The eyes are a very powerful tool for successfully navigating nature, and they go far beyond simply providing a view of the world. The membrane that gives color to the eyes, the iris, contracts or dilates depending on the ambient light, and the window through which light enters is what we know as the pupil.
Their geometric arrangement has long been the subject of debate. In 2015, a group of researchers from the University of California, Berkeley, conducted a detailed study aimed at answering the question of why animals have different types of pupils.
To do so, scientists analyzed the pupil shape of more than two hundred different terrestrial species and concluded that pupil morphology was directly related to the animal's ecological niche.
The vertical pupil helps to ambush
These researchers observed that the vertical slit pupil is a common pattern found in predators, animals that hunt stealthily and have to remain motionless for long periods of time while observing their prey. Cheetahs, for example, have this type of pupil.
This pupil arrangement allows them to better control the incoming light and accurately gauge the distance between themselves and their prey without having to move their heads. Vertical pupils are useful for determining the probability of success and whether or not the energy expenditure of running is worth it.
Thanks to the vertical pupil, they can calculate the depth of field through two mechanisms: measuring the degree of blurriness of objects located in front of and behind the focused object, and measuring the distance of the objects through the three-dimensional image that the brain is able to reconstruct from the information it receives from each retina (stereopsis).
Rectangular pupils for panoramic vision
Gazelles, like most ungulate mammals, have rectangular pupils arranged horizontally, which allow them to see more clearly what is on the ground, calculating the precise moment when they have to start running.
Thanks to this pupil arrangement, more light enters from the sides than from above or below, creating a panoramic vision that allows them to detect the presence of predators. The horizontal pupil also improves the image quality of objects and helps the eye reduce glare from overhead.
Some herbivores have taken an additional evolutionary step and have elongated ridges on the upper part of the eye, known as the corpus nigra or iridescent granules, which act as a visor to minimize the entry of light from above and prevent glare.
On the other hand, herbivores enjoy an ocular adaptation known as cyclovergence, which basically involves the eyeball rotating in the eye socket so that the long axis of the pupil is always parallel to the plane of the ground. This unique feature allows their eyes to always see far into the distance, regardless of whether they are grazing or not.
Furthermore, the eyeballs are located on both sides of the head to provide a much larger field of vision, with only one blind spot, located behind the hindquarters. This explains, for example, why horses, when they sense danger behind them, defend themselves by kicking.
Round and W-shaped pupils
There are animals, such as wolves, that don't require such complex optical apparatus because their hunting tactic is daytime and consists of exhausting their prey. For this reason, like us, they have rounded pupils.
Of all the types of pupils, perhaps the most astonishing is that of cuttlefish (
Cuttlefish
) which takes on a practically circular shape when the animal is in dark environments and a 'W' shape when there is a lot of light, with which it amplifies the contrast of the image and assesses more precisely the distance at which the prey is located.
Finally, night geckos have an irregular, jagged pupil that helps them hunt in a variety of environments. As the popular saying goes, the eyes are the mirror of the soul.
Pedro Gargantilla is an internist at the El Escorial Hospital (Madrid) and author of several popular books.