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Loud calls in Male Sulawesi Crested Black Macaques (Macaca nigra): Differences in Acoustic Structure and Response of Group Mates
Christof Neumann, Diploma student, University of Leipzig/German Primate Center, Germany - 2007 completed
 

The Sulawesi crested black macaque (Primates; Macaca nigra ) is unique among all macaque species that have been studied so far since only in this species loud calls (long-distance calls) are uttered by all adult males within a group. M. nigra is therefore particularly interesting to study the function of this trait. In order to examine these calls with regard to a potential function as a sexually selected signal, this diploma thesis intents to study loud calls of free ranging M. nigra by means of behavioral observation and acoustic analysis. First, it was investigated whether loud calls given in different contexts differ in acoustic structure and thus need to be studied separately. In a second step, it was assessed whether structural differences between calls from individual males were distinct as would be expected for sexually selected signals. Finally, I investigated whether adult males and females were able to discriminate between variants in loud call features that are related to individuality.

I collected data of two groups of wild M. nigra during seven months in the Tangkoko Nature Reserve, Sulawesi, Indonesia. I analyzed 503 loud calls given by 24 adult males in six contexts (feeding, resting, locomotion, mating, aggression, social positive) and extracted 42 acoustic parameters. Discriminant function analyses (DFA) was conducted to assess whether loud calls can be correctly assigned to its context, rank group or emitter based on its acoustic characteristics. Furthermore, playback experiments have been carried out to study whether males could discriminate between calls of males that were ranking higher or lower than the target male. This included experiments with manipulated loud calls as the attempt to artificially alter the caller’s rank. Likewise, females were tested regarding their ability to discriminate between calls of high- and low-ranking males. In addition, it was studied whether receptive females respond stronger to loud calls than non-receptive females. Experiments were conducted with both sexes in order to address the question whether loud calls function in inter- or intra-sexual selection.

DFA yielded a correct assignment of calls to their context of 18.1% at a chance level of 16.7%. With a modified sample in which the number of contexts was reduced, the correct classification result was 44.6% (chance level: 33.3%). Calls of different individuals were correctly assigned in 71.4% of the cases (chance level: 3.8% – 5.5%). Correct classification of the caller’s rank group (high, intermediate, low) was 83.3% (chance level: 33.3%).

Playback experiments revealed that males tended to react stronger to calls of males that were lower ranking than themselves. This pattern was also observed if calls were ‘artificially altered in rank’. None of these playback experiments resulted in a difference between responses that was statistically significant. Likewise, receptive females did not discriminate between calls of high and low-ranking males. Receptive females, however, reacted stronger to loud calls than nonreceptive females (one tailed sign test: exact p = 0.055, N = 10).

The results suggest that in M. nigra only one type of loud call exists with no pronounced variation between the examined contexts. In contrast, calls show significant individuality, which is the basis for a sexual signal. Support for loud calls as a sexually selected signal comes from the result that receptive females show greater interest in loud calls than non-receptive females. The results also suggest that since dominance rank is encoded in loud call structure, rank information is perceived by male listeners and seems to be important for their response. However, it remains unclear whether discrimination is based on direct assessment of call features or whether it is mediated through the assessment of a calling male’s identity. This question must be addressed in future research.