Quirks & Quarks

Why do some animals have slit shaped pupils?

Animals have evolved different shaped pupils to serve purposes particular to their lifestyle, such better depth perception or enhanced night vision.

Different shaped pupils can provide benefits in low light or for predators or prey.

Alligators have vertical pupils, all the better to see you with (Fernando Cortes / Shutterstock)

This week's Question comes from Derek Sotheby in Burlington, Ontario. He asks:

Why do some animals have slit-like pupils and some have round pupils? What are the advantages and disadvantages of each?

Christopher Dutton, an adjunct professor at the Ontario Veterinary College of the University of Guelph says slit-like pupils have evolved independently in a variety of vertebrate species. There are both vertical and horizontally slitted pupils. Vertical pupils can be found in dogs, cats, vipers, geckos, crocodiles and some birds. Goats, sheep, deer, horses, frogs and toads have horizontal pupils. 

One advantage to slit-like pupils is that they allow the iris to contract and expand more dramatically. This is a useful trait for nocturnal species whose eyes are designed for low-light levels. It also protects their eyes from the bright light of day.

Another advantage is that slit-shaped pupils also work well with lenses that have evolved multi-focal structures that better allow animals to see colour at night. This can be important for animals looking for brightly coloured fruit in the dark. 

Many prey species like this deer have horizontal pupils (Shutterstock / Mircea Costina)

Vertical slits enable ambush predators to optimize their depth perception. Horizontal slits enable prey species to optimize the field of view and image quality of horizontal contours. 

But round and slitted are not the only pupil shapes out there. There are pupils that are crescent shaped, triangular, and pupils with bumps and even pinholes. 



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