Horses and Sheep and their amazing Eye Movements

– [Man] Look closely at
this slow-motion sequence of a sheep pitching its head up and down. You will see that the pupils
in its eyes are slits. And if you look really closely,
you’ll see that the slits stay nearly parallel to the ground as the sheep rotates its head. This is a remarkable ability, and it’s certainly something
that we, as humans, cannot do. Horses, goats, and deer
have the same ability. Here is a photo of a horse
with its head up and down, and you can see that the elongated pupil has remained parallel to the ground. If we artificially rotate
this image of its head down so that the head becomes horizontal, then we can see how strange
the pupil looks now. So, why have horses and sheep
evolved to be able to do this? Many grazing animals have their eyes on the side of their head,
unlike ours, which point forward. This helps them to see all around. Their elongated pupil gives
them an effective field of view that is wider than if the
pupil were vertically elongated and is shorter than if the
pupil were vertically elongated. They can therefore see
nearly all around themselves; forward, left, behind, and right. Most grazing animals are
prey for other animals, so it’s important for them to see predators approaching from any direction. The horizontally elongated pupil helps these prey animals to
see all around along the ground by enhancing the amount
of light entering the eye in the forward and backward directions, while decreasing the amount of light entering the eye from above. The pupil also enhances the
image quality of the ground, which again is important to
help them flee from predators. So, there are advantages to these animals in having horizontal slit pupils, but there is then a problem. What happens when they bend
their heads down to eat? The natural thing would be
for their eyes to rotate with their head so that the pupils became close to vertical
relative to the ground. This wouldn’t be good
because it would enhance the effective field of
view above and below, rather than along the ground. But they have evolved to
be able to rotate both eyes to keep the slits approximately
parallel to the ground as their heads pitch up and down. This eye movement is
know as cyclovergence. It’s opposite in
direction in the two eyes. It’s an amazing ability
that these animals have. We can make such movements, but over a very much smaller
range than the grazing animals. Why don’t you see if
you can see this effect in grazers yourself when
you’re out and about?


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