Animal pupils are attuned to lifestyle.
The pupil is the gap in the iris of the eye that allows light to traverse to the retina, where it is absorbed and employed for vision. The muscles in the iris vary pupil size, altering the amount of light input.
The pupil shapes of terrestrial animals vary tremendously depending upon their ecological niche: their way of garnering food and time of day they are active.
Human pupils are circular because the iris muscles are arranged in a ring that contracts evenly toward the center. This gives an even focus across the entire field of view. But circular pupils cannot constrict as tightly as other pupil shapes.
Chameleons also have circular pupils, but they can move each eye independently, affording simultaneously looking in 2 different directions with a full 360° view. The chameleon mind makes sense of the disparate visual scenes the eyes present by collating the collage.
Nocturnal animals need large, light-sensitive eyes that would be overwhelmed in broad daylight, so they evolved an extra set of muscles to pull the pupil into a narrow slit during the day.
Predatory animals, such as cats and many snakes, need to know how far away their prey are. Vertical pupils create an astigmatic (skewed) depth of field: vertical contours at different distances are sharp, whereas horizontal contours are blurry. This helps ambush predators, and those active day and night, to accurately estimate distances via vertical contours while still sensing horizontal movement.
Like felines, crocodiles are ambush predators. Both have vertical slit pupils to gauge distance and good nighttime vision. The crocodile goes one further.
Crocodile eyes have a layer of reflective, mirrored crystals behind its retinas. During the day, a pigment in these crystals acts like a pair of sunglasses. But at night, the pigment cells retract, allowing the crystals to reflect light back onto the retina, amplifying image strength. This lets a crocodile see in the dark.
Crabs also have vertical pupils, albeit often with unmovable eyes mounted on stalks sitting atop their heads. Crab vision is not sharp, but good enough to send it scuttling for its burrow when danger comes into view – and to catch what it can for food.
To be on the lookout, prey animals need to see over a wide range. Coupled with laterally placed eyes (on the sides of the head), horizontal pupils create a panoramic view that facilitates predator detection across uneven terrain. Goats, for instance, can see 320° with no blind spot. The tradeoff is that image sharpness is poor at the periphery.
Rabbit eyes are mounted high on the sides of their heads, affording vision from nearly every direction, including above them. Like humans, rabbits’ round pupils have limited contraction. Though rabbit vision is not very sharp, they have decent depth perception. Rabbit sight is designed to effectively detect predators from any direction.
Some animals evolved elaborate pupil shapes to fit their circumstance.
Cuttlefish have a smoothly curved W-shaped pupil that balances bright light from above with the darkness below. Though cuttlefish cannot see color, they have the most acute polarizing vision of any animal.
Cuttlefish have 2 acute vision spots (foveae): one oriented forward, the other backward. The eye changes focus by shifting the position of the entire lens with respect to the retina, instead of reshaping the lens as in mammals. Unlike vertebrate eyes, no blind spot exists in cuttlefish because the optic nerve is positioned behind the retina.
Cuttlefish have stereopsis (stereoscopic vision). They discern depth/distance via their mind calculating input from both eyes.
Nocturnal geckos have large eyes and pupils, with acute photoreceptors and a short focal length (the distance from the center of an eye’s lens to the point where the light converges) that affords color vision in low-light conditions. Gecko eyes are 350 times more sensitive to light than human eyes.
Gecko eyes have multifocal lenses which lets different portions of the lens focus distinct wavelength ranges onto photoreceptors. This sharpens color vision. The mind mystically merges the separate color swatches so that the representation reflects what the gecko is looking at.
In bright light the gecko pupil contracts to 4 pinholes along a vertical slit. The tiny portals channel light to the different parts of the multifocal lens. The multiple pupils generate several images on the retina with varying levels of focus, which the gecko mind employs to judge distance.
Insects with compound eyes dispense with pupils altogether. Insect minds fabricate coherent scenes from many thousands of visual inputs. The diverse designs of these eyes are optimized for the distinct lifestyles these 6-leggers live.
Pupils exemplify that evolution is an orchestrated adaptation to functional traits. A localized force of coherence that resides with the energy complex that defines an organism learns and malleably adapts future generations to novel designs. Antecedent traits and estimation of environmental advantage serve as backward-looking and forward-looking guides respectively. Whence evolution proceeds.
References:
Ishi Nobu, The Elements of Evolution, BookBaby (2019).
Martin S. Banks, “Why do animal eyes have pupils of different shapes?,” Science Advances (7 August 2015).
Jochen Smolka & Jan M. Hemmi, “Topography of vision and behaviour,” Journal of Experimental Biology (2009).
Luis Villazon, “Why do animal pupils come in different shapes?,” BBC Science Focus (2020).
Susan Blackmore, “Why don’t prey animals have eyes in the backs of their heads?,” BBC Science Focus (2020).