Ground squirrel’s secret weapon is a warm tail

Confronted by a hungry rattlesnake, a California ground squirrel chucks pebbles and dirt at its enemy and menacingly waves its tail.

Then it really turns on the heat.

Using infrared cameras, scientists at UC Davis have found that ground squirrels warmed their tails as much as 12 degrees to silently warn rattlesnakes, which can detect the tiniest of temperature changes.

The report Tuesday in the Proceedings of the National Academy of Sciences provides an unusual demonstration of how animal behavior can evolve in response to specific threats.

Rattlers and squirrels are locked in an epic struggle for which squirrels are surprisingly well equipped. They are immune to rattlesnake venom and aren’t afraid to use their sharp teeth.

Squirrels deploy their defenses to keep rattlesnakes from noshing on squirrel pups, a favorite rattlesnake meal. Scientists examining the contents of rattlesnake stomachs have found that in the spring and summer -- the months when pups are born -- infant squirrels account for about two-thirds of the rattlers’ diet. The snakes don’t try to eat adult squirrels.

Researchers long ago noticed that squirrels used their tails to wave off rattlers, even at night when the effort seemed useless. But snakes’ heat sensors don’t require sunlight.

On a hunch, scientists staged a confrontation between a snake and a squirrel, separating the adversaries with a wire mesh while recording the action on infrared video. The squirrel’s tail shot to 82 degrees, which made the animal’s infrared image look bigger.

To study the snake’s reactions, researchers created a robot from a taxidermy squirrel. As the robosquirrel’s tail grew warmer, the snake’s body posture shifted from a slithering offensive mode to a coiled defensive position.

Lead author Aaron S. Rundus, who did the study to earn his doctorate and is now a researcher at the University of Nebraska at Lincoln, surmised that squirrels heat their tails by shunting warmed blood into them. The reaction is probably triggered by rattlesnake smells or sounds, he said.

denise.gellene@latimes.com