Pupil shape in vertebrates ranges from circular to vertical, with multiple
phylogenetic shifts in this trait. Our analyses challenge the widely held view
that the vertical pupil evolved as an adaptation to enhance night vision. On
functional grounds, a variable-aperture vertical pupil (i) allows a nocturnal
species to have a sensitive retina for night vision but avoid dazzle by day by
adjusting pupil closure, and (ii) increases visual acuity by day, because a
narrow vertical pupil can project a sharper image onto the retina in the
horizontal plane. Detection of horizontal movement may be critical for
predators that wait in ambush for moving prey, suggesting that foraging mode
(ambush predation) as well as polyphasic activity may favour the evolution of
vertical pupil shape. Camouflage (disruption of the circular outline of the eye)
also may be beneficial for ambush predators. A comparative analysis in snakes
reveals significant functional links between pupil shape and foraging mode, as
well as between pupil shape and diel timing of activity. Similar associations
between ambush predation and vertically slit pupils occur in lizards and
mammals also, suggesting that foraging mode has exerted major selective
forces on visual systems in vertebrates.