The adaptive utility of Richardson’s ground squirrel (Spermophilus richardsonii) short-range ultrasonic alarm signals

Abstract

travel farther, are louder, and they contain frequencies to which conspecific and allospecific recipients are more sensitive. In our first experiment, we presented an alarming stimulus to 103 squirrels to examine the effect of threat proximity on signal type. The ratio of ultrasonic to audible alarm calls increased with increasing distance from the stimulus. We conclude that the size of the active space influences signalling strategy and that squirrels emitting ultrasonic calls can signal conspecifics to the exclusion of distant predators. As recipients of ultrasonic calls must be close to the signaler, one context in which ultrasonic calling may be most adaptive is during natal emergence, when juveniles are particularly abundant, highly vulnerable to predation, and clustered in space. In our second experiment, we broadcast ultrasonic alarm signals to emerging juveniles and found that they, like older individuals, respond to calls by increasing vigilance. We discuss the adaptive utility of multiple signalling strategies in light of our findings.

Species-specific and shared features in vocal repertoires of three Eurasian ground squirrels (genus Spermophilus)

“…Hare, personal communication). Given the rare occurrence and poor propagation of the very high-frequency ultrasound, the functioning of ultrasound alarms as specific "cryptic" call to warn nearby conspecifics of potential danger (Wilson and Hare 2006), it is hardly probable for S. suslicus. A more probable function may be the enhanced structural variability of alarm calls to prevent habituation to such stimuli.…”

“…This vocalization is produced with sudden exhalation concurrent with opening of the mouth, in the absence of audible sound excluding the faint sounds of rushing air, and function probably as a short-range alarm signal (Wilson and Hare 2006). Audible alarm calls of Richardson's ground squirrel are similar in the acoustic structure to alarm calls of speckled ground squirrel S. suslicus (Davis 1984;Volodin 2005).…”

Ultrasonic Vocalizations Emitted by Flying Squirrels

“…When squirrels forage together they may opt to communicate with entirely ultrasonic signals in order to share food quality information while simultaneously decreasing the probability of being detected by avian predators. This appears to be the case for Richardson's ground squirrels ( Urocitellus richardsonii ), who have adopted a strategy of alternating between pure ultrasonic, audible, and ‘mixed’ alarm call signalling [11]–[12]. Mixed alarm calls were emitted by ground squirrels when predators were at a distance so neither the sonic nor ultrasonic components of the signal could easily be detected by the predator but would be audible to nearby conspecifics.…”

“…When predators were detected at intermediate distances, ground squirrels switch to emitting purely ultrasonic signals that would be less likely to reach the predator but would still be heard by nearby conspecifics and thereby elicit a response. When predators were detected at close range, ground squirrels switch to an alarm call in the sonic frequency range, giving the call a greater propagation distance and increasing its warning impact [11], [12].…”

“…For example, some studies have examined the role of ultrasound in alarm signalling [10]–[12], whereas others have documented how variation in ultrasonic vocalization structure can be used for additional functions [13]. A variety of studies have shown that mice, voles, and rats use ultrasound during courtship, mating, and child rearing [13]–[16].…”