Staring into the eyes of a cat can be a mesmerizing. The different shapes and expressions of each cat create distinctions between the breeds, as some have large round eyes, and others have almond-shaped eyes. But one aspect of the feline’s eyes that are most peculiar are the distinct shape of the pupil. Many animal have pupils which contrast greatly with those that humans possess.  Dogs and birds endearingly have round pupils, goats have rectangle pupils, and cat pupils are shaped as a marquise or diamond, pointing sharply at both ends, or expanding widely with a pointy top and bottom of the pupil.   Why are the shapes of pupils so varied between species, and how do they differ in function?  Here is a brief look at the design of both canine pupils and feline pupils.

Cat Pupils

First of all, pupils are the part of the eye that “reads” or absorbs light. Because light can fluctuate hour-to-hour, the pupil either extends or shrinks in order to adjust to the light at any given time.

Domestic cats have diamond-like pupils that can shrink to a small sliver, and expand to a broad pupil shape, while wild cats (such as tigers and lions) have small round pupils. Here are several theories of why this is the case, and what these pupil shapes are able to accomplish.

The University of California at Berkeley studied over 214 land-dwelling species and how their eyes function.  Animals who would most often become prey were likely to have extended, square-shaped pupils, to give them a wide panoramic view to be able to spot approaching danger.  The study concluded that animals who are aggressive predators who hunt low to the ground would be more likely to have slit-shaped pupils, so as to focus on their object of pursuit. Larger animals of prey (such as tigers, wolves, hawks) who have a higher vantage points while hunting, would be more likely to possess a circular shaped pupil.  Since domestic cats stalk prey by crouching low to the ground, their eyes are able to gauge distances before aiming to pounce upon prey. This diamond/slit shaped pupil also aids domestic cats for hunting both day and night, as their pupils can almost collapse into themselves, and avoid being blinded by the brightness of the sun, while also expanding to absorb light and shapes in what would appear like pitch-darkness to others.  The feline’s pupil shape is perfectly designed to provide the skills he needs to survive.

Dog Pupils

Like humans, dogs have rounded pupils. A circle-shaped pupil is generally believed to correlate to a monofocal optical function that is most commonly found in animals that hunt during the day and primarily sleep at night. Since dogs are naturally inclined to hunt while the sun is out, they have less of a need for strong night vision than domestic cats do. Surprisingly the status of a dog’s round pupil is unclear since some dogs have photoreflective reflexes (such as the Dachshund), while other breeds actually have circular ring-like structures (like the Schnauzer).  However, the round shaped pupils of canines could contribute to why humans emotionally connect to their dogs, and enjoy staring into their “puppy eyes.”  The wide, round pupils are familiar, sympathetic, and almost familial compared to the eyes of other creatures.

Pupils of goat, owl, and snake

Pupils of goat, owl, and snake

Other Animal Pupil Shapes

  • Horses: oval pupils
  • Eagles/Owls/Hawks: round pupils
  • Snakes: slit pupils (Except snakes like the Emerald rat snake, or the Japanese vine snake who has either round or misshapen horizontal pupils).
  • Dolphins: crescent-moon pupils
  • Alligators: slit pupils
  • Sheep: rectangle pupils
  • Sharks: diamond-shaped
  • Seal: pear-shaped pupils
  • Lemur: large eyes with tiny round pupils
  • Giraffe: round pupils
  • (Some) Frogs: heart-shaped pupils
  • Cuttlefish: w-shaped pupils
  • Wolves: round pupils
  • Foxes: diamond-shaped pupils
  • Octopi: flat, horizontal-line pupils