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Proceptive vocalisations in orang-utan females1798IntroductionPrimate females can substantially change their sexual behaviour across the menstrual cycle. According to Beach (1976), these changes in behaviour can be divided into three categories: attractivity, receptivity and proceptivity. Attractivity refers to the stimulus effect of the female on males. For example, genital swelling might increase attractivity of a female (Dixson, 2012). Proceptivity refers to female appetitive sexual behaviours, such as approaching males, vocalising, presenting, and affiliation with males. Receptivity concerns behaviours that facilitate copulation, such as employing the correct body postures (Hrdy & Whitten, 1986). In some primates, increases in attractivity, proceptivity and receptivity concord with the occurrence of ovulation. However, sexual behaviour in primates is not rigidly controlled by ovarian hormones and most female primates engage in mating throughout the menstrual cycle. Therefore, the relationship between fertility and sexual behaviour is not always straightforward in primates (Dixson, 2012). Orang-utans (genus Pongo) are among the primate species for which there is evidence consistent with the idea that female sexual behaviour changes across their 30-day menstrual cycle. While recent evidence suggests that attractivity and receptivity do not change substantially across the cycle (Durgavich et al., 2023), data from both captive and field studies suggest that orang-utan females show a change in proceptivity across their menstrual cycle. In captivity, a zoohoused Sumatran orang-utan (Pongo abelii) female showed a 4-6 day-lasting increase in proceptive behaviour, such as following, grooming, and touching the genitals of the male (Maple et al., 1979). According to the authors, these periods occurred approximately 26-30 days apart, which suggests that they coincided with ovulation. Similarly, in a restricted access study, orang-utan females were proceptive towards males specifically during their ovulation: when females had control over access to males, they chose to spend time with the male mostly during their ovulation (Nadler, 1995). Furthermore, field data suggest that Bornean orang-utan females (Pongo pygmaeus) show a qualitative change in proceptivity across their menstrual cycle (Knott et al., 2009): females seemed to make no distinction when mating with either flanged or unflanged males when they were not fertile, but specifically mated with dominant flanged males during ovulation. Altogether, these results suggest that orang-utans show qualitative and quantitative changes in proceptivity that co-occur with ovulation.Tom Roth.indd 179 08-01-2024 10:42