STRESS PHYSIOLOGY

Unfortunately, there is no single bio-indicator that reflects the ‘stress status’ of an animal. The physiological stress response is the cumulative response of a suite of existing physiological systems and the specific physiolog­ical components may vary depending on the nature of the stressor (Moberg and Mench 2000; NRC 2008).

It is generally recognised that there are three broad dimensions of the physiological stress response: the auto­nomic nervous system (ANS), the neuroendocrine system and the immune system (Moberg and Mench 2000; NRC 2008). As knowledge of physiology, and especially molecular biology, expands, additional factors are gain­ing attention (e.g. antioxidants, heat-shock proteins, micro-RNA), but this chapter will focus on the three dimensions listed above.

One of the most studied aspects of stress physiology in Australian mammals is the male die-off seen in several dasyurid species (Bradley 2003; Naylor et al. 2008). Sev­eral dimensions of physiology have been investigated during this extreme stress response. During the breeding season, males stop eating and simultaneously increase their energy expenditure through fighting and copulation. They experience weight loss, hair loss, parasitic infections, gastric ulceration, anaemia, spermatogenic and renal fail­ure, and ultimately death. At the endocrine level, this pre­cipitous decline is caused by an increase in testosterone, which in turn causes a decrease in corticosteroid-binding globulin and an increase in free cortisol (Bradley 2003; Naylor et al.

1.1 The autonomic nervous system

The ANS is the branch of the parasympathetic nervous system that regulates the function of internal organs and is generally not under conscious control. The ANS is often the first physiological system recruited to respond to a stressor and activation occurs quite rapidly (Moberg and Mench 2000; Fair et al. 2014). When the CNS per­ceives a stressor, it causes the adrenal medulla to release catecholamines (adrenaline, noradrenaline and dopa­mine), which in turn causes an increase in heart rate, respiration rate, blood pressure and GI activity (Fair et al. 2014; Romero and Wingfield 2015). This is often referred to as the ‘flight or fight’ response. Because the response of the ANS to stressors is so transient, it is thought to be a better indicator of acute stress than chronic stress (Moberg and Mench 2000).

1.1.1 Autonomic nervous system activity as a measure of stress in Australian mammals

There are limited studies of the response of the ANS to stressors in Australian mammals. To some extent, this is because catecholamines are rapidly released and quickly degraded, making them difficult to study. Most of the existing studies focus on basic description of the ANS or the role of the ANS in thermoregulation (e.g. Clements et al. 1998), rather than stress per se. Recent studies in common bottle-nosed dolphins (Tursiops truncatus) and pantropical spotted dolphins (Stenella attenuata) found that catecholamines were elevated in response to chase, encirclement and tagging, indicative of a typical mam­malian stress response (St Aubin et al. 2013; Fair et al. 2014). In sugar gliders (Petaurus breviceps), subordinate males have higher catecholamine levels in response to the odour of a dominant male but not a castrated male or female (Stoddart and Bradley 1991). However, in all these studies it is difficult to determine whether elevated cat­echolamine levels represent a threat to an animal’s overall health.

1.2 The neuroendocrine system

Stress is associated with changes in numerous hormones, including glucocorticoids, prolactin, oxytocin, vasotocin, growth hormone, thyroid hormones and antidiuretic hormone (Moberg and Mench 2000; Romero and Wing­field 2015; Bradshaw 2017). The response of the neuroen­docrine system is generally slower and more prolonged than that of the ANS, making it more useful as a monitor­ing tool. Although several neuroendocrine pathways respond to stressors, the release of glucocorticoids (GCs e.g. cortisol or corticosterone) by the hypothalamic-pitu­itary-adrenal (HPA) axis is the most studied and has almost become synonymous with stress in the literature (Romero and Wingfield 2015). However, it is problematic to equate GCs with stress for two reasons. First, the HPA axis is not the only system that responds to stressors and it may not respond to all types of stressors. Second, responding to stressors is not the primary function of the HPA axis: the HPA axis contributes to regulation of the circadian rhythm, metabolism (glucose regulation and nitrogen balance) and reproductive function (Moberg and Mench 2000; Fanson and Parrott 2015; Romero and Wingfield 2015). By equating stress with HPA activation and ignoring both the other components of the stress response and the other roles of GCs, we are likely missing opportunities to enhance our understanding of stress physiology and distinguish between acute and chronic stress. Although some studies integrate multiple dimen­sions of stress physiology (Fair et al. 2014; Bradshaw 2017), many focus solely on GCs.

1.2.1 Hypothalamic-pituitary-adrenal activity as a measure of stress in Australian mammals

The focus on GCs as a response to stressors in Australian mammals is a fairly recent development, with the exception of the dasyurid die-off described above.

Recent studies have largely focused on the role of GCs in the stress response (Hing et al. 2014; Narayan 2017). There is substantial evidence that capture, handling and trans­port can lead to short-term increases in adrenal activity in Australian mammals (Fair et al. 2014; Hing et al. 2014; McMichael et al. 2014; Fanson et al. 2017). Adverse envi­ronmental or habitat conditions (e.g. water shortages, degraded habitat) are associated with elevated adrenal activity in several species (Hing et al. 2014; Bradshaw 2017). For example, koalas (Phascolarctos cinereus) exhibit greater adrenal activity (as measured via faecal glucocorti­coid metabolites [FGM]) in drier environments or during periods of lower rainfall (Davies et al. 2013b; Davies et al. 2014). In several species, the increase in adrenal activity associated with poor environmental conditions may be linked to food availability and/or population density (Ayres et al. 2012; Moore et al. 2015; Parry-Jones et al. 2016).

The health of an animal may also affect adrenal activ­ity, but the directionality is not consistent. In eastern grey kangaroos (Macropus giganteus), adrenal activity is higher in individuals with ‘lumpy jaw’ disease (Sotohira et al. 2017). Conversely, among koalas undergoing reha­bilitation, the sickest individuals exhibited the lowest levels of adrenal activity (T Keeley pers. comm.). In zoo animals, management procedures such as handling or visitor exposure are sometimes associated with an increase in adrenal activity (Hogan et al.

The clinical implications of changes in GCs (particu­larly excreted GCs) are not straightforward. It is often assumed that GCs are directly related to stress and that elevated GC levels are deleterious or indicative of chronic stress, but there is little empirical evidence to support this (Dickens and Romero 2013). Adrenal activity and steroid metabolism can vary with sex, age, reproductive status, season, energy expenditure, and/or diet. Consequently, when trying to interpret fluctuations in GC levels or differ­ences between individuals or populations, it is important to incorporate the other regulatory roles of GCs and also integrate other measures of animal condition or wellbeing. The few studies that have integrated measures of body con­dition have generally found a negative correlation with GCs (Burgess et al. 2013; Hing et al. 2017a; Hing et al. 2017b; Parry-Jones et al. 2016) but not always (Moore et al. 2015). Taking a broader approach and using multiple measures to monitor stress physiology will undoubtedly prove to be more informative than focusing on GC levels alone.

1.3 The immune system

Changes in immune function are often thought to be a consequence of other physiological changes in response to stressors, but are increasingly being recognised as a direct and independent component of the stress response (Moberg and Mench 2000). Changes in immune function are often slower to occur and thus measures of immune function are sometimes thought to be more indicative of chronic stress (Johnstone et al. 2012). Immunology of Australian mammals is reviewed in Chapter 7.

1.3.1 Immune indicators as a measure of stress in Australian mammals

Recent work with agile antechinus (Antechinus agilis) found that animals living in fragmented landscapes had lower total leucocyte counts, higher neutrophil/lympho- cyte ratios and a greater percentage of eosinophils, which was interpreted as being indicative of chronic stress (Johnstone et al. 2012). In more heterogeneous habitats dominated by shrubs and woody debris, male antechinus had higher Hb-HCT residuals, which are expected to positively correlate with body condition (Johnstone et al. 2011). Females had higher neutrophil/lymphocyte ratios in habitats with a greater proportion of edge habitat and were also less likely to be trapped in these environments (Johnstone et al. 2011). However, neither of these meas­ures showed a strong association with fat reserves, a measure of body condition. In tammar wallabies, lym­phocyte count strongly correlates with body mass index and may be a good indicator of stress-mediated changes in haematology. However, lymphocyte counts did not vary across three populations that differed in human dis­turbance, rates of reproduction and survival (Robert and Schwanz 2013).

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