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Proceptive vocalisations in orang-utan females1938However, both Temmy’s and Eloise’s vocalisations seem to differ from the lork call in several aspects. Altogether, our observations suggest that orang-utan females might have a more extensive vocal repertoire that has been assumed. While we want to be careful to ascribe a function to the vocalisations, we do think it is interesting to provide suggestions. The most likely function of the calls would be mate attraction. Especially for relatively solitary animals like orangutans, it might be essential to coordinate movement. Long calls have such a coordinative function (Delgado et al., 2008; Mitani, 1985), but it could well be that female vocalisations also contribute. Although proceptive vocalisations seem to be rare in primates (Dixson, 2012), females of at least two other relatively solitary primate species seem to produce proceptive vocalisations that can travel relatively long distances (grey mouse lemur, Microcebus murinus: Buesching et al., 1998; aye-aye, Daubentonia madagascariensis: Sterling & Richard, 1995). Given that Eloise’s vocalisation was relatively loud, the mate attraction hypothesis would be a likely candidate to explain her vocalisations. However, Temmy’s call seems to travel relatively short distance. Potentially, short-distance vocalisations like Temmy’s communicate mating motivation or induce arousal in males. Such vocalisations have also been described in other primate species (Dixson, 2012). For example, gelada (Theropithecus gelada) females produce pre-copulatory vocalisations, especially during ovulation (Roberts et al., 2017). The length of these calls is a predictor of copulation duration (Zanoli et al., 2022), suggesting that these calls are an indicator of female sexual motivation. Thus, female proceptive vocalisations have previously been described in other primate species, and are potentially present in orang-utans.It is important to note, though, that these functional accounts are also in conflict with an important aspect of orang-utan sexuality, namely concealed ovulation in females. Orang-utan females show no outward indicators of ovulation (Knott & Kahlenberg, 2011). It has been suggested that concealment of ovulation reduces infanticide risk through paternity confusion (Kunz et al., 2022). If concealing ovulation is indeed beneficial for females, ovulation-induced vocalisations could be detrimental. Therefore, it is also possible that ovulation-induced vocalisations have negative fitness consequences, which would explain why such vocalisations have not previously been described in wild orang-utans. Our cases concern two females from the zoo-housed population, and such housing is often associated with changes in selective pressures (Williams & Hoffman, 2009). Specifically, orang-utan females are often housed with only one breeding male in zoos, which Tom Roth.indd 193 08-01-2024 10:42