Validating faecal glucocorticoid metabolite analysis in the Virunga mountain gorilla using a natural biological stressor

The continued degradation of primate habitat worldwide is forcing many primate populations into small protected forest islands surrounded by high-density human populations. One well-studied example is the critically endangered mountain gorilla (Gorilla beringei beringei). Decades of monitoring and research on Rwanda's mountain gorillas offer a unique opportunity to use non-invasive endocrine analysis to address pressing questions about the conservation of this endangered population. The aims of our study were as follows: (i) to validate field and laboratory methods for assessing stress through faecal glucocorticoid metabolite (FGM) analysis using inter-social unit interactions as a natural stressor; (ii) to determine the excretion lag times between interactions and detectable stress response in faeces; and (iii) to determine whether there are circadian patterns of FGM excretion. We collected ~6000 faecal samples from 127 known gorillas in 10 habituated groups, monitored by the Dian Fossey Gorilla Fund's Karisoke Research Center over 21 months in 2011 and 2012. Extracted FGMs were measured using a cortisol enzyme immunoassay (R4866; C. J. Munro). Results revealed cause-effect relationships between inter-unit interactions and increased FGMs (relative to individual pre-event samples) between 20 and 140 h after interactions, with the peak most often occurring on day 3. There was no evidence of circadian patterns in FGM concentrations, as previously shown in many species with long gut passage times. However, baseline FGM concentrations were lower in adult males than in adult females, and variation was associated with the collection month, indicating possible seasonal variation. This study provides a biologically validated, field-friendly faecal hormone metabolite extraction and laboratory enzyme immunoassay analysis method for non-invasive monitoring of adrenocortical activity in Virunga mountain gorillas. The methods are useful for future evaluation of a variety of environmental and human-induced potential stressors in this critically endangered population.