Mating patterns and sexual swellings in pair-living and multimale groups of wild white-handed gibbons, Hylobates lar

Abstract

White-handed gibbons usually live in monogamous pairs, but at Khao Yai National Park, Thailand groups often contain two adult males. We studied mating and sexual behaviour (i.e. proceptivity, receptivity and attractivity) of 12 females in relation to the females' fertile phases as assessed by faecal progestogen analysis. Females' mating activity, in pairs and multimale groups, exceeded the fertile phase and extended well into gestation and, in one exceptional case, into lactation. Whereas copulation frequency was skewed towards one male and peaked during the periovulatory period, no significant difference between fertile and nonfertile phases of the menstrual cycle was detected. Similarly, frequencies of female sexual behaviours, such as proceptivity and receptivity, did not differ across menstrual cycle phases and were common during pregnancy but absent during lactation. However, female attractivity in the form of sexual swellings directly affected copulation frequency, in that copulations were concentrated in the period when females were maximally swollen. Our data suggest that female sexual behaviours do not provide reliable information on the precise timing of the fertile phase to males. Because copulation frequencies were closely associated to sexual swelling stage during both ovarian cycles and pregnancy, we advocate that gibbon females display such visual signals to manipulate male mating behaviour. The results suggest that sexual swellings enable females to mate with multiple males during times when they are not fertile, perhaps to benefit from paternity confusion or to bias copulations towards preferred males when highly fertile to acquire ‘good genes’.