Repetitive calls of juvenile Richardson's ground squirrels (Spermophilus richardsonii) communicate response urgency

Abstract

Richardson's ground squirrels, Spermophilus richardsonii, produce both repetitive and non-repetitive antipredator calls. While many hypotheses have been advanced to explain non-repetitive calls, the function of repetitive calling has received relatively little attention. We presented juvenile Richardson's ground squirrels with a predator model at distances ranging from 1 to 8 m and recorded the subsequent repetitive calls on digital audiotape. The rate of calling was inversely correlated with the distance between the model and the caller, with distance explaining almost 24% of the variation in call rate. To determine whether call recipients use that information, we manipulated the intersyllable latency of a single repetitive call exemplar to form 3 test stimuli varying only in call rate. Across 16 Richardson's ground squirrel colonies to which these calls were broadcast, the proportion of squirrels assuming the highly vigilant, alert posture increased with the rate of the repetitive call presented. Hence, juvenile Richardson's ground squirrels appear to communicate the proximity and presumably the degree of threat posed by potential predators.

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

“…Consistent with this, many signalling systems have been demonstrated not to be reflexive, but rather, sensitive to intrinsic and extrinsic factors, such as the signaler's life history (Abrams 1983), the presence or absence of a suitable audience (Karakashian et al 1988), or the location of a given predator (Warkentin et al 2001). The great tit (Parus major, Linnaeus, 1758), for example, produces an 8--kHz 'seet' alarm call when its primary predator, the European sparrowhawk (Accipiter nisus, Linnaeus, 1758), is distant, but halts signal production when that predator approaches the signal's audible range (Klump and Shalter 1984).…”

“…By integrating information about predator distance (Warkentin et al 2001) and the presence or absence of nearby familiar or related conspecifics (Hare 1998b;Hare and Atkins 2001), callers could use exact information (but see Koops and Abrahams 1998) or simple rules of thumb (Bouskila and Blumstein 1992) to select the optimal strategy for a given situation (Maynard Smith 1978;Abrams 1983). A signaler's strategy could even be adaptable to situational changes, such as shifts in predator location or changes in the abundance of nearby squirrels, if the caller continually assesses the situation (e.g.…”

“…8 kHz), often repeated for several minutes, and capable of propagating throughout an entire colony (Koeppl et al 1978;Hare 1998a). Calls convey to conspecifics valuable information (Pulliam 1973) about the identity (Hare 1998a) and reliability (Hare and Atkins 2001) of the signaler, as well as the proximity of the predator (Warkentin et al 2001;Sloan et al 2005). In addition to 'audible' calls, squirrels also produce and respond to 'ultrasonic' (fundamental frequency: ca.…”

“…pursuit--deterrent signalling: Woodland et al 1980;Shelley and Blumstein 2005) may be optimal strategies in their respective situations. As squirrels are capable of gauging the proximity of a threat (Warkentin et al 2001), it is also possible that signal type depends on the location of the predator relative to the caller. By issuing short--range ultrasonic calls when a predator is beyond the active space of ultrasonic, but not audible, alarm calls, squirrels may be able to signal nearby conspecifics to the exclusion of the presumptive predator.…”

Richardson's ground squirrels (Spermophilus richardsonii) do not communicate predator movements via changes in call rate

“…While both call types warn conspecifics of potential danger (e.g. Sherman 1977;Warkentin et al 2001), the nature and extent of information encoded within repeated alarm calls remains unknown. The tonic communication hypothesis predicts that repetition of syllables serves to prolong residual vigilance in receivers during and after a predatory threat.…”