You know you have stumbled onto a question that we don’t have a lot of answers to, when every Google search result refers you to the same piece of published research. That unfortunately is the case regarding animals and their pupil shape. You would think, “Why do cats have vertical slits for pupils?” is a question that has been asked and answered many times over. Well, it has definitely been asked, it is just that science does not have all the answers yet. What we have can best be described as hypotheses. And these stem from a study published in 2015 by a team of researchers at the University of California led by Professor Martin Banks.
To start off, let’s refresh our memory on the functions of the pupil and retina. The retina is the layer of cells on the back wall of our eye. Light entering the eye hits the retina and in response, the retina sends electrical impulses to the brain, eventually allowing us to see. The pupil is simply the hole in our eye that controls how much light gets through to the retina. So, as you might have already realized, how much the retina gets illuminated depends directly on two factors, one, the area of pupil dilation (how wide open the pupil is) and two, the intensity of the light.
Now, what we have known since the mid-1900s is that slit pupils allow better control of the amount of light entering and striking the retina. This is because, while circular pupils like ours can change their area by about 15-times (between bright and dim conditions), slit pupils like those of cats and geckos undergo area changes hundreds of times that (150-fold in cats and 300-fold in geckos).
This capability is essential to species such as these which are primarily nocturnal but also exhibit daytime activity, since they need to dilate sufficiently in dark conditions to maximize available light, while constricting in bright conditions to prevent too much light from entering. However, this is true of all slit pupils. What it does not explain is why animals like cats have slits that are oriented vertically.
Analyzing a database of 214 different animals (excluding birds and fish), Professor Banks and his team found an incredibly strong correlation between the orientation of slit pupils and whether the animal in question was predator or prey. To be more precise, within the database, amongst those animals with slit pupils, those with vertical slits tended to be ambush predators, many of which are predominantly nocturnal. Those with horizontally oriented slits tended to be herbivores, i.e., prey. Let’s leave these horizontal slit-pupiled animals for another time and focus on why ambush predators such as our domestic cats may have evolved vertical slits for pupils.
Vertical versus horizontal slit pupils.
Of paramount importance to an ambush predator is the ability to accurately judge the distance between itself and its prey.
In binocular vision systems (organisms with two eyes), there are three main ways in which this sort of distance judging can be accomplished.
The first is a method called stereopsis which uses the 3D effect of having two separated eyes, each with its own slightly different point of view. This is what we humans primarily use to measure distance. Focus on your finger with both eyes, then close one eye at a time. That “side-to-side jump” between views is what makes stereopsis work (at this point, it is also important to keep in mind that most predators that use sight for hunting typically have eyes on the front of their head).
For stereopsis to be effective, the brain must know that it is looking at the same object with both eyes, which means that one eye has to match a feature of the object with the same feature in the other eye (think of the finger again). Typically, animals are symmetrical about a vertical axis, which essentially means that you would have to split an animal front-on for the two halves to be symmetrical (apologies on that rather gory example). What this also shows is that the eyes of front-facing animals, including our own and most sight-reliant predators, are parallel to each other on a vertical plane. If our eyes had evolved to be one above the other, then they would be parallel to each other on a horizontal plane. Therefore, horizontal displacements, as with our finger “jump” can more easily be measured by using vertical shapes rather than horizontal shapes (try the finger exercise again, first with the finger pointing up and then pointing sideways to see this in action). Hence, for stereopsis to work best, sharp resolution of vertical lines and contours of a subject is important. What this also tells us about stereopsis is that it is most effective at judging the distances of vertical shapes and contours.
Second, there is defocus blur which refers to objects at different distances being blurred out differently. This effect is best visualized using a camera, which when focused on an object, blurs out the back and foreground (depending on the camera’s aperture setting).
To really understand defocus blur, let’s take the example of the camera again. Bokeh is a much-loved and much sought-after effect where the subject is in sharp focus and the back and foreground is blurred out. You would also know that such an effect is accomplished by widening the camera’s aperture (essentially the pupil of the camera). So, in terms of an eye, the wider the pupil is, the more a subject can be discriminated against the rest of the scene. Hence, to maximize defocus blur, wide open pupils work best. To see how this would come in handy, put yourself in the paws of a jungle cat crouched on the ground, stalking a rodent. As the cat gazes along the ground, vertical shapes become foreshortened. To understand foreshortening best, place a page of square-ruled paper (like from an old maths exercise book), or any chequered material for that matter, on a table and look at it from above. Now go lower and look at it with your eyes just above the level of the table. The vertical lines leading away from you look much shorter, while the horizontal lines are not as affected. What has happened is that the prominence of the horizontal lines of the paper, which in the case of the jungle cat is the ground, have increased. And just like stereopsis was effective at judging the distances of vertical shapes, good defocus blur thanks to wide open pupils, allows a greater depth of field, which in turn increases focus discrimination of horizontal lines and contours thus allowing better judgement of distances along the ground.
Finally, imagine driving by in a car. The pavement next to the road appears to move much faster than the trees and buildings further away. This effect is called motion parallax and is the third method of distance estimation for binocular organisms.
An ambush predator cannot use Motion Parallax for obvious reasons. Ambush relies on the element of surprise. If the predator has to move about trying to gauge the distance between it and its quarry, surprise goes out of the window along with dinner. Therefore, these creatures rely mainly on stereopsis and defocus blur for distance estimation. At this point, I’m sure you are probably wondering how all this relates to vertically aligned slit pupils. Well, we are almost there.
Now herein lies a bit of a contradiction. The requirements for stereopsis and defocus blur seem to be mutually exclusive. Let’s take the camera example again to understand why this appears to be the case. While a large aperture (large pupil dilation) ensures focus discrimination of a subject for defocus blur, smaller apertures would (pupil constriction) ensure greater image sharpness which is necessary for stereopsis. So how can both be achieved at the same time? It sounds impossible. Or is it?
Let’s take a step back and look at the camera aperture again. It is circular in shape. A circle cannot be narrow and wide at the same time. But what if it was not a circle? Imagine a pupil that is a vertical slit.
It is very narrow horizontally which is ideal to perform sharp focusing on vertical shapes, thus facilitating stereopsis for estimating the distance of items perched (vertical) on the ground, like a rodent. At the same time, it is wide vertically for great defocus blur on horizontal lines and contours thus allowing distance estimation along the foreshortened ground. And guess what? That is exactly the shape of a cat’s pupil!
But there is more. Some of you may be wondering, well hold on a minute, big cats like leopards and tigers are also ambush predators, but they have circular pupils.
What’s the deal with that?
Excellent question. Let’s go back to the finger example. But instead of a finger, let’s use our whole palm this time. Hold your palm away from you at arm’s length. The whole palm is clear and sharp. Now bring it to within a few inches of your face. You can no longer focus on the entire palm, just a small part that your eyes focus on. This is exactly the same scenario that small cats like domestic cats and fishing cats and other small ambush predators face. But instead of your palm, they are looking at the ground and at very close quarters. Therefore, they have a much greater need to reduce the blurring of vertical shapes for stereopsis. On top of this, their low viewpoint means that more of the ground is blurred out which would promote the use of defocus blur to measure distances along the ground. These conditions favour a pupil shape that is a vertical slit for short, frontal-eyed, ambush predators such as small cats, whereas for taller predators, like the big cats, these requirements are not as important. In fact, Among 65 frontal-eyed, ambush predators assessed in the study, 44 had vertical pupils. Of these, 82% had shoulder heights less than 42cm.
To conclude, it is important to remember that all we have discussed are theories at the moment. The fact that there are several exceptions within the animal kingdom to the above conclusions point toward many other factors contributing to pupil shape and orientation beyond simply if an animal is predator or prey, big or small. There is also the question as to where circular pupils fit in. Although the evidence is not as strong, the results suggest that large predators which are primarily diurnal (active during the day), but may also show some nocturnal activity, including humans, tend to have circular pupils as it provides a good overall compromise. Continued research will unlock more clues to our questions about animals and their pupil shapes in the future, at which point, this article will need updating.
So, watch this space and the next time you stare into your cat’s vertical slit-pupils, just remember that it probably has no idea why you are doing it.
References:
Banks, M., Sprague, W., Schmoll, J., Parnell, J. and Love, G., 2015. Why do animal eyes have pupils of different shapes?. Science Advances, [online] 1(7), pp.e1500391-e1500391. Available at: <https://advances.sciencemag.org/content/1/7/e1500391>.
