The adaptive utility of Richardson's ground squirrel ultrasonic alarm signals

When confronted by predators, Richardson's ground squirrels (Spermophilus richardsonii) produce either audible (ca. 8 kHz) or ultrasonic (ca. 48 kHz) alarm vocalizations that warn conspecifics of impending danger. Because atmospheric attenuation of sound is frequency-dependent, audible calls travel farther than their ultrasonic counterparts, and can therefore be detected by conspecifics and allospecifics residing at greater distances. In our first experiment, we demonstrated that squirrels exploit the short-range propagation of ultrasound by facultatively producing ultrasonic alarm calls when predators are distant and unlikely to detect them. Of course, the intended recipients of ultrasonic alarm calls share equally with their predators the challenge of detecting a rapidly attenuating signal, and can therefore benefit from those signals only when they are within the signal's limited active space. Accordingly, one context in which short-range signaling may be highly adaptive is during natal emergence, when juvenile squirrels are particularly abundant, highly vulnerable to predation, and uniquely clustered in space. In our second experiment, we broadcast ultrasonic alarm signals to emerging juvenile squirrels and found that they, like older individuals, respond to those calls by increasing vigilance. We discuss the adaptive utility of employing multiple signaling strategies.