Tourism impacts on fecal glucocorticoid metabolites of free-ranging mountain gorillas in Bwindi Impenetrable National Park, Uganda

Mountain gorilla tourism plays a vital role in their conservation; however, increasing tourism intensity may elevate gorilla stress levels. Despite this concern, knowledge regarding its physiological effects remains limited. This study non-invasively assessed the impact of tourism on gorilla stress by measuring fecal glucocorticoid metabolites (fGCMs). Fecal samples were collected from habituated gorilla groups in Bwindi Impenetrable National Park, Uganda, between 2017-2018. Concentrations of fGCMs were analyzed using a field-friendly extraction method and enzyme immunoassay. To assess the specific impacts of tourism, we focused on the Rushegura group, which is frequently visited by tourists, and used Bayesian linear mixed-effects models to examine associations between log-transformed fGCMs (log-fGCMs) and tourism intensity (daily tourist numbers and visit frequency). Additionally, we performed a inter-group comparison with the Mubare group, which was visited less frequently but underwent more dynamic social changes, to evaluate how log-fGCMs levels differed across different group contexts. In the Rushegura group, the number of tourists and visits frequently exceeded the maximum limits established by the tourism guidelines (i.e., one visit of eight people per day). In this group, log-fGCMs increased with the number of tourists and were higher during days with multiple visits. The model incorporating visit frequency demonstrated better predictive performance. On the other hand, Mubare gorillas showed higher log-fGCMs than Rushegura gorillas, likely reflecting the influence of social instability. These findings underscore the importance of adhering to the tourism guidelines--specifically regarding group size and visit frequency--to minimize physiological stress and associated risks, such as disease transmission. Although this pilot study lacks site-specific biological validation for this subspecies, the results provide preliminary insights into the physiological stress responses of Bwindi mountain gorillas to tourism. Further long-term, large-scale studies involving multiple gorilla groups are necessary to better understand how tourism and other factors (e.g., habituation status and social factors) influence their stress levels.