The alarm vocalizations of Brants' whistling rat, Parotomys Brantsii

[摘要] ENGLISH ABSTRACT: The alarm calls of a colonial rodent, Brants' whistling rat (Parotomys brantsii), wereinvestigated at Goegap Nature Reserve in the Northern Cape, South Africa. At thissite P. brantsii occurs in sympatry with the congeneric P. littledalei, and the alarmcalls in response to humans was compared between the two species. Parotomysbrantsii's typical call in reaction to a human is a simple whistle, characterized by aninitial upward frequency sweep and high frequency plateau, whereas the alarm whistleof P. littledalei has three overlapping components. Parotomys littledalei's calls aresignificantly shorter and lower in dominant frequency than those of P. brantsii, andhave a significantly wider frequency bandwidth. These distinctions were attributed tospecies discrimination and habitat preferences. The acoustic adaptation hypothesis issupported in part, as P. littledalei, which inhabits a closed habitat in comparison withP. brantsii, has calls which are lower in frequency than P. brantsii calls, but contraryto the hypothesis, P. brantsii calls show less frequency modulation than those of P.littledalei. The high pitch of both species' vocalizations may be a convergent featuremaking the callers difficult for predators to locate.The alarm call repertoire of P. brantsii was investigated by presenting members of theP. brantsii colony with models of some typical predators, a raptor and puff adder, aswell as a human observer. Virtually no variation was detected in the structure of alarmwhistles elicited by different predators, indicating that P. brantsii did not identifydifferent predator types by means of vocalizations, even though behavioural responsesshowed that P. brantsii do distinguish between different predators and non-predators.Note duration varied in conjunction with the physical reaction of the caller. When the caller bolted towards safety, the whistle was significantly shorter than when itremained aboveground. During an encounter with a snake or far-off human the callerwould utter relatively long duration calls, signifying a low-risk situation, while highrisksituations such as a nearby human or raptor provoked short calls, emitted justbefore the whistling rat bolted underground. I regard P. brantsii's alarm call repertoireas a graded urgency-based system, indicating threat level perceived rather thanpredator type. This system is widespread among ground squirrels and is usuallyassociated with animals inhabiting a two-dimensional environment.Playback experiments were conducted to determine the meaning of alarm calls toconspecifics, focusing specifically on the effect of variation in single call duration andfunction of alarm-calling bouts. All alarm calls induced heightened vigilance inreceivers, but the behavioural response did not differ between short and long singlecalls. Multiple calls lead to significantly longer periods of high vigilance than singlecalls even after calling bouts have ended, showing that one of the functions ofmultiple calls is to maintain high vigilance in conspecifics for relatively long periods.The non-iconic nature of their alarm calls and the undiscriminating response ofreceivers to different calls emphasizes the importance of contextual clues in thecommunication system of animals living in an open habitat like that of P. brantsii.

[发布机构] Stellenbosch University