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

Abstract

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.

Social and ecological factors alter stress physiology of Virunga mountain gorillas (Gorilla beringei beringei)

“…As previously shown (Eckardt et al, ), females had significantly higher baseline FGM concentrations than males, and older animals had lower baseline FGM levels than younger animals (Table ).…”

“…Interunit interactions can cause acute elevation of FGM levels in mountain gorillas (Eckardt et al, ). Such interactions have increased sixfold in the study subpopulation of Virunga mountain gorillas between 2006 and 2011 (Caillaud et al, ).…”

“…As part of a long‐term monitoring project initiated in the study population in April 2011 (for detailed methods see Eckardt et al, ), we collected weekly fecal samples from each gorilla when the group was not exposed to obvious external stressors (e.g., interunit interactions or being caught in a snare), though some weeks were missed for some individuals due to time limits on data collection. Hereafter, these are referred to as “baseline samples.” Baseline FGM levels maintain reactive homeostasis of the HPA axis through change (Dickens & Romero, ; Romero et al, ).…”

“…We used a field‐friendly, standardized protocol for fecal hormone extraction that was previously validated for the Virunga gorilla population (Eckardt et al, ). Hormone extracts were dried and stored frozen at −20°C until shipping at ambient temperature to the Davee Center for Epidemiology and Endocrinology at the Lincoln Park Zoo (Chicago, IL, USA) for further analysis.…”

“…This will create a better understanding of their adaptive capacity to future and current social and environmental variability. We use noninvasive monitoring of FGM, which is widely recognized as an useful measure of stress (Ayres et al, ; Millspaugh & Washburn, ; Sapolsky, Romero, & Munck, ; Walker, ) and validated previously in this study population (Eckardt et al, ). In addition to establishing the current relationship between social and ecological variation and stress physiology in this gorilla population, these analyses will pave the way for additional longitudinal monitoring for potentially problematic changes in baseline stress levels in this endangered species.…”