Simulating physiological flexibility in the acute glucocorticoid response to stressors reveals limitations of current empirical approaches

Abstract

Wild animals often experience unpredictable challenges that demand rapid and flexible responses. The glucocorticoid mediated stress response is one of the major systems that allows vertebrates to rapidly adjust their physiology and behavior. Given its role in responding to challenges, evolutionary physiologists have focused on the consequences of between-individual and, more recently, within-individual variation in the acute glucocorticoid response. However, empirical studies of physiological flexibility are severely limited by the logistical challenges of measuring the same animal multiple times. Data simulation is a powerful approach when empirical data are limited, but has not been adopted to date in studies of physiological flexibility. In this article, I develop a simulation that can generate realistic acute glucocorticoid response data with user specified characteristics. Simulated animals can be sampled continuously through an acute response and across as many separate responses as desired, while varying key parameters. Using the simulation, I develop several scenarios that address key questions in physiological flexibility. These scenarios demonstrate the conditions under which a single glucocorticoid trait can be accurately assessed with typical experimental designs, the consequences of covariation between different components of the acute stress response, and the way that context specific differences in variability of acute responses can influence the power to detect relationships between the strength of the acute stress response and fitness. I also describe how to use the simulation tools to aid in the design and evaluation of empirical studies of physiological flexibility.

Endocrine flexibility can facilitate or constrain the ability to cope with global change

“…Given these challenges, progress will require a combination of traditional field-based approaches, consideration of different experimental design and sampling schemes [19] and collaboration between empiricists and theoreticians to generate novel hypotheses, confirm empirical findings and guide empirical work towards collecting critical missing data. For example, experimental treatments that combine samples from multiple individuals can describe group-level differences in hormonal reaction norms that circumvent the limitations of repeated sampling within individuals [30,118].…”

“…Field-based observation and capture will remain a central focus of endocrine flexibility and recent studies have demonstrated that these approaches can be successful despite the data hungry nature of reaction norm models [26]; in the future, it may be possible to repeatedly measure plasma hormone levels with bio-loggers, which would be a breakthrough for studies of endocrine flexibility. At present, excellent models for study design and necessary sample sizes can be borrowed from the behavioural reaction norm literature [52,117] and several recent papers detail particular considerations for physiological traits like hormones [18,19,51].…”

“…Recent studies have begun to shed light on the macroevolutionary patterns of hormone levels in relation to climate (e.g. [17]), but collecting these data is challenging and it is even more difficult to measure endocrine flexibility because cross-sectional studies or measurements under a single set of conditions may not capture critical variation in flexibility [18][19][20].…”

Endocrine flexibility can facilitate or constrain the ability to cope with global change.

“…Field-based observation and capture will remain a central focus of endocrine flexibility and recent studies have demonstrated that these approaches can be successful despite the data hungry nature of reaction norm models [26]; in the future, it may be possible to repeatedly measure plasma hormone levels with bio-loggers, which would be a breakthrough for studies of endocrine flexibility. At present, excellent models for study design and necessary sample sizes can be borrowed from the behavioral reaction norm literature [52,117] and several recent papers detail particular considerations for physiological traits like hormones [18,19,51].…”

“…Unfortunately, most physiological traits are not preserved in traditional specimens or reported by community scientists and few resources exist to evaluate whether there have been similar long-term changes in these response systems. Recent studies have begun to shed light on the macroevolutionary patterns of hormone levels in relation to climate [e.g., 17], but collecting these data is challenging and it is even more difficult to measure endocrine flexibility because cross-sectional studies or measurements under a single set of conditions may not capture critical variation in flexibility [18][19][20].…”