Measuring Hair Cortisol Concentrations to Assess the Effect of Anthropogenic Impacts on Wild Chimpanzees (Pan troglodytes)

Abstract

Non-human primates face major environmental changes due to increased human impacts all over the world. Although some species are able to survive in certain landscapes with anthropogenic impact, their long-term viability and fitness may be decreased due to chronic stress. Here we assessed long-term stress levels through cortisol analysis in chimpanzee hair obtained from sleeping nests in northwestern Uganda, in order to estimate welfare in the context of ecotourism, forest fragmentation with human-wildlife conflicts, and illegal logging with hunting activity (albeit not of primates), compared with a control without human contact or conflict. Concerning methodological issues, season [F(2,129) = 37.4, p < 0.0001, r2 = 0.18] and the age of nests [F(2,178) = 20.3, p < 0.0001, r2 = 0.11] significantly predicted hair cortisol concentrations (HCC). With regard to effects of anthropogenic impacts, our results neither showed elevation of HCC due to ecotourism, nor due to illegal logging compared to their control groups. We did, however, find significantly increased HCC in the fragment group compared to chimpanzees living in a nearby intact forest [F(1,88) = 5.0, p = 0.03, r2 = 0.20]. In conclusion, our results suggest that hair cortisol analysis is a powerful tool that can help understanding the impact of anthropogenic disturbances on chimpanzee well-being and could be applied to other great ape species.

Fig 1

Illustration of (a) waning and (b) nest age effect on hair cortisol concentration. (a) Line plot illustrating hair cortisol concentrations (HCC) along four consecutive 1-cm-segments. Data are shown for a representative subsample from 12 chimpanzee sleeping nests with different nest age classes. HCC was significantly different between segments [χ²(3) = 27.7, p < 0.0001, r² = 0.07] with HCC decreasing towards the distal end of the hair shaft. (b) Boxplots with 1.5 IQR showing HCC from 181 sleeping nests depending on the age class of the nest during hair sampling. Planned contrasts indicated that HCC was significantly higher in new vs. recent and old nests [t(2) = 6.3, p < 0.0001, r² = 0.95] whereas HCC was only borderline significant between recent and old nests [t(2) = 1.85, p = 0.07, r² = 0.63].

Fig 2. Differences in hair cortisol concentrations between seasons and chimpanzee communities with diverse anthropogenic impacts.

Residualized mean hair cortisol concentrations (accounting for nest age effect) with 95% CI in different chimpanzee groups and seasons. (a) Chimpanzees did not exhibit significantly more cortisol due to tourism or logging in comparison to the control group without human contacts. A significant effect of seasonality [F(2,129) = 37.4, p < 0.0001, r² = 0.18] was presumably unrelated to human impacts. (b) The comparison of hair cortisol concentration between chimpanzees living in an intact forest and those in a forest fragment with severe human-wildlife conflicts revealed significantly elevated HCC in the latter group [F(1,88) = 5.0, p = 0.03, r² = 0.20].