Institution: University of California--Davis
Title of Talk: In the heat of battle: Infrared signaling by ground squirrels puts rattlesnakes on the defensive
California ground squirrels (Spermophilus beecheyi) are a favourite prey of rattlesnakes, but they are not always easy targets. Adults are resistant to snake venom and so can be quite bold, often driving off a snake with an aggressive display that involves swinging their tail like a windscreen wiper. What is unique about this display is that it seems to use both visual and infrared cues. Rattlesnakes are exquisitely sensitive to heat, and a squirrel will heat up its tail by as much as 2 oC during tail-flagging. To test how much the rattlers' response is influenced by the heat signal, Rundus teamed up with Davis mechanical engineer Sanjay Joshi to rig a stuffed squirrel with a motorized, heat-controlled tail.
The evolution of communicative signals involves a major bottleneck; only those signals will evolve that effectively stimulate the sensory systems of their targets. Therefore, sensory specializations of target animals are important sources of selection on signal structure. During encounters with the rattlesnakes that prey upon their young, California ground squirrels (Spermophilus beecheyi) add an infrared component to their tail-flagging signal that targets the infrared sensory specializations of these snakes. Here I report on the effects of this infrared signal on the predatory behavior of northern Pacific rattlesnakes (Crotalus oreganus). Wild-caught rattlesnakes were observed during simulated predatory encounters with a biorobotic ground squirrel model cts of this signal. Rattlesnakes increased their orientation toward the squirrel model under the infrared condition, and emitted the defensive activities of rattling, coiling, and cocking-to-strike only when infrared was present. This is the first report of an animal communicative system that exploits the infrared modality.