There are a lot of different pupil shapes among vertebrates (and some invertebrates, too).
The eye itself is kind of a weird misshapen organ, particularly in land animals where it has had to compensate for, you know, the fact that it originally evolved in the water. Light passes differently through water than it does in air, not to mention that now we have to worry about our lenses- which have to be moist to properly function- drying out.
But the focus (ha ha) today is on the pupil, the transparent bit inside the iris that allows light to enter the eye. Without it, our eyes would be functionless. With it, there are a whole bunch of different ways that animals can shape their vision- and their pupil- to their advantage.
Of course, no two scientists seem to agree on exactly what these advantages are.
Essentially what the pupil does is allow light to pass through the eye and onto the retina at the back of the eye. In bright light, the terrestrial vertebrate pupil contracts (becomes smaller) to reduce the amount of light coming in; conversely, in dim light the pupil expands to increase the amount of light coming in. It’s functionally very similar to the aperture on a camera.
Most fish pupils do not dilate. Their means of controlling how much light enters their eye is relegated further back, within the retina. The exception to this are shark and ray species, which have evolved pupils that can contract or expand. This has led to some pretty bizarre shapes. We’ll discuss those a bit more later.
The Function of Vertical Slit Pupils
There is a fascinating tendency for certain shapes to pop up again and again in species of animals with certain lifestyles. For example, vertical slit pupils have evolved independantly in small canids, small felids, vipers, geckos, crocodilians, galagos, slow lorises, and skimmer birds. All of these species are predators, are nocturnal or crepuscular (active at dawn and dusk) and do not stand very high off the ground.
Again, in all these groups, slit pupils evolved separately. It must be a powerful adaptation.
So what advantages do slit pupils confer? There are actually a number of different theories right now.
The simplest and most basic theory looks at an animal’s functional anatomy. Slit pupils are found most often in animals with eyes that are exposed to highly variable light conditions- i.e., nocturnal or crepuscular animals. Slit pupils allow the iris to contract or expand more dramatically- a human’s round pupil can expand to allow light to be 10-fold more intense compared to its smallest size, while a cat’s pupil expands to 135-fold intensity. This allows the eyes of nocturnal creatures, which are designed to take in much more light than those of diurnal creatures, to close down and protect themselves during the daytime.
This explanation has never been perfect, however. Many nocturnal species have round pupils which are actually quite good at contracting to very small sizes. Take the tarsier, for example.
I like to call that the “where is your god now” effect. In fact, round pupils provide the clearest nighttime images when compared to all other shapes.
So what advantages, other than dramatic dilation, does the vertical slit pupil allow? Recently, a theory has been proposed that the slit pupil aids predatory animals in seeing color in different light conditions. This is based off of studies of fish eyes, which are adapted to highly variable underwater light conditions.
Colors are produced by light bouncing off of objects, and each color represents a different wavelength, from the long infrared to the very short ultraviolet. Because these wavelengths are different, each color reaches the pupil at a different speed. This isn’t much of a problem when there’s lots of light, but when the light is very dim, it can result in a loss of color vision. Think of how much harder it is to make out the colors of an object in the dark.
In fish eyes, this detriment can be corrected by having a pupil with different focal lengths- i.e., different sharpness in the way light is angled towards the fovea. By bending the light at different angles in different focal zones, the fish can get all colors to hit its retina at the same time.
Let’s now go back to the terrestrial eye and compare the constriction of a slit pupil with that of a round pupil. With the pupil divided into different zones, how these zones are exposed to light changes depending on the way the pupil constricts.
When the pupil constricts in a circular way, the outermost (black) zone of the pupil is completely blocked, meaning that in bright conditions an animal would be unable to see colors with longer wavelengths. But when the pupil constricts in a vertical way, all three zones of the pupil still have some exposure. In other words, an organism with a slit, multifocal pupil could maintain better color vision at both low and high levels of light.
Animals with rounded pupils almost never have these different focal zones (there are some exceptions in certain species of snakes and rodents) because, obviously, the broader zones would be blocked in high-light conditions.
Color is important specifically for predators because it allows them to contrast a hidden prey animal against its surroundings. It’s also important for nocturnal frugivores such as the galago and slow loris, as fruit tends to be brightly colored.
However, this color explanation STILL doesn’t resolve every issue surrounding vertical slit pupils. We would expect this to be a very powerful adaptation for all nocturnal predators, yet the consistent pattern is that it is seen in small or low-to-the ground animals. Large cats and large canids do not have slit pupils.
So what’s the difference? Well, there’s one more thing that the shape of the pupil confers: the shape and depth of an image. Animal eyes adapted to low light are usually characterized by a short length of focus (i.e., they can’t see as far). A vertical pupil allows an animal to have a longer length of focus during the daytime thanks to its elongated but thin shape.
A vertical pupil also allows a small predator to see horizontal movement in sharp focus- important when spotting prey from low to the ground. Spotting movement like this is of particular importance to ambush predators (like snakes, crocodiles, and small cats). It’s less useful for taller predators because their head is higher in respect to the horizontal plane of the ground.
There is yet one more advantage that vertical pupils might confer, and that is crypsis: a round pupil is more distinctly noticeable than a vertical pupil.
Horizontal Slit Pupils
Like vertical pupils, horizontal slit pupils have evolved independently in many groups of animals. These include even-toed ungulates as well as all equids, mongooses, some rays, some frogs and toads, Japanese vine snakes, and octopi.
The similarities between the members of this group are not as clear-cut as those between the vertical pupil group. The ungulates are large diurnal herbivores, while the mongooses, amphibians, rays, snakes, and octopi are all small carnivores, some of whom are nocturnal. The rays and octopi are even fully aquatic!
Perhaps one commonality in all of these creatures is that they can all be considered prey animals, and they all have their eyes located on the sides of their heads. What’s the connection? Well, as in the vertical pupils, the pupil shape and orientation can have an effect on their depth of field. In this case, horizontal pupils sacrifice some sharpness with the advantage of an extremely wide- nearly 360 degree in some species- field of vision.
Obviously, this is useful for a prey animal. They need much less to see the predator clearly than they do to spot the predator at all and run. (In fact, most prey animals would probably prefer not seeing a predator very closely.) Similar to the ways that vertical slit pupils are better at seeing horizontal motion, horizontal slit pupils see vertical motion more sharply, a better way of spotting distant predators.
There are other factors to consider as well: I mentioned earlier that most animals with vertical slit pupils were ambush predators- well, the majority of animals with horizontal pupils are active foragers, whether they are prey or predators. The wider field of vision likely aids with that.
Other uses for slit pupils apply irrespective of orientation- many animals with horizontal pupils also have multifocal lenses, enabling them to see color in many different light levels. Horizontal pupils can also expand to become very wide and round, though they rarely close as tightly as vertical slit pupils. Most of the animals are diurnal and do not need to block out quite so much light.
Again, no single theory fully explains why these shapes have evolved; it’s likely a combination of all of them.
Other Shapes and Weird Wiggles
Ok, now we get to the fun bit. There are some really weird pupil shapes out there, especially in aquatic animals. There are crescent, u, or w-shaped pupils; pupils with weird bumps and wiggles and pinholes; pupils that can constrict into pear or triangle shapes. These unusual eyes can be found in the aforementioned rays and sharks, cuttlefish, cetaceans, pinnipeds, snakes, and geckos. Some ungulates such as horses also have surprisingly wiggly pupils when examined closely.
Let’s talk about pinholes first.
Pinholes occur when the pupil is shaped in such a way that when it fully contracts it leaves several tiny gaps. When light shines through these tiny gaps, multiple images shine onto the retina if the object being viewed is either too far away or too close. Only at the perfect distance will a single image be displayed. This allows the gecko to precisely line up its distance from a prey animal before it strikes.
Pinholes are present in vertically-inclined gecko pupils and the u-shaped pupils of some rays and skates, but cetaceans such as dolphins also have them. I couldn’t find a good image of a dolphin’s pupil contracting, so I drew one.
In the case of cetaceans, the two pinholes help them to look both forwards and backwards at the same time.
This is also a feature of the w-shaped cuttlefish eye, though it does not form pinholes. Instead, they have two separate fovea instead of one on their retinas, allowing for two separate images to form: one looking forwards and one looking backwards.
The cuttlefish eye is also specialized at contracting for changing light levels.
The u-curved or crescent-shaped pupils that cuttlefish as well as many rays, skates and sharks have function in a similar way as the horizontal pupils do: they have a wide depth of field. However, the curved shape also causes them to perceive light in different ways- n-shaped if they are looking beyond it and u-shaped if they are looking in front of it, with the size of the curve increasing with distance. This, like the pinholes, allows them to accurately gauge their distance from an item and bring it into a strike zone.
Finally, here are some more pictures of weird pupils because I couldn’t find enough research to explain everything.
Read on: Evolution is awesome, isn’t it? Want to know about other diverse organs, like boobs? Or how about whether or not the same traits can evolve twice? Tiny changes in developmental timing probably lead to some of these bizarre pupil shapes. And just look at the diversity of animals in the canine and bat families! For more crazy evolution check out my posts on sexual selection and brood parasitism.
Brischoux, F., Pizzatto, L., & Shine, R. (2010). Insights into the adaptive significance of vertical pupil shape in snakes. Journal of evolutionary biology,23(9), 1878-1885.
Land, M. F. (2006). Visual optics: the shapes of pupils. Current biology, 16(5), R167-R168.
Malmström, T., & Kröger, R. H. (2006). Pupil shapes and lens optics in the eyes of terrestrial vertebrates. Journal of Experimental Biology, 209(1), 18-25.
Mass, A. M., & Supin, A. Y. (2007). Adaptive features of aquatic mammals’ eye. The Anatomical Record, 290(6), 701-715.
Murphy, C. J., & Howland, H. C. (1990). The functional significance of crescent‐shaped pupils and multiple pupillary apertures. Journal of Experimental Zoology, 256(S5), 22-28.
Sprague, W., Helft, Z., Parnell, J., Schmoll, J., Love, G., & Banks, M. (2013). Pupil shape is adaptive for many species. Journal of Vision, 13(9).