Female multiple copulations among wild Sichuan snub-nosed monkeys (Rhinopithecus roxellana) in Qinling, China

Abstract

Mating strategies are the key to understanding both reproduction and evolution. The majority of the previous work on the mating strategies of the Sichuan snub-nosed monkey (Rhinopithecus roxellana), a polygynous species, has concentrated on the character of sexual behavior between heterosexual individuals, mating patterns, and the seasonality of mating and births in captivity 7] and in wild. The importance of multiple copulations of them, both wild and captivity, has so far been neglected. In this study we present some preliminary data on female multiple copulations among wild Sichuan snubnosed monkeys in the Qinling Mountains, attempting to explain the significance of this phenomenon. This study was carried out in the Yuhuangmiao region, located in Zhouzhi Nature Reserve on the northern slope of the Qinling Mountains, intermittently, from March 8, 2003 to May 22, 2004. Monkey groups were provisioned using apples, radishes, and corns at a stable feeding site three times per day (10:00 AM, 12:00 PM and 2:00 PM) during the observation. This manipulation was to coax the animals from a distance of 0.5 50 m. One male unit (OMU), such as the BaZiTou unit and LuoPan unit, as well as individual animals within each unit could be distinguished . We collected the sexual behavior data with the focal animal sampling and behavior sampling. During the observation period, there were 38 females and 6 males in our group. For each male unit, the total number of adult females and the sub-adult female fluctuates from 4 to 10. Only one male was traced as long as possible every day from dawn till dusk. Besides the focal male, we noted other males’ copulations as more as possible. The mating season was between late September and December in accordance with previous authors. A total of 733 hours of observations were conducted from March 2003 to May 2004. During this period a total of 399 copulations (Table 1), including 22 multiple copulations (Table 2), were observed. Table 2 shows that: (i) female multiple copulations occurred throughout the year but were more frequent during the mating season than the non-mating season; (ii) the number of female multiple copulations per hour ranged from two to five; (iii) the mean duration of female multiple copulations per hour were greater in the mating season (19.67±1.29 s, n=17) than in the non-mating season (9.8±2.46 s, n=5); the difference in the mean duration of female multiple copulations between mating season and non-mating season was very significant (t=3.62, p=0.0008<0.01, t-test); and (iv) during the mating season both adult and sub-adult females were the main initiators to display the typical sexual behavior of multiple copulations, whereas during the non-mating season males become the primary initiators. Sperm competition is less important since usually only a single adult male is present in the polygynous mating system. In these cases, sperm might be a “limiting resource” for females within the one-male group, and female-female competition for conceptions might occur consequentially. As for the Sichuan snub-nosed monkey (Rhinopithecus roxellana), we suggest that multiple copulations initiated by females support the Small’s viewpoint that females might compete for the limited sperm reserves available, while behaviours of male Sichuan snub-nosed monkeys show their attitude about sperm allocations to some extent. In the present study, all adult females involved in multiple copulations during the mating season of 2003 gave birth between March and April of the next year 2004 while those not involved in multiple copulations did not gave birth. This suggests that by increasing the incidence of multiple copulations in the mating seasons female monkeys aim to increase the possibility of impregnations. These results are consistent with those of Andreas. Andreas pointed out that females benefited from multiple copulations via the assurance of fertility.