Balancing Reproduction and Survival: Seasonal Body Mass Dynamics in a High-Altitude Primate (Rhinopithecus bieti)

Seasonal body mass fluctuations in mammals reflect fundamental trade-offs between ecological constraints and reproductive effort, yet few studies have simultaneously linked these dynamic changes with activity budgets in high-altitude primates inhabiting extreme-temperature environments. We examined the effects of environmental stress and mating effort on body mass dynamics in black-and-white snub-nosed monkeys (Rhinopithecus bieti). Using monthly non-invasive monitoring over a full annual cycle, we obtained 464 body mass records from free-ranging adults (127 male records, 337 female records). Both sexes showed marked seasonal variation: body mass peaked in spring and autumn, declined moderately in winter (males: −7.4%, females: −2.9%), and dropped more sharply during the mating season (males: −11.1%, females: −12.9%). Male mass loss was positively correlated with the number of mates. Seasonal shifts in activity budgets accompanied these patterns: in summer, males increased movement and females increased both movement and social interaction, while both sexes reduced resting; in winter, both sexes increased feeding time, with females additionally reducing social activities. These findings reveal two distinct phases of body mass loss. Body mass decline was more pronounced during the mating season than in winter, a pattern that may be associated with higher energetic costs related to reproductive activity. Our study demonstrates how high-altitude primates balance survival and reproduction through flexible energy allocation, offering key insights into their seasonal energy allocation resilience in provisioned high-altitude habitats.