Ground squirrels use an infrared signal to deter rattlesnake predation

Rundus and colleagues (2007) report a previously undescribed modality of interspecific communication: infrared radiation. California ground squirrels use a tail-flagging display to confront snakes, but when interacting with northern Pacific rattlesnakes (which are sensitive to infrared) the squirrels include an infrared component that is absent from displays to snakes that lack infrared sensitivity. Using a biorobitic squirrel, the authors demonstrated that the signal is received by rattlesnakes, which changed their behavior from predation to defense in response.

Airborne acoustic signals from the squirrels are believed to be undetectable by the snakes, and squirrels use visual signals when interacting with snakes. Tail-flagging seems to serve a deterrent function (like stotting in gazelles), placing snakes on the defensive. The authors suggest that the infrared component of the tail-flagging display increases the conspicuousness and efficacy of the signal to rattlesnakes. Because infrared radiation is released and captured by biological tissue as heat, the infrared signal of ground squirrels may be produced by increasing blood flow from the body core to the tail. Squirrels increased their emission of infrared radiation from the tail by statistically significant levels during trials with rattlesnakes but not with infrared-insensitive gopher snakes, while overall activity levels and number of tail-flagging bouts did not vary between trials, nor did temperature differences appear in other body regions. These results suggest precise control over the release of infrared radiation by the ground squirrels.