Stress physiology in marine mammals: how well do they fit the terrestrial model?

Abstract

Stressors are commonly accepted as the causal factors, either internal or external, that evoke physiological responses to mediate the impact of the stressor. The majority of research on the physiological stress response, and costs incurred to an animal, has focused on terrestrial species. This review presents current knowledge on the physiology of the stress response in a lesser studied group of mammals, the marine mammals. Marine mammals are an artificial or pseudo grouping from a taxonomical perspective, as this group represents several distinct and diverse orders of mammals. However, they all are fully or semi-aquatic animals and have experienced selective pressures that have shaped their physiology in a manner that differs from terrestrial relatives. What these differences are and how they relate to the stress response is an efflorescent topic of study. The identification of the many facets of the stress response is critical to marine mammal management and conservation efforts. Anthropogenic stressors in marine ecosystems, including ocean noise, pollution, and fisheries interactions, are increasing and the dramatic responses of some marine mammals to these stressors have elevated concerns over the impact of human-related activities on a diverse group of animals that are difficult to monitor. This review covers the physiology of the stress response in marine mammals and places it in context of what is known from research on terrestrial mammals, particularly with respect to mediator activity that diverges from generalized terrestrial models. Challenges in conducting research on stress physiology in marine mammals are discussed and ways to overcome these challenges in the future are suggested.

Variation in adrenal and thyroid hormones with life-history stage in juvenile northern elephant seals (Mirounga angustirostris)

“…Several studies have suggested that aldosterone is an important component of the stress response in marine mammals, being released in response to an ACTH challenge or perceptual stress (Champagne et al, 2015;Gulland et al, 1999;Houser et al, 2011;Thomson and Geraci, 1986). The regulation of aldosterone release by the HPA axis may reflect the need to alter kidney perfusion and lung perfusion during diving, inhibiting typical RAS regulation (Atkinson et al, 2015). In contrast to the juveniles in the current study, aldosterone concentrations increased across the breeding fast in adult male elephant seals (Champagne et al, 2006;Ortiz et al, 2006) in association with RAS activation suggesting that this hormone may be under dual regulation by the HPA axis and RAS.…”

“…EPI and NE are also involved in the dive response in marine mammals (Atkinson et al, 2015). The splenic contraction and circulatory adjustments (e.g., bradycardia) that occur in diving pinnipeds are associated with increased catecholamine concentrations (Cabanac et al, 1997;Hance et al, 1982;Hochachka et al, 1995).…”

“…While EPI increases metabolic rate, enhances glycogenolysis, and increases release of glucose into the blood stream, NE plays key roles in homeostasis of blood volume and blood pressure (Atkinson et al, 2015). Previously, juvenile northern elephant seals exhibited sex differences in both fuel metabolism and energy expenditure during fasting (Kelso et al, 2012).…”

Multi-year longitudinal profiles of cortisol and corticosterone recovered from baleen of North Atlantic right whales (Eubalaena glacialis)

“…Differentiating the potential physiological impacts of these various stressors has proven difficult, partly due to a relative dearth of techniques for assessing adrenal activity and stress physiology in large cetaceans (Atkinson et al, 2015;Hunt et al, 2013). In terrestrial wildlife, physiological effects of natural and anthropogenic stressors are often monitored via assay of glucocorticoids (GCs) -cortisol, corticosterone and related hormones.…”

“…These results suggest that baleen plates might also contain longitudinal, multi-year, time series of other steroid hormones. Information on glucocorticoids is particularly desirable given management needs for methods of assessing effects of natural and anthropogenic stressors in large whales (Atkinson et al, 2015;Hunt et al, 2013). The National Academies of Sciences has recently recommended further research on baleen hormones for assessment of stress in cetaceans (National Academies of Sciences, Engineering, and Medicine, 2016).…”