Cortisol concentrations in hair are used increasingly as a biomarker of long-term stress in free-ranging wildlife. Cortisol is believed to be integrated into hair primarily during its active growth phase, typically occurring over weeks to months or longer periods, depending on latitude. Cortisol concentrations in hair thus reflect the activity of the hypothalamic-pituitary-adrenal axis over this time. However, local, independent cortisol secretion within the skin, which includes hair follicles, may also contribute to cortisol levels in growing hair. Methodological differences between studies include the measurement of cortisol in only the hair shaft (i.e. follicle absent, as with shaved hair) versus the whole hair (i.e. follicle present, as with plucked hair). If the concentration of cortisol in the follicle is high enough to influence the overall hair cortisol concentration (HCC), this could confound comparisons between studies using different types of hair samples (hair shafts vs. whole hair) and collection methods. Here, we test the hypothesis that cortisol present in follicles influences HCC. We compared HCC in paired subsamples of hair with and without follicles from 30 free-ranging Scandinavian brown bears (Ursus arctos) and observed significantly greater HCC in samples with follicles present. The effect of follicles remained significant also with sex and age of sampled bears taken into account in a linear mixed model. Finally, we provide an overview of collection methods and types of hair samples used for HCC analysis in 77 studies dealing with stress in wild mammal species. Our findings highlight the need to unify methods of hair collection and preparation to allow for valid comparisons, and to optimize labour input in ecophysiological studies.

中文翻译：

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卵泡重要吗？测试毛囊对头发皮质醇水平的影响。

散发野生动植物中，头发中皮质醇的浓度越来越多地用作长期压力的生物标记。据信皮质醇主要在其活跃的生长阶段整合到头发中，这通常发生在数周至数月或更长时间，具体取决于纬度。因此，头发中的皮质醇浓度反映了这段时间内下丘脑-垂体-肾上腺轴的活动。但是，包括毛囊在内的皮肤内局部，独立的皮质醇分泌也可能会助长头发中的皮质醇水平。两次研究之间的方法学差异包括仅测量毛干（如毛发不存在时的毛囊）与整个头发（如毛发不存在时的毛囊）中的皮质醇。如果毛囊中的皮质醇浓度足够高，足以影响整体头发皮质醇浓度（HCC），则可能会混淆使用不同类型的头发样本（发干与整发）的研究与采集方法之间的比较。在这里，我们测试了卵泡中皮质醇影响肝癌的假说。我们比较了30个自由放养的斯堪的纳维亚棕熊（Ursus arctos）的成对子样本中有和没有卵泡的HCC，观察到有卵泡的样本中HCC明显更高。在线性混合模型中，考虑到熊的性别和年龄，卵泡的影响仍然很明显。最后，我们提供了用于处理野生哺乳动物物种压力的77项研究中用于HCC分析的头发样本的收集方法和类型的概述。