In this blog post, we’ll examine how the size and shape of the pupil are regulated, how the pupil’s shape affects the light entering the eye, and, as a result, how the pupil’s shape contributes to an animal’s survival.
Animals gather visual information through their eyes. In particular, the pupil and iris—which are responsible for the first stage of visual information processing—play crucial roles. The pupil acts as a passageway for light entering the eye, while the iris regulates the shape and size of the pupil. The size of the pupil adjusts in response to changes in external light levels, ensuring that an appropriate amount of light enters the eye. The focus of this article is on the shape of the pupil. While human pupils are circular, some animals, such as cats and goats, have flat pupils that are not circular. Pupil shapes can be broadly categorized into circular, vertical, horizontal, and other forms. Let’s examine how the size and shape of the pupil are regulated, how the shape of the pupil affects the light entering the eye, and specifically explore the advantages that this shape provides to animals.
Pupil size is regulated by the movement of the iris muscles, which is controlled by the midbrain and the autonomic nervous system; as a result, the pupil either contracts or dilates. In species with circular pupils, such as humans, the iris contains the circular muscle—a ring-shaped muscle surrounding the pupil—and the radial muscles, which extend radially. When the parasympathetic nervous system is activated, the circular muscle contracts and the radial muscles relax, causing the pupil to constrict; when the sympathetic nervous system is activated, the opposite mechanism occurs. However, in circular pupils, there is a limit to the range of adjustment achievable through the contraction of the circular muscles. Furthermore, to reduce the size of the pupil while maintaining its circular shape, the circumference must also decrease accordingly; consequently, circular pupils take relatively longer to regulate the amount of light entering the eye, which can result in reduced light sensitivity.
In contrast, organisms with elongated pupils possess linear muscles. Straight muscles contract and relax very quickly in the horizontal or vertical directions, and their range of movement is also large. Comparing humans and cats, the difference in area between a contracted and relaxed human pupil is about 15-fold, whereas for a cat’s vertically elongated pupil, that ratio ranges from 135- to 300-fold. However, because diffraction occurs when light enters through an elongated pupil, vertical pupils are at a disadvantage when receiving horizontally entering light, and horizontal pupils are at a disadvantage when receiving vertically entering light, making distortion more likely to occur.
It was found that 44 out of 65 species of ambush predators—animals that lie in wait to launch surprise attacks—have vertically elongated pupils. The reason for this is “depth of field.” Depth of field refers to the degree of blurriness when an object is out of focus; due to diffraction, vertical pupils have a shallow depth of field in the vertical direction, causing images to blur easily, but a deep depth of field in the horizontal direction, so images do not blur easily. This makes them advantageous for judging the distance to an object. Additionally, because vertical pupils dilate significantly in low-light conditions, the amount of light reaching the retina increases, allowing these animals to see their surroundings clearly even in the dark—a trait that is advantageous for nocturnal activity. Consequently, vertical pupils are highly advantageous for hunting among small felids, such as cats and wildcats, as well as ambush predators active day and night, such as snakes, crocodiles, and lizards.
However, tigers and lions—which belong to the same family—have round pupils rather than vertical ones. According to data, 82% of the 44 species with vertical pupils are small animals with a shoulder height of less than 42 cm, whereas only 17% of the 19 species with round pupils fall into this category. This difference is due to the distance between the eyes. The brain interprets the differences in information received from both eyes to process visual information; the smaller the distance between the eyes, the smaller the difference in information. Therefore, small animals with a relatively narrow interpupillary distance experience less image distortion caused by vertical pupils, whereas larger animals with a shoulder height exceeding 42 cm perceive greater image distortion; it can be inferred that they have round pupils, which result in less variation in depth perception.
In contrast to vertical pupils, horizontal pupils are primarily found in prey animals and have an elongated horizontal shape. While horizontal pupils suffer from severe vertical distortion of light, they can accommodate light entering from both sides, allowing for a wider field of view. Prey animals with a wider field of view are more likely to spot predators. Therefore, horizontally elongated pupils are advantageous for survival in prey animals such as goats, mongooses, and frogs. In fact, because animals with horizontal pupils often have eyes positioned on both sides of their heads, they can secure a field of view approaching nearly 360 degrees.
The shape of the pupil varies depending on the type of iris muscle, and different pupil shapes affect the light entering the eye, which has had a significant impact on animal survival. Light entering through a circular pupil is relatively uniform from all directions, providing a stable image, whereas an elongated pupil distorts light entering vertically or horizontally relative to the axis, creating differences in depth perception. As a result, most large animals have round pupils. Vertical pupils are advantageous for judging the distance to a target, thereby enhancing a predator’s hunting ability, while horizontal pupils widen the field of view from side to side, increasing a prey’s chances of escape. Although differences in pupil shape may seem very minor, it is surprising that they have provided such significant survival advantages for certain animals. It also makes one wonder how pupil shape affects the survival of animals living not only on land but also in water or in the air.
