Squirrels can heat their bushy tails to warn off infrared-sensitive snakes hungry for their pups. Infrared video showed that the tails of adult California ground squirrels, which are fearsome defenders able to resist snake venom, warmed by several degrees when threatened by rattlesnakes, which detect infrared via so-called pit organs in their noses. But no heating occurred while the rodents confronted gopher snakes, which lack such heat seekers. A stuffed squirrel also elicited defensive reactions from rattlesnakes if the mock critter's tail was artificially heated and wagged. The results, published online August 17 by the Proceedings of the National Academy of Sciences USA, mark the first discovery of one animal communicating with another by infrared. Still, the defense is not perfectly effective: earlier studies have found that a rattlesnake's diet is 70 percent squirrel pup.