Ground squirrels use an infrared signal to deter rattlesnake predation

Abstract

The evolution of communicative signals involves a major hurdle; signals need to effectively stimulate the sensory systems of their targets. Therefore, sensory specializations of target animals are important sources of selection on signal structure. Here we report the discovery of an animal signal that uses a previously unknown communicative modality, infrared radiation or ''radiant heat,'' which capitalizes on the infrared sensory capabilities of the signal's target. California ground squirrels (Spermophilus beecheyi) add an infrared component to their snake-directed tail-flagging signals when confronting infrared-sensitive rattlesnakes (Crotalus oreganus), but tail flag without augmenting infrared emission when confronting infrared-insensitive gopher snakes (Pituophis melanoleucus). Experimental playbacks with a biorobotic squirrel model reveal this signal's communicative function. When the infrared component was added to the tail flagging display of the robotic models, rattlesnakes exhibited a greater shift from predatory to defensive behavior than during control trials in which tail flagging included no infrared component. These findings provide exceptionally strong support for the hypothesis that the sensory systems of signal targets should, in general, channel the evolution of signal structure. Furthermore, the discovery of previously undescribed signaling modalities such as infrared radiation should encourage us to overcome our own human-centered sensory biases and more fully examine the form and diversity of signals in the repertoires of many animal species.animal communication ͉ signal evolution ͉ multimodal communication

Ground squirrel tail-flag displays alter both predatory strike and ambush site selection behaviours of rattlesnakes

“…Ground squirrels increase the temperature of their tail when tail-flagging towards infrared-sensitive rattlesnakes [19]. However, we found that tail-flagging did not influence the accuracy of snake strikes, suggesting that sensory confusion is an unlikely mechanism by which tail-flagging deters snake strikes.…”

“…When encountering rattlesnakes, both pup (recently weaned) and adult ground squirrels approach, often within striking distance, and invariably tail-flag (wave tail side-to-side) [16 -18]. Ground squirrels increase the temperature of their tail when tail-flagging towards infrared-sensitive rattlesnakes, but not when tail-flagging towards non-infrared-sensitive gopher snakes [19]. This suggests that tail-flagging serves a specific function for communicating with snake predators.…”

The alarm call system of two species of black-and-white colobus monkeys (Colobus polykomos and Colobus guereza).

“…For example, it has been shown that predator-specific alarm calls can also benefit the signaler directly (Cheney & Seyfarth, 1981). Predators often differ with regard to sensory and psychological specializations, which is likely to act as a direct selection factor in the evolution of form and patterning of alarm signals (e.g., Owings & Morton, 1997;Rundus, Owings, Joshi, Chinn, & Giannini, 2007).…”

Predator-deterring alarm call sequences in Guereza colobus monkeys are meaningful to conspecifics

“…In this view, the form and patterning of alarm signals are determined by the sensory bias of the predator, regardless of conspecific receivers (e.g. Marler 1965Marler , 1967Rundus et al 2007). Under this hypothesis, signals are arbitrary for conspecifics in terms of their morphology, although receivers can form associations with specific contexts, that is, they can become meaningful to them.…”