Infrared video showed that California ground squirrels' tails warmed by several degrees, up to 28 degrees Celsius (82 degrees Fahrenheit), when threatened by northern Pacific rattlesnakes, which detect the infrared glow from small mammals using so-called pit organs in their noses. But no heating occurred while the rodents defended against gopher snakes, which lack such heat seekers, according to a report in Proceedings of the National Academy of Sciences USA.
The authors found that a stuffed squirrel also elicited defensive reactions from rattlesnakes, including coiling and rattling, if the dead critter's tail was artificially heated and wagged—but not if waved back and forth unheated.
The result marks the first discovery of one animal communicating to another by infrared, says behavioral biologist Aaron Rundus of the University of Nebraska–Lincoln School of Biological Sciences.
Seemingly skittish, adult squirrels are fearsome defenders. Aided by a natural blood protein that counteracts snake venom, they may approach within inches of a threatening snake, throwing dirt with their front paws and even biting or swiping at the snake's tail, which can leave a nasty infection. "Sometimes you feel a little bad for these rattlesnakes," says Rundus, who conducted the experiments while working toward his PhD at the University of California, Davis, with adviser Donald Owings.
During these encounters, which may last from minutes to an hour, the squirrel frequently bushes and wags its tail. Researchers have noticed that squirrels wag or "flag" their tails more frequently when snakes approach in the dark than during daytime, suggesting that flagging is not simply a warning to neighboring squirrels.
Flagging alone probably reminds snakes of past squirrel lashings, Rundus says. "It's a conspicuous signal that you've been detected and you're probably going to be harassed," he notes, "and potentially harmed if you stay in this area." The researchers add that a heated tail may make the squirrel appear larger and more intimidating, and would be more visible at night than during the heat of the day.
The source of the heating is unclear, but Rundus says the squirrel probably shunts warmer blood from the body to the tail, as it does to maintain its body temperature of 31 degrees Celsius (88 degrees Fahrenheit).
Squirrels and snakes have coexisted for more than a million years in California, matching one another step for step in their evolutionary struggle for survival, he adds. When snakes developed venomous bites, squirrels evolved antivenom. The researchers say squirrels may distinguish infrared-sensitive rattlesnakes from gopher snakes and other predators based on rattlers' distinct odor and sound.
Sadly for squirrel lovers (but comforting to viper devotees), Rundus notes that these weapons are not perfectly effective: Earlier studies have found that a rattlesnake's diet is 70 percent squirrel pup.