Daily defecation outputs of mountain gorillas (Gorilla beringei beringei) in the Volcanoes National Park, Rwanda.

Increasing population density can increase infectious disease risk and thus reduce population growth and size. Host-parasite interactions of threatened animals that remain in small protected forest fragments therefore need to be monitored carefully. Due to extreme conservation efforts, the mountain gorilla (Gorilla beringei beringei) population in the 450-km Virunga Massif has more than doubled since 1973, reaching 604 individuals in 2016. To better understand changes in the transmission risks of soil-borne and other enteric pathogens for mountain gorillas, we determined defecation outputs of different age-sex classes and the diurnal variation in feces deposition. We weighed 399 wet fecal samples deposited at nest sites and on trails between nest sites by gorillas of varying age and sex, determined by lobe diameter, from five social groups (n = 58 gorillas) that range in the Volcanoes National Park, Rwanda. We found increasing daily average defecation outputs with increasing age-sex class (infants, 435 g; juveniles, 1346 g; medium-sized gorillas, 2446 g; silverbacks, 3609 g). Gorillas deposited two- to threefold the amount of feces at nest sites compared to on trails, suggesting that nest sites may function as hotspots for enteric pathogen infections through direct contact or when gorillas ingest foods contaminated with infectious larvae during site revisits in intervals matching the maturation period of environmentally transmitted gastrointestinal parasites. In combination with ranging and demographic data, these findings will enable the modeling of spatiotemporal variation in soil contamination and infectious disease risk for Virunga gorillas as their population density continues to increase.