Browsing by Author "Le Roux, Aliza"
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- ItemThe alarm vocalizations of Brants' whistling rat, Parotomys Brantsii(Stellenbosch : Stellenbosch University, 2001-12) Le Roux, Aliza; Cherry, M. I.; Jackson, T. P.; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.ENGLISH ABSTRACT: The alarm calls of a colonial rodent, Brants' whistling rat (Parotomys brantsii), were investigated at Goegap Nature Reserve in the Northern Cape, South Africa. At this site P. brantsii occurs in sympatry with the congeneric P. littledalei, and the alarm calls in response to humans was compared between the two species. Parotomys brantsii's typical call in reaction to a human is a simple whistle, characterized by an initial upward frequency sweep and high frequency plateau, whereas the alarm whistle of P. littledalei has three overlapping components. Parotomys littledalei's calls are significantly shorter and lower in dominant frequency than those of P. brantsii, and have a significantly wider frequency bandwidth. These distinctions were attributed to species discrimination and habitat preferences. The acoustic adaptation hypothesis is supported in part, as P. littledalei, which inhabits a closed habitat in comparison with P. brantsii, has calls which are lower in frequency than P. brantsii calls, but contrary to 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 feature making the callers difficult for predators to locate. The alarm call repertoire of P. brantsii was investigated by presenting members of the P. brantsii colony with models of some typical predators, a raptor and puff adder, as well as a human observer. Virtually no variation was detected in the structure of alarm whistles elicited by different predators, indicating that P. brantsii did not identify different predator types by means of vocalizations, even though behavioural responses showed 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 it remained aboveground. During an encounter with a snake or far-off human the caller would utter relatively long duration calls, signifying a low-risk situation, while highrisk situations such as a nearby human or raptor provoked short calls, emitted just before the whistling rat bolted underground. I regard P. brantsii's alarm call repertoire as a graded "urgency-based" system, indicating threat level perceived rather than predator type. This system is widespread among ground squirrels and is usually associated with animals inhabiting a two-dimensional environment. Playback experiments were conducted to determine the meaning of alarm calls to conspecifics, focusing specifically on the effect of variation in single call duration and function of alarm-calling bouts. All alarm calls induced heightened vigilance in receivers, but the behavioural response did not differ between short and long single calls. Multiple calls lead to significantly longer periods of high vigilance than single calls even after calling bouts have ended, showing that one of the functions of multiple calls is to maintain high vigilance in conspecifics for relatively long periods. The non-iconic nature of their alarm calls and the undiscriminating response of receivers to different calls emphasizes the importance of contextual clues in the communication system of animals living in an open habitat like that of P. brantsii.
- ItemCommunication in the yellow mongoose, Cynictis penicillata(Stellenbosch : University of Stellenbosch, 2007-12) Le Roux, Aliza; Cherry, M. I.; Manser, M. B.; University of Stellenbosch. Faculty of Science. Dept. of Botany and Zoology.Improved anti-predator protection has been postulated to be the primary advantage of sociality in the family Herpestidae. Therefore, the yellow mongoose, Cynictis penicillata, is considered an anomaly in the family because it may den socially with conspecifics, cooperating in the rearing of young and territory defence, but inevitably forages alone. I studied the communicative and anti-predator behaviour of a population of yellow mongooses which exhibited a lower degree of sociality than populations studied elsewhere. The yellow mongoose’s flexible social nature was evident in its vocal repertoire. Although its vocal repertoire was smaller and less context-specific than those of social mongooses, it had a large proportion (over 50%) of affiliative vocalizations, suggesting that individuals show a higher degree of cooperation than strictly solitary species. During predator encounters yellow mongooses used a simple urgency-based alarm call repertoire, indicating high and low urgency threat with two separate call types. The social environment strongly affected the alarm communication of yellow mongooses – vocal alarms were displayed almost exclusively by individuals in a group, whereas the visual alarm (a raised tail) was displayed by solitary individuals, when predators were outside the range from which they were potentially dangerous. This was a clear demonstration of the ‘audience effect’ – a phenomenon whereby animals adjust their communicative signals depending on the audience that is present. Until this study, the audience effect has only been demonstrated in obligate social species. The yellow mongoose’s social flexibility was further reflected in its territorial scent marking behaviour. In contrast to high density populations, where subordinate individuals contribute significantly to territory defence and scent marking, only the dominant male marked and defended territory borders in this low density population. Dominant males appeared to overmark the small number of cheek marks that females deposited, especially during the breeding season, which suggests that cheek marks were used in mate guarding. The yellow mongoose showed less flexibility in responses to conspecifics while foraging: the presence of group members appeared to make foragers more nervous, as individuals increased vigilance and decreased foraging success when group members were nearby. This could not be attributed to foraging competition, which happened very rarely. Yellow mongooses relied on a form of vigilance that allowed them to continue foraging while remaining alert, which contrasted with meerkats, Suricata suricatta, that had to interrupt foraging in order to be vigilant. The foraging patterns of yellow mongooses and meerkats differed markedly, and both species appeared to be inflexible in these patterns. I have proposed, therefore, that rigid vigilance patterns of vigilance are the reason why yellow mongooses forage alone, despite showing other cooperative tendencies. This study highlights that the selective forces acting on group living and group foraging are very different, and that the group-size effect – which postulates that individual vigilance declines as group size increases – may not be applicable to species adapted to solitary foraging.