Please note that this story is an update on a study ScienceNOW first covered last year (ScienceNOW, 15 August 2006).
The best defense against rattlesnakes appears to be a heated tail. When confronted with the vipers, California ground squirrels pump blood into their vigorously waving tails, creating a warning beacon of infrared radiation. The findings represent the first known animal communication system based on infrared light.
Over the course of millions of years, rattlesnakes and other so-called pit vipers have evolved infrared-sensitive pit organs on their heads. The organs detect the body heat of warm-blooded animals and help the snakes pinpoint squirrels and other prey. In response, squirrels have evolved better defenses: When a snake goes for their babies, for example, the squirrels scare it off by waving their tails vigorously, biting, or throwing dirt.
Knowing that squirrels can alter blood flow to their tails, behavioral biologist Aaron Rundus and colleagues at the University of California (UC), Davis, wondered whether the rodents had evolved an even more impressive defense system--one that took advantage of the snake's infrared senses. Using adult, wild-caught ground squirrels and an infrared camera, the team filmed a series of 10-minute encounters that pitted the rodents against rattlesnakes and infrared-insensitive gopher snakes. The researchers placed the animals in adjoining cages that faced each other through a wire mesh.Proceedings of the National Academy of Sciences.
To confirm that a toasty tail was key to the defense, the researchers repeated the experiments using a robotic squirrel with a heatable, waveable, tail. The reptiles spent about 20% less time in aggressive, predatory posture--switching to defensive, coiled positions--when confronted with a heated tail rather than a cold one, the team reports. That difference is small but reliable, says Richard Coss, a behavioral biologist at UC Davis who consulted with Rundus on the research. Rundus isn't sure yet why the squirrels don't turn their tails on in all snake encounters, though he suspects that there is a cost to the squirrel from losing body heat.
"The paper is top-notch," says Aaron Krochmal, a behavioral biologist at the University of Houston in Texas. Scientists have studied the snake-squirrel system for a long time, he says, but no one ever suspected this unique form of signaling. "When I read it, I said, 'I wish I'd thought of that.'"
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