Vertical pupils help snakes stalk their prey

London (ANI) - Vertical pupils don't just help snakes see at night, but also help them stalk prey without being seen.

Richard Shine and Francois Brischoux at the University of Sydney, Australia, scoured the literature and found that vertical pupils on most animals become round in low light.

This challenges common theory that vertical pupils evolved to give animal's night vision.

Thus, the researchers compared the pupil shape of 127 Australian snake species with Shine's data on their hunting behaviour and the time of day that they are most active.

This revealed that snakes that ambush their prey and hunt at night tend to have vertical pupils, while round pupils were more characteristic of diurnal snakes that actively seek out and pursue their prey.

Overall, however, pupils were more strongly associated with hunting behaviour than nocturnal activity.

By studying the evolutionary relationship between the various species, the team found that vertical and circular pupils evolved several times in different groups of snakes, and that hunting behaviour was the strongest driver for pupil shape.

Ted Maddess a biologist at the Australian National University in Canberra, who was not involved in the study, says the findings provide "good evidence" for this, reports New Scientist.

The slim, vertical pupil probably helps ambush hunters stalk prey at night by making objects at a distance from the snake's hideout appear sharper, said Brischoux.

Just like a smaller aperture on a camera lens, a smaller pupil creates a deeper field of focus.

And the downside, however, is that it lets less light in.

Vertical pupils are smaller in the horizontal plane only, meaning they offer a greater depth of field in the horizontal plane but still let enough light into the eye for night vision.

Seeing a wide horizontal field in focus may help a snake waiting to ambush prey because it stops them having to move forward to see prey in focus - and risk giving away their position.

Brischoux suggests vertical pupils might also assist with camouflage by mimicking surrounding grasses.

According to Shine, the relationship between pupil shape and hunting technique is likely to apply to other animals with vertical pupils too, such as many cats and foxes.

The study has been published in the Journal of Evolutionary Biology.