Know why kittens have vertical pupils

Pupils' shape and size can reveal whether your kitten is a hunter or the hunted, research shows.

The study also said that species with pupils that are vertical slits are more likely to be ambush predators that are active both day and night.

"For species that are active both night and day, like domestic cats, slit pupils provide the dynamic range needed to help them see in dim light yet not get blinded by the midday sun," said lead researcher Martin Banks, professor of optometry at University of California-Berkeley.

The vertical slits of domestic cats and geckos undergo a 135 and 300-fold change in area between constricted and dilated states, while humans' circular pupils undergo a mere 15-fold change, the study said.

In contrast, those with horizontally elongated pupils are extremely likely to be plant-eating prey species with eyes on the sides of their heads.

Circular pupils were linked to "active foragers", or animals that chase down their prey.

For ambush predators with vertical-slit pupils, the authors noted the importance of accurately gauging the distance animals would need to pounce on their prey.

Researchers identified three cues generally used to gauge distance -- binocular disparity, motion parallax and blur.

Binocular disparity and blur work together with vertically elongated pupils and front-facing eyes. Vertical-slit pupils maximise both cues, the researchers said.

However, vertical pupils are not equally distributed among ambush predators.

"A surprising thing we noticed is that the slit pupils were linked to predators that were close to the ground," said William Sprague, a postdoctoral researcher in Banks' lab.

"So domestic cats have vertical slits, but bigger cats, like tigers and lions, don't. Their pupils are round, like humans and dogs," he added.

Vertical pupils appear to maximise the ability of small animals to judge distances of prey, said the findings published in the journal Science Advances.