The importance of individual heterogeneity for interpreting faecal glucocorticoid metabolite levels in wildlife studies

Coppes, J.; Kämmerle, J.L.; Willert, M.; Kohnen, A.; Palme, R.; Braunisch, Veronika (2018). The importance of individual heterogeneity for interpreting faecal glucocorticoid metabolite levels in wildlife studies. Journal of Applied Ecology, 55(4), pp. 2043-2054. Blackwell Scientific Publications 10.1111/1365-2664.13140

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1. As a non‐invasive and inexpensive method, the use of faecal glucocorticoid metabolites (FGM) analysis in wildlife research is increasing. Various environmental factors have been shown to influence FGM levels, or faecal corticosteroid metabolites (FCM) levels in birds, but most studies do not account for inter‐individual variance, which we hypothesized may substantially affect results.

2. We combined FCM analysis with genetic analysis to identify the sex and individual's identity in samples collected in three consecutive winters; with repeated samples per individual, across the entire range of an endangered population of capercaillie Tetrao urogallus in south‐western Germany. Using generalized additive mixed models, we modelled FCM levels as a function of sex, season and environmental covariates at two spatial scales: location and home range. We compared two models: one including information on the individual animal and the other excluding this information (i.e. naïve model) to assess the influence of individual heterogeneity on the results obtained.

3. Models accounting for inter‐individual differences explained 44.0% and 45.1% (at the location and home‐range scale respectively), while only very little (4.0% and 5.1%, respectively) was explained by the environmental predictors. When ignoring individual effects, the model results changed considerably with other, previously non‐informative predictors, becoming significant.

4. In the full models, accounting for inter‐individual variance, weather conditions had no effect at either scale. FCM levels were negatively correlated with habitat quality at the sampling location, while human recreation at the home‐range scale led to elevated FCM levels. In the naïve models, two additional predictors appeared significant: one weather variable at the local scales and two at the home‐range scale. In all models, seasonal FCM patterns differed significantly between males and females.

5. Synthesis and applications. By combining faecal corticosteroid metabolites (FCM) analysis with genetic individual assessment, we demonstrate that individual heterogeneity can explain most of the variance in faecal corticosteroid metabolites levels and that ignoring this information can lead to erroneous conclusions when testing for environmental stressors. We therefore stress the importance of identifying individuals when studying faecal corticosteroid metabolites in wildlife and recommend combining faecal corticosteroid metabolites analyses with genetic analyses to adequately address this issue.

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