Reproductive strategy of bachelors in a snub-nosed monkey (Rhinopithecus bieti) all-male unit

Snub-nosed monkeys exhibit a rare multilevel social system composed of several one-male units (OMU) and at least one all-male unit (AMU). The AMU comprises males who are blocked from access to females by resident males in the OMUs, and how these satellite males achieve reproductive success is still unclear. To investigate their reproductive strategies, we focused on the AMU in a band of provisioned black-and-white snub-nosed monkeys (Rhinopithecus bieti) in Yunnan, China. Behaviors that AMU males use to gain access to females (i.e. immigration, male takeover, and sexual interaction with females) were recorded and compared with resident OMU males to explore how AMU bachelors achieve reproductive success when they are denied stable access to females. We found that in response to solicitation from females, adult and sub-adult members of the AMU responded more actively than resident males, and the bachelors actively initiated mating with females when the latter's resident male was temporarily absent. These mating opportunities mostly coincided with the peak mating season in OMUs, and probably allowed bachelors to sire some offspring. We also found that for some AMU adults, taking over an OMU is the main strategy used to gain stable access to females, and these males repeatedly migrate between bands. AMU members therefore show multiple strategies that allow them to gain some degree of reproductive success.

Seasonality of reproduction of wild black-and-white snub-nosed monkeys (Rhinopithecus bieti) at Mt. Lasha, Yunnan, China

Black-and-white snub-nosed monkeys (Rhinopithecus bieti) inhabit one of the harshest habitats by any nonhuman primate. Reliable predictive cues to initiate reproduction may be particularly critical for R. bieti because they inhabit such seasonally energetically challenging environments. To better understand the seasonal distribution of and predictive cues to reproduction, we collected breeding and birth data in a population of R. bieti at Mt. Lasha in Yunling Nature Reserve, Yunnan, China, from January 2008 to May 2010, using a combination of 10-min instantaneous scans and ad libitum observations. We examined variations in temperature, rainfall, and food availability, as well as photoperiod differences between Mt. Lasha and the more northerly Xiaochangdu to identify environmental influences on the timing of reproductive events. Our data show the area exhibited distinct seasonal fluctuations in rainfall, temperature, and food availability. Mating occurred year-round, but peaked in August, coinciding with the end of the period of highest temperatures and food availability, and during the peak rainfall. Copulation frequency peaked 1 month after corresponding peaks in staple foods, rainfall, and minimum temperatures, and 3-4 months after peaks in high-quality foods. Births were significantly seasonal, with a birth peak from mid-February to early April, and a mean birth date of 14 March. Eleven births occurred in 41 days in 2009, and 16 births occurred in 52 days in 2010. Births occurred during periods of increasing temperatures and food availability. Our findings are suggestive of at least one environmental control of conception timing, and support the notion that food availability during key reproductive stages is an ultimate factor for birth seasonality, but provide no supporting evidence for photoperiod during the conception season as a proximate cue to reproduction in R. bieti.

Birth seasonality and interbirth interval of captive Rhinopithecus bieti

This study, which is based on 10 years of birth records, shows that black-and-white snub-nosed monkeys (Rhinopithecus bieti) in captivity display marked birth seasonality. The birth season starts in December and ends in June, with a peak from March to May, and a median birth date of April 10. More male infants than female ones are born in captivity. More males were born at the Kunming Institute of Zoology (KIZ) than at the Kunming Zoo (KZ). Of 17 interbirth intervals (IBIs), 29% were from females that had lost an infant at <1 year of age or experienced stillbirth, and 71% were from females whose infant survived more than 1 year. The mean IBI for the former group (428+/-SD 87 days) was significantly shorter than that for the latter group (706+/-71 days), in agreement with reports of other Colobine species. Infant mortality was lower in captivity than in the field, which may reflect the relatively stable food availability and climate in captivity compared to the harsh conditions in the wild.