Page 182 - Demo

Chapter 8180Orang-utans are also characterised by long calls: acoustic signals that are produced by flanged males or males who are developing flanges (Delgado et al., 2008). These calls reach a loudness of approximately 100 decibels (Lameira & Wich, 2008) and carry for about 1km (Mitani, 1985). These calls are thought to play an important role in regulating social interactions (Delgado et al., 2008), given that orang-utans are semi-solitary and only sporadically interact with conspecifics (Roth et al., 2020; van Schaik, 1999). Two main functions have been suggested, which are not mutually exclusive (van Schaik & van Hooff, 1996); first, long calls might facilitate inter-individual spacing, especially between males (Delgado et al., 2008; Mitani, 1985). Second, long calls might function to attract females (Mitra Setia & van Schaik, 2007). Accordingly, Mitra Setia & van Schaik (2007) reported that females approached male long calls, and especially the calls of local dominant males. Altogether, it seems that long calls do not only play a role in intrasexual competition, but also in intersexual competition.When it comes to female vocalisations in the context of mating, remarkably few examples of proceptive vocalisations have been described in primates (Dixson, 2012). This also applies to orang-utans. Hardus et al. (2008) describe two vocalisations that females produce during copulation: mating squeals and fear squeals. However, we have found no mentions of proceptive vocalisations, i.e., vocalisations that precede mating events. Nonetheless, production of proceptive vocalisations might be beneficial for orang-utan females, through either coordinating movement with preferred males (long- or intermediate-distance calls) or communicating their intention or motivation to mate (short-distance calls) in order to reduce male ambivalence towards them (O’Connell et al., 2020). Especially in habitats with bad visibility vocalisations can aid in coordination and social interaction (Delgado, 2006) and reduce ambiguity during social approaches.Here, we report two cases of increased proceptivity in ovulating zoo-housed Bornean orang-utan females. Our first case concerns Temmy, who showed a strong bodily and vocal response to male long call playbacks. We observed this phenomenon during a playback experiment in which she at first did not participate. She was temporarily housed in the outside enclosure while we conducted our playback experiment in the indoor enclosure, and was likely in her peri-ovulatory period. We opportunistically played long call recordings on 4 consecutive days while she was in the outdoor enclosure and videorecorded her behaviour. Approximately 2 months later, we repeated this on two consecutive days when she was most likely not ovulating. Our second case concerns Eloise, Tom Roth.indd 180 08-01-2024 10:42