Glucocorticoid manipulations in free‐living animals: considerations of dose delivery, life‐history context and reproductive state

Abstract

Summary1. Experimental glucocorticoid (GC) manipulations can be useful for identifying the mechanisms that drive life-history and fitness variation in free-living animals, but predicting the effects of GC treatment can be complicated. Much of the uncertainty stems from the multifaceted role of GCs in organismal metabolism, and their variable influence with respect to life-history stage, ecological context, age, sex and individual variation. 2. Glucocorticoid hormones have been implicated in the regulation of parental care in many vertebrate taxa but in two seemingly contradictory ways, which sets up a potential GC-induced 'reproductive conflict'. Circulating GCs mediate adaptive physiological and behavioural responses to stressful events, and elevated levels can lead to trade offs between reproductive effort and survival (e.g. the current reproduction vs. survival hypothesis). The majority of studies examining the fitness effects of GC manipulations extend from this hypothesis. However, when animals are not stressed (likely most of the time) baseline GCs act as key metabolic regulators of daily energy balance, homoeostasis, osmoregulation and food acquisition, with pleiotropic effects on locomotor activity or foraging behaviour. Slight increases in circulating baseline levels can then have positive effects on reproductive effort (e.g. the 'cort' fitness/adaptation hypotheses), but comparatively few GC manipulation studies have targeted these small, non-stress induced increases. 3. We review studies of GC manipulations and examine the specific hypotheses used to predict the effects of manipulations in wild, breeding vertebrates. We argue that given the dichotomous function of GCs the current 'reproduction vs. survival' paradigm is unnecessarily restrictive and predicts only deleterious GC effects on fitness. Therefore, a broader set of hypotheses should be considered when testing the fitness effects of GC manipulations. 4. When framing experimental manipulation studies, we urge researchers to consider three key points: life-history context (e.g. long vs. short lived, semelparous vs. iteroparous, etc.), ecological context and dose delivery.

Short-term and long-term effects of transient exogenous cortisol manipulation on oxidative stress in juvenile brown trout

“…Elevated levels of GCs can reduce an individual's ability to forage, avoid predators and grow (Wingfield et al, 1998), therefore limiting the energetic resources available for defence against and repair of oxidative damage. Studies that manipulate circulating levels of GCs via exogenous manipulations are becoming increasingly common, not only to understand fundamental aspects of organismal function but to also understand the ecology of stress in wild animals Crossin et al, 2016). Elevated levels of GCs have been suggested to increase oxidative stress via an elevation in metabolic rate, which causes an increased flux of electrons at the level of the electron transport chain (Wingfield et al, 1998;Roussel et al, 2004).…”

“…Though a single (transient) exogenous manipulation of cortisol is a common method for studying 'stress', it fails to fully emulate a stress response per se in that it does not include the process of the organism perceiving a stressor and the associated neuroendocrine cascade . Nonetheless, this approach does have merit for testing the effects of experimental elevation of GCs on organismal biology Crossin et al, 2016). Given that increased GC levels have been shown to increase metabolic rate and may reduce the availability of resources to fight oxidative stress, we predicted that growth rate will be lower and predation higher in fish manipulated with cortisol relative to control and sham treatment groups.…”

Stress and food deprivation: linking physiological state to migration success in a teleost fish

“…There is increasing interest in experimentally manipulating GCs in wild animals (Sopinka et al, 2015;Crossin et al, 2016) to simulate different natural and anthropogenic stressors. To our knowledge, this is one of the first studies to directly compare and contrast the consequences of a natural challenge (i.e.…”

“…Through analysis of tissue samples intended to characterize baseline or stress-induced GC levels (see Dantzer et al, 2014) to various GC manipulation studies (reviewed in Sopinka et al, 2015;Crossin et al, 2016), researchers have started to elucidate what is now termed 'the ecology of stress' (Boonstra, 2013a). Yet, many challenges remain, as studies often use GC manipulations to simulate semi-chronic or chronic stressors that may not be ecologically relevant (Crossin et al, 2016;Sopinka et al, 2015). Natural experiments, where natural processes are re-created directly in the field, avoid this type of issue (i.e.…”

Relações entre comportamento vocal, imunidade e níveis plasmáticos de corticosterona e testosterona em sapos (Rhinella granulosa)

“…Entretanto, em R. granulosa, a aplicação de 4,2µg de corticosterona levou a aumento da CBP em 7 vezes, constante por até 10h após a aplicação, enquanto animais que receberam aplicação de maior dose tiveram aumento da CBP menor e mais breve. Esses resultados indicam que dose, duração e frequência do aumento dos níveis plasmáticos de glicocorticoides são relevantes não só na modulação da resposta imune, mas devem exercer também efeito no ajuste mais fino de padrão de intensidade e duração de resposta observada (Crossin et al 2015;McCornick & Langkilde, 2015).…”