Hair cortisol and stable carbon and nitrogen isotope ratios in barbary macaques (Macaca sylvanus) from Gibraltar

Prolonged exposure to the stress hormone cortisol can precipitate a catabolic state in animals resulting in a negative nitrogen balance and increased excreted nitrogen, possibly affecting stable nitrogen and carbon isotope ratios in animal tissues. Here we test the hypothesis that stress, as measured by hair cortisol, is a significant contributor to variation in δ¹⁵ N and δ¹³ C values in hair. The results of the present study have important implications for the use of stable carbon and nitrogen isotope ratios measured in animal and human tissues to make inferences regarding feeding ecology and diet.

Samples containing roughly 10-20 hairs (approx. 4-6 cm long) were collected from 49 free ranging macaque monkeys (Macaca sylvanus) in Gibraltar. Hair samples were analyzed for stable carbon and nitrogen isotopes by continuous-flow isotope-ratio mass spectrometry. Cortisol extracted from matched hair samples was measured using gas chromatography/mass spectrometry. Cortisol and stable isotope ratios were compared statistically using nonparametric Kruskal-Wallis and Mann-Whitney U tests, as well as Spearman correlation.

Correlation analysis revealed that although hair δ¹⁵ N and δ¹³ C values were significantly correlated with each other, neither was correlated with cortisol concentrations. Cortisol concentrations were not correlated with age-adjusted body mass index. Age category was correlated with δ¹³ C values but not with δ¹⁵ N values.

The results of our study suggest that the cumulative stress experienced by these macaques under typical social and environmental conditions is not a significant contributor to variation in δ¹⁵ N and δ¹³ C values, nor does it have a significant effect on relative body mass.