Research overview: the wide pupil area range in domestic cats

Cats are often noted for an exceptionally large ratio between their fully constricted and fully dilated pupil areas, a consequence of the slit geometry. This overview surveys measurements and discussion of that range, frequently cited as far greater than the modest ratio possible with a round human pupil. The slit can close to a fine line in bright sun and open to a broad aperture in near-darkness, matching the cat's need to operate from daylight to deep dusk. The survey connects this range to the protective and light-gathering demands of a tapetum-equipped, rod-rich retina that is both sensitive and vulnerable to bright light. Documenting the constriction range turns the visible behavior of a cat's pupil, narrowing and widening dramatically with conditions and arousal, into a quantitative feature of its visual adaptation to a broad span of light intensities.