I investigated patterns of mating and reproduction in wild female Phayre’s leaf monkeys (Trachypithecus phayrei crepusculus) at Phu Khieo Wildlife Sanctuary (Thailand). This study had three major goals: (1) First, I validated the use of fecal hormone metabolites (estrogens and progestins) to assess reproduction and provided the first estimates of reproductive parameters such as gestation and menstrual cycle length (including the separate follicular and luteal phases). (2) I then examined the effect of phytosteroids and nutrition on female reproduction. (3) Finally, I examined patterns of sexual behavior and mate choice during fertile (around ovulation) and non-fertile receptive periods (during cycling and gestation).
In relation to my first goal, I found that patterns of excreted hormone metabolites could indeed be used to assess reproductive function in Phayre’s leaf monkeys. Both fE and fP levels showed expected patterns related to cycling and conception. However, patterns of fP could not be used to adequately assess the end of the luteal phase. Furthermore, a seasonal increase in fP levels obscured biological patterns based on this hormone for part of the study. Using fE patterns instead to estimate reproductive parameters, I found that gestation was 205 days (N = 7). Because previous studies have suggested that seasonally elevated progestins might affect ovarian function, I restricted the analysis of menstrual cycle parameters to the time preceding the seasonal fP elevation, and found a cycle length of 28 days (N = 10), with a 15-day follicular phase (N = 10) and a 12-day luteal phase (N = 12). Estimates of gestation and menstrual cycle parameters were generally consistent with previous studies on colobines. However, the 205-day gestation length adds to a growing body of data suggesting that colobines and cercopithecines differ in their allometric scaling of body mass to gestation length (Borries et al., unpublished data).
In relation to the effect of phytosteroids on reproductive function, I found that periods of elevated progestins coincided with the availability of leaves and fruits from the genus Vitex, and these periods were not only associated with longer cycle lengths and follicular phases, but also longer inter-mating intervals (intervals preceding receptive periods) and a greater probability of conception. These results were consistent with previous studies suggesting that elevated progestins and Vitex consumption might impair ovarian function and suppress sexual behavior (e.g., Higham et al., 2007), but increase the probability of conception should ovulation occur (Westphal et al., 2004). However, results also show that nutritional status cannot be ruled out as an additional factor explaining reproductive patterns, at least related to conception. Specifically, when ovulations occurred, conceptions were also more likely during months when physical condition was increasing, and these months overlapped the period of elevated progestins. Because food availability in general might affect reproduction via both avenues—nutrition and phytosteroid consumption—these results suggest that the effects of the two may be difficult to disentangle (Knott, 2001).
Finally, I investigated patterns of mating and mate preference in relation to fertile (periovulatory) and non-fertile periods (cycling and post-conceptive), focusing specifically on the phase of the study when two males (one adult and one subadult) were sexually active. I found that consistent with studies on other primates, females were receptive throughout cycling and even during gestation. Receptive periods were around four days long (N = 59), with an average inter-mating interval of 17 days (N = 46). Neither the length of the receptive period nor the inter-mating interval differed when compared across fertile receptive periods, non-fertile cycling receptive periods, and post-conceptive receptive periods. However, receptive periods overlapping the periovulatory phase were distinguished from non-fertile receptive periods by higher rates of proceptive and receptive behaviors. Moreover, females were more proceptive towards the adult male during fertile periods, but towards the subadult male during non-fertile periods. These results were consistent with the hypothesis that females balanced a strategy of paternity confusion, coupled with conveying some accuracy on the timing of ovulation. A high lactation to gestation ratio, coupled with the absence of infanticide suggests that non-fertile receptivity might function successfully to confuse paternity and decrease the occurrence of infanticide in this population. (Abstract shortened by UMI.)