Hair cortisol and its potential value as a physiological measure of stress response in human and non-human animals

Abstract

There is considerable interest in the potential for measuring cortisol in hair as a means of quantifying stress responses in human and non-human animals. This review updates the rapid advancement in our knowledge of hair cortisol, methods for its measurement, its relationship to acute and chronic stress, and its repeatability and heritability. The advantages of measuring cortisol in hair compared with other matrices such as blood, saliva and excreta and the current theories of the mechanisms of cortisol incorporation into the fibre are described. Hair cortisol as a measure of the physiological response to stress in a variety of species is presented, including correlations with other sample matrices, the relationship between hair cortisol and psychosocial stress and the repeatability and heritability of hair cortisol concentrations. Current standards for the quantification of hair cortisol are critically reviewed in detail for the first time and gaps in technical validation of these methods highlighted. The known effects of a variety of sources of hair cortisol variation are also reviewed, including hair sampling site, sex, age and adiposity. There is currently insufficient evidence to conclude that cortisol concentration in hair accurately reflects long-term blood cortisol concentrations. Similarly, there is a lack of information surrounding the mechanisms of cortisol incorporation into the hair. This review highlights several directions for future research to more fully validate the use of hair cortisol as an indicator of chronic stress.

Sheep wool cortisol as a retrospective measure of long-term HPA axis activity and its links to body mass

“…Using wool cortisol to monitor HPA axis activity as an indicator of sheep welfare would be of great benefit because wool presents a potential advantage over hair given that wool (as opposed to hair) fibres grow almost continuously and may be used as an indicator of a cortisol 'timeline' or 'retrospective calendar of HPA activity' [6,25]. However, segmental wool cortisol analysis, i.e.…”

“…the cutting of samples in segments that differ with respect to their proximity to the animal's skin), has, to our knowledge, not yet been conducted. In addition, wool differs markedly from hair with regards to follicle and fibre characteristics [25] which, in turn, differ also between breeds and also within a sheep's body (e.g. [38,39]).…”

“…[38,39]). Together, these features of wool offer potential for new insight into the mechanisms of cortisol incorporation into the fibre as well as sources of hair cortisol variation [25].…”

“…In recent years, hair cortisol analysis has become a commonly used tool for the retrospective assessment of long-term HPA axis activity in numerous mammal species, including, for example, non-human primates [7,[8][9][10][11][12][13], humans [14,15,16], carnivores [17][18][19][20], ungulates [21,22], lagomorphs [23], rodents [24] (for reviews see e.g. [25,26]). Hair sample collection is relatively easy and non-invasive, and although in most cases the animal will have to be held or restrained (but see [10,17]), there is no impact of handling on hair cortisol concentrations.…”

Exploring hair cortisone concentration as a novel tool to assess chronic stress in sheep with tick-borne fever

“…The Norwegian White has crossbred wool, and the mean wool growth rates are probably more similar to the growth rate of the Wensleydale x Blackface lambs than to the Merino weathers. The rate of wool fibre elongation may also be influenced by nutrition, photoperiod, reproduction, stress, and parasites (Burnard et al, 2017). The wool length was longer in Week 0 and 15 compared to week 3 and 6, which may have resulted in less specific hair steroid measurements in Week 0 and 15 compared to Week 3 and 6.…”