Food availability influences birth seasonality at a small spatial scale in endangered golden monkeys (Cercopithecus mitis kandti)

Objectives: Studying reproductive seasonality helps us understand changes in the energetically demanding periods of pregnancy and lactation. We investigated how diet variability and key food plant phenology relate to mating and birth seasonality in both remaining populations of endangered golden monkeys in the Virunga massif and the Gishwati forest in Rwanda. Material and Methods: Using scan and ad libitum sampling, we recorded feeding, mating, and births in two social groups (K and M) living in Volcanoes National Park (VNP), in the Virunga massif, from 2004 to 2018, and in one group (G) in the Gishwati forest in 2017-2018. We also monitored bamboo shoot and fruit availability in the groups' home ranges in 2017-2018. Results: Mating was observed year-round but peaked four to 6 months prior to each group's respective birth season. Despite the two VNP groups ranging only 16 km apart, they had different birth seasons. Females from group K gave birth during the late bamboo growing season, from September to December, while females from group M gave birth during the early bamboo shooting season, from February to April. This pattern was linked to differences in the availability and consumption of bamboo shoots between low-elevation (group K) and high-elevation (group M) habitat. In group G, births occurred from March to April, coinciding with the period of high fruit availability and consumption in the Gishwati forest. Discussion: Our findings suggest that food availability shapes golden monkey birth seasons, even at a small spatial scale. Current changes in key food plant regeneration, potentially driven by climate change, need to be closely monitored to inform golden monkey conservation management.