Infrared Communication by Squirrels

August 17, 2007

California ground squirrels (Spermophilus beecheyi) have evolved a special way to defend their young against rattlesnakes: they heat their tails, deterring the snakes, researchers find. No other species is known to communicate with infrared radiation, the authors write.

Squirrels wave their tails at snakes—a behavior called tail flagging—in order to appear bigger and signal aggression. It has long been a paradox that squirrels will wave their tails at rattlesnakes even more vigorously in the dark than in daylight. Rattlesnakes can sense infrared radiation, and Aaron Rundus and colleagues suspected that signaling invisible to the human eye was taking place.

Using an an infrared imaging video camera, the researchers filmed squirrel-rattlesnake and squirrel-gopher snake encounters in the laboratory. The squirrels turned on the heat in tail-waving displays of aggression when confronted by Northern Pacific Rattlesnakes (Crotalus oreganus oreganus). Faced with gopher snakes (Pituophis melanoleucus), which cannot detect infrared, the squirrels waved their tails, but kept them cool. The researchers also confronted rattlesnakes with a stuffed robotic squirrel model fitted with a heatable tail. The rattlesnakes were wary of the waving robotic tail, but especially so when it was heated to the same temperature as the tail of a live squirrel engaging a rattlesnake. According to the authors, the squirrels appear to have evolved the ability to heat their tails at will especially to communicate with rattlesnakes.

[Chipmunks (Tamias striatus) and Gray squirrels (Sciurus carolinensis) have been observed by other researchers tail flagging at timber rattlesnakes (Crotalus horridus), causing the snakes to abandon their attacks, but these studies did not include infrared measurements.]

Abstract adapted from a PNAS press release, © 2007, PNAS

“Ground squirrels use an infrared signal to deter rattlesnake predation,” Aaron S. Rundus, Donald H. Owings, Sanjay S. Joshi, Erin Chinn, Nicolas Giannini, August 17, 2007. PNAS Paper, available free, includes infrared video

Aaron Rundus, Department of Psychology, University of California, Davis