keeping the pupil aligned with the ground. This remarkable eye
movement, which is in opposite directions in the two eyes, is known as
cyclovergence. Each eye in these animals rotates by 50 degrees, possibly
more (we can only make the same movement by a few degrees).