Abstract

Stress levels of individuals are documented using glucocorticoid concentrations (including cortisol) in blood, saliva, urine or faeces, which provide information about stress hormones during a short period of time (minutes to days). In mammals, use of hair cortisol analysis allows for the assessment of prolonged stress over weeks and months and provides information on chronic stress levels without bias associated with handling. Here, we validate hair cortisol analysis in wild rodents using exogenous adrenocorticotrophic hormone (ACTH challenge) and apply the technique to evaluate stress in eastern chipmunks inhabiting logged and natural sites. Chipmunks were subjected to a mark-recapture study and injected weekly with ACTH (Synacthen Depot) or saline, with hair being collected at the conclusion of the challenge. Subsequently, faecal and hair samples were collected from chipmunks occupying logged and natural sites to assess the utility of hair cortisol in comparison with faecal cortisol metabolites. Following extraction, cortisol concentrations were quantified in hair and faecal extracts by enzyme immunoassay. Hair cortisol concentrations were significantly elevated in samples from ACTH-injected chipmunks compared with saline-injected control animals (five times higher). Chipmunks inhabiting logged sites had increased faecal cortisol metabolite concentrations compared with those in natural sites, but no differences were observed in hair cortisol concentrations. Faecal cortisol metabolite levels were positively correlated with hair cortisol levels in chipmunks. Hair cortisol levels reflect changes in circulating cortisol levels and can be used to evaluate the adrenal stress response, and thus stress, in natural populations. Nonetheless, because of the differences in the temporal scale of stress that hair and faeces represent, we caution the use of hair cortisol for detecting differences in physiological stress when comparing individuals within populations and suggest that it is best suited to examining population-level differences.

Highlights

  • The mean (±SD) mass of chipmunks injected with adrenocorticotrophic hormone (ACTH) was 90.0 ± 8.80 g, whereas chipmunks injected with saline had a mean mass of 87.4 ± 12.15 g on initial capture

  • Final hair samples were collected from n = 12 eastern chipmunks (n = 5 injected with ACTH and n = 7 injected with saline)

  • Saline-injected chipmunks were injected a mean of 5.71 times (SD = 3.04) over a mean of 54.86 days (SD = 20.84), whereas ACTH-injected chipmunks were injected a mean of 5.40 times (SD = 1.82) over 64.2 days (SD = 13.77)

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Summary

Introduction

A linear mixedeffect model was run to test the null hypothesis that there is no difference between faecal cortisol metabolite concentrations of the chipmunks from logged or natural sites using the lme4 (Bates et al, 2013) in the R environment (R ­version 2.15.1; R Core Team, 2012). A linear model was run to test the null hypothesis that there is no difference in hair cortisol concentrations of the chipmunks from logged or natural sites.

Results
Conclusion
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