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Chapter 8192account for ovulatory status. Therefore, it remains to be established whether wild orang-utan females indeed show a stronger approach tendency during ovulation, like Temmy did. Nevertheless, Knott et al. (2009) suggest that ovulating females show an increase in encounters with in flanged males, resulting in more copulating events with them. Thus, male long calls might be an important stimulus that guides ovulating females towards preferred flanged males, and future studies can further elucidate the mate attraction function of long calls by comparing female responses to male long calls at different points during their menstrual cycle. Eloise’s case supports the notion that ovulating orang-utan females have an important role in facilitating mating. In the period that we described, Eloise showed proceptive behaviour in the form of following the male and actively soliciting him. This is in line with results from restricted-access tests (Maple et al., 1979; Nadler, 1995): when orang-utan males have free access to females, mating events are quite evenly spread across the menstrual cycle. However, when females can control access, they specifically approach males during ovulation. This is also supported by the fact that wild orang-utan females selectively mate with flanged males during their peri-ovulatory period (Knott et al., 2009). Thus, it might be the case that matings are mostly characterised by receptivity outside the peri-ovulatory period (Durgavich et al., 2023), while they are characterised by increased proceptivity followed by receptivity during the peri-ovulatory period. These proceptive behaviours could include actively approaching long calls, following males, and soliciting. In addition, ovulation-induced vocalisations could be considered proceptive behaviours.Up until now, relatively few vocalisations have been described that are unique to orang-utan females (Hardus et al., 2008). Two of these unique vocalisations occur in the context of mating, namely the mating squeal and the fear squeal. However, to our knowledge, there are no descriptions of female ovulation calls. In this report, we described two seemingly new vocalisations that could possibly be considered ovulation calls. Both Temmy and Eloise produced vocalisations that showed resemblances in spectral and temporal pattern to the male long call. However, Temmy’s vocalisations seemed to travel a relatively short distance, while Eloise’s vocalisations seem much louder, and could be considered middleor even long-distance calls (sensu Hardus et al., 2008). As far as we know, only one other female long-distance vocalisation has been described for orang-utans, namely the lork call (Hardus et al., 2008). According to Rijksen (1978) this call might function as an advertisement call, or as an expression of agonistic state. Tom Roth.indd 192 08-01-2024 10:42