Page 189 - Demo

Proceptive vocalisations in orang-utan females1878were ovulating at the same time as Eloise, she would be supplanted and pushed away by the females. Otherwise, these females would not intervene. Her ovulation lasted approximately 3-4 days, with the vocalizations being most intense during the first day or two of the cycle and slowly tapering off.The fact that her vocalisations started 2 weeks after her menstruation, and coincided with an increase in proceptivity as well, suggests that the vocalisations were induced by ovulation. This conclusion is further supported by the fact that disappearance of the vocalisations co-occurred with Eloise’s transition towards her post-reproductive period. Thus, when Eloise’s menstrual cycle became less regular, her calls became less frequent and eventually disappeared.Vocalisation analysisDuring all playbacks that we performed in the period of study 1 for which we suspect Temmy was in the peri-ovulatory period, Temmy performed vocalisations. We were able to capture three good quality recordings of the vocalisation that Temmy produced. These calls consisted of short series of 5 to 6 consecutive bark-like vocalisations (hereafter pulses; Figure 3). Each call lasted on average 4.2 seconds (SD=0.3), in the longest recording these calls were repeated consecutively for more than 3 minutes. Each bark-like pulse within the call had an average duration of 690ms long (SD=48ms), with an average fundamental frequency of 250 Hz. These parameters remained consistent across all of Temmy’s vocalisations. Only the intensity of the calls decreased sharply after 65 seconds of vocalising. Still, the softer calls remained similar in the other acoustic parameters. Interestingly, the calls that Temmy emitted in response to the playbacks seem to be different from any calls generally known to be uttered by orang-utans. To validate this, we compared Temmy’s calls to known sounds in the orang-utan repertoire: the grumble, complex call, rolling call, gorkum and lork call. These comparisons were based on 4 main features, call duration (s), peak frequency (Hz), fundamental frequency (Hz) and inter-onset-interval length (s) (Figure 3). The complex call, rolling call and gorkum all have one high frequency burst in the 3500 Hz range, something that is missing from Temmy’s vocalisations during the playbacks. However, based on the temporal structure, these calls are rather similar. Temmy’s vocalisations might be a variation on the complex call that lacks the high burst and where the emphasis is mainly on the low-frequency grunts. Based on the frequency range, there is more similarity with the grumble, a call with a quick repetition of short and low pulses. These same pulses seem to occur Tom Roth.indd 187 08-01-2024 10:42