Socioecological correlates of energy balance using urinary C-peptide measurements in wild female mountain gorillas

• Quantifying energy balance in wild animals is fraught with difficulties.
• Urinary C-peptides are a valuable biomarker of energy balance in primates.
• We measured C-peptide levels in female mountain gorillas in Rwanda.
• C-peptide levels were significantly influenced by habitat, season and day time.
• Ecological variability can affect energy balance even in herbivorous species.

Maintaining a balanced energy budget is important for survival and reproduction, but measuring energy balance in wild animals has been fraught with difficulties. Female mountain gorillas are interesting subjects to examine environmental correlates of energy balance because their diet is primarily herbaceous vegetation, their food supply shows little seasonal variation and is abundant, yet they live in cooler, high-altitude habitats that may bring about energetic challenges. Social and reproductive parameters may also influence energy balance. Urinary C-peptide (UCP) has emerged as a valuable non-invasive biomarker of energy balance in primates. Here we use this method to investigate factors influencing energy balance in mountain gorillas of the Virunga Volcanoes, Rwanda. We examined a range of socioecological variables on energy balance in adult females in three groups monitored by the Karisoke Research Center over nine months. Three variables had significant effects on UCP levels: habitat (highest levels in the bamboo zone), season (highest levels in November during peak of the bamboo shoot availability) and day time (gradually increasing from early morning to early afternoon). There was no significant effect of reproductive state and dominance rank. Our study indicates that even in species that inhabit an area with a seemingly steady food supply, ecological variability can have pronounced effects on female energy balance.