March 10, 2020 / Modified mar 10, 2020 1:57 p.m.

Study explains how rattlesnakes catch rainwater on their backs

For the western diamond-backed rattlesnake, the advantage might lie in a labyrinth of grooves on its scales.

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Several species of rattlesnake in the Southwest drink raindrops from their skin. But how do they keep water from running off their scales?

A new study in the journal ACS Omega could provide an answer.

Many plants and animals in dry environments sport special surface structures that gather water from rain, dew or even fog.

Arizona's western diamond-backed rattlesnake (Crotalus atrox) harvests raindrops by coiling up and flattening its back. But its real advantage might lie in a labyrinth of grooves on its scales.

Co-author Konrad Rykaczewski, an associate professor of mechanical engineering at Arizona State University, said these channels are very effective at keeping water droplets from sliding off.

"You can have a half-centimeter droplet sitting on a vertical side of the snake, and it doesn't come off. I mean, try putting a droplet that big on your water cup or anything, it just slides off," he said.

Because they used shed skins from road-killed snakes, the scientists could not test how coiling or back-flattening contribute to water-harvesting.

Researchers used high-speed cameras to capture water drops as they struck the scales of a desert kingsnake (Lampropeltis splendida), a Sonoran gopher snake (Pituophis catenifer)) and a western diamond-backed rattlesnake.

The first two snakes occur in the same geographic area as the third, but are not known to harvest rain. They provided a basis for comparison.

The desert kingsnake has fairly smooth scales on its back, while the Sonoran gopher snake, like the western diamond-backed rattlesnake, has scales with a central ridge, or keel.

As droplets strike the rattlesnake's skin, they split into smaller droplets or beads that stick. As more droplets join them, they form pools.

If the pools grow large enough, they will slide off. But, until then, the snake has a water source it can gather with minimum risk or energy expenditure.

Conversely, water striking the other two snakes immediately formed small puddles and lost water to runoff.

The keel did not appear to interact much with the water droplets, either on the gopher snake or the rattlesnake.

Conversely, the rattlesnake's ridges were ideally suited to their task.

"If the channels were just as a little bit deeper, water wouldn't be able to penetrate all the way down. Consequently, you would end up with little air pockets and the surface would become super hydrophobic," said Rykaczewski, meaning the water droplets would simply bead up and roll off.

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