POTENTIAL NEUROENDOCRINE INDICATORS OF CHRONIC STRESS

As mentioned above, all physiological systems that respond to stressors also have important day-to-day functions, so it can be difficult to distinguish between normative fluctuations, acute (but healthy) stress responses and chronic stress responses associated with deterioration of health.

3.1 Dampened fluctuations in glucocorticoids

There is increasing evidence from the medical literature that patterns of GC expression may be more indicative of an individual’s condition than absolute GC concentrations. For example, in humans the amplitude of circadian fluc­tuations in GCs is dampened in several psychiatric condi­tions or situations associated with chronic stress (Fries et al. 2009). There is also evidence that dampened fluctua­tions in GCs across the oestrous cycle are associated with reproductive failure (Fanson and Parrott 2015). In female Tasmanian devils (Sarcophilus harrisii) that bred success­fully, there was a peak in GCs associated with ovulation, followed by a pronounced decrease in GCs (Keeley et al. 2012). However, in females that mated but did not success­fully produce pouch young, the ovulatory peak in GCs was delayed by ~4 d and the subsequent decrease was damp­ened. Therefore, monitoring patterns of GC secretion rather than absolute concentrations may provide more insight into when an animal is experiencing chronic stress.

3.2 Ratio of cortisol to corticosterone

There is some evidence that the ratio of cortisol to corti­costerone may provide an index of chronic stress. The adrenal glands produce multiple GCs, but there is typi­cally one ‘dominant’ GC produced in higher concentra­tions (Hancock 2010). This dominant GC varies across species; in most mammals the dominant GC is cortisol.

Table 8.1. Summary of published studies using non-invasive (faecal and urinary) glucocorticoid measures in Australian mammals

Population type (Pop) is managed care (MC) or free-ranging (FR), which includes wild animals that were temporarily confined and animals in large, naturalistic enclosures. ‘Sex' indicates whether the study included only females (F), only males (M), or both sexes (B). Focus of the study briefly summarises the focus of the study ('Methodological' = testing some aspect of sample collection or storage; 'Descriptive' = describing normative fluctuations in GCs (not necessarily stress-related differences)).

¹Russell etal. 2022; ²Hogan etal. 2012; ³Fanson etal. 2017; ⁴Keeley etal. 2012; ⁵Florent etal. 2023; ⁶Jensen etal. 2019; ⁷Swinston etal. 2020; ⁸Narayan etal. 2012; ⁹Evans etal. 2013; ¹⁰Narayan etal. 2014; ¹¹Dowle etal. 2013; ¹²Davies etal. 2013a; ¹³Davies etal. 2013b; ¹⁴Johnston etal. 2013; ¹⁵Narayan etal. 2013; ¹⁶Davies etal. 2014; ¹⁷Webster etal. 2017; ¹⁸Narayan 2019; ¹⁹Narayan and Vanderneut 2019; ²⁰Charalambous etal. 2021; ²¹Santamaria etal. 2021a; ²²Santamaria etal. 2021b; ²³Santamaria etal. 2021c; ²⁴Pahuja and Narayan 2023b; ²⁵Pahuja and Narayan 2023a; ²⁶Hogan etal. 2011; ²⁷Descovich etal. 2012; ²⁸Du etal. 2017; ²⁹Du etal. 2018; ³⁰Sengupta 2019; ³¹Weiss and McDonald 1966; ³²Cope etal. 2022; ³³Scheun etal. 2019; ³⁴Dimovski etal. (In press); ³⁵Oates etal. 2007; ³⁶Hing etal. 2016a; ³⁷Hing etal. 2016b; ³⁸Hing etal. 2017b; ³⁹Hing et al. 2017a; ⁴⁰Page etal. 2019; ⁴¹Batson etal. 2017; ⁴²Stead-Richardson etal. 2010; ⁴³Sherwen etal. 2015; ⁴⁴Brunton etal. 2020; ⁴⁵McKenzie and Deane 2005; ⁴⁶Schwanz and Robert 2014; ⁴⁷Moore etal. 2015; ⁴⁸Williams-Kelly etal. 2023; ⁴⁹Rowell etal. 2022; ⁵⁰McMichael etal. 2014; ⁵¹Parry-Jones etal. 2016; ⁵²Burgess etal. 2013; ⁵³Hogg and Rogers 2009; ⁵⁴Constable etal. 2006; ⁵⁵Mercera etal. 2021; ⁵⁶Ayres etal. 2012; ⁵⁷Steinman etal. 2020; ⁵⁸Steinman and Robeck 2021; ⁵⁹Hunt etal. 2019; ⁶⁰D'Agostino etal. 2022; ⁶¹Valenzuela-Molina etal. 2018