Looking into the eyes of various animal species, you will notice that some pupils have unusual shapes. Why is that? It turns out that the shape of the pupil represents the behavior of the animal within its ecosystem.
The pupil is a hole located in the center of the iris that allows light to pass through and reach the retina, activating photoreceptor cells and starting the image processing.
The pupil appears black because the light passing through is absorbed directly by the tissues inside the eye or absorbed after being diffusely reflected within the eye, unable to escape through the narrow pupil. Their sizes change in response to brightness, certain drugs, and emotional states.
Vertical Pupils
The basic state of pupils varies significantly among animal species. For example, domestic cats have vertically elongated pupils; they are skilled hunters at night. In darkness, their pupils dilate significantly, maximizing available light. When it is bright, they constrict into vertical slits.
In fact, cat pupils are highly flexible, with their maximum area being 135 times larger than their minimum area, while our pupils can only contract and dilate up to 15 times. When the vertical slit pupils receive light, they create vertical clear viewing zones. The cat’s brain processes images from each eye and combines them to form a complete image, helping them accurately identify prey.
Many other predatory species also have vertical pupils. This may be because vertical pupils are particularly useful in detecting prey at close distances that these predators target.
Horizontal Pupils
The world looks very different through the eyes of many herbivorous species with horizontally elongated pupils, such as goats, sheep, and horses. Instead of being positioned at the front like cats, goats’ eyes are located on either side of their heads, giving them nearly 360-degree vision and creating horizontal clear viewing zones.
This helps goats detect abnormalities such as lurking predators, as well as providing them with a wide field of view to identify obstacles while fleeing. In fact, goats keep their pupils level with the horizon by rotating their eyeballs as they move their heads up and down.
Round Pupils
Why do humans have round pupils? Vertical or horizontal pupils help animals focus sharply in a specific direction. Scientists believe that, rather than focusing sharply on a single point with the surroundings blurred, round pupils allow us to see a broader picture with relatively detailed observations, enabling more general observations.
This is particularly useful for foraging in environments with complex landscapes, such as picking fruit from trees, hunting, and recognizing faces in social settings. Round pupils are also found in many animal species such as dogs, wolves, and bears.
Other Pupil Shapes
The pupils of chameleons constrict into vertical slits with numerous small holes in bright light conditions. Each small hole reflects a sharp, distinct image onto the chameleon’s retina. Scientists believe that comparing these images can help chameleons gauge distance without needing to move.
Mantises and other insects and crustaceans have pseudo-pupils. The black dot you see on their eyes is not a pupil but merely an optical illusion. Mantises have compound eyes made up of thousands of light-sensing units. When they face you, some of the cells turn black because they are absorbing most wavelengths of incoming light, but they do not actually have pupils.
Typically, depending on the species’ behavior, we see them sharing a common pupil shape; however, there are exceptions. For example, the Pallas’s cat and the meerkat are both ambush predators, but the Pallas’s cat has round pupils, while the meerkat has horizontal pupils.
It must be acknowledged that we have only discovered a few pupil shapes. Other shapes, such as crescent-shaped or heart-shaped pupils, exist. Notably, in the case of the cuttlefish, they have round pupils in darkness, but when exposed to light, they change to a W-shape. Why is that? There is still much more for us to explore to find answers in this world.
