Ever noticed that some animals have slit-shaped pupils in their eyes? This gives them a number of advantages, says Stuart Blackman.
Slit-shaped pupils have a far greater range of contraction and expansion than circular ones, so suit species, such as cats and geckos, that are active both day and night.
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Slits may be vertical or horizontal. Herbivores with side-mounted eyes (sheep and horses, for example) usually have horizontal ones, which provide a panoramic view of approaching predators.
Their eyes even rotate in their sockets so that the slits remain horizontal when the animal puts its head down to feed.
On the other hand, vertical slits enable predators with forward-facing eyes to best judge precise distances to prey, though circular pupils do the job better for larger predators, such as lions and humans, with eyes set higher above the ground.