Some individuals quickly up- and downregulate hormones (dotted line) relative to others (dashed and dotted line). (B) Individuals vary in endocrine speeds and scope. The rates at which an individual can up- and downregulate hormone levels (‘Speed up’ and ‘Speed down’, respectively) are their endocrine speeds. The range of hormone levels from lowest to highest concentration within an individual is their endocrine scope (‘Scope’). After a certain hormone level is reached, the body begins to downregulate and clear the hormone. After the stimulus is perceived, hormone production is upregulated. (A) Before a stimulus is perceived, the focal hormone is at baseline levels. Lendvai et al., 2014).Įndocrine flexibility. The correlation between the intercept and the slope can also be quantified as a measure of the independence of these two metrics (e.g. Using the reaction norm approach, endocrine flexibility is derived from the intercept and slope of hormone concentrations measured across contexts. Endocrine flexibility might be quantified by calculating endocrine scope and speed or by using reaction norms (see Glossary Guindre-Parker, 2020 Malkoc et al., 2021). Individuals may also exhibit endocrine flexibility in either their baseline or induced hormone levels across environmental contexts ( Guindre-Parker, 2020).
Endocrine scope and speed are measures of flexibility in the increases (and decreases) in hormone levels from baseline to induced levels. Our broad definition of endocrine flexibility includes endocrine scope and speed (see Glossary Fig. 1A Taff and Vitousek, 2016), but also includes flexibility in baseline and induced hormone concentrations ( Fig. 1B Guindre-Parker, 2020). We use the term flexibility, rather than plasticity, to acknowledge its reversibility and ability to occur throughout an individual's lifetime, in contrast to phenotypic plasticity (see Glossary), which is sometimes defined as an irreversible change in phenotype induced during development ( Pigliucci, 2005 Hau et al., 2016). We consider endocrine flexibility to be a sub-type of phenotypic flexibility, which is broadly defined as reversible variation in trait expression within an individual's lifespan ( Hau et al., 2016). Individuals and species vary in their endocrine responses to external cues in important ways that we can describe and quantify. As examples, hormone levels can change in response to fluctuating environmental conditions, social interactions, and disease exposure, and these changes in hormone levels then stimulate modifications of behavior and physiology. To maintain homeostasis, hormones mediate interactions between the external and internal environments.
Mathematical modeling is not yet widely employed in endocrinology, but can be used to identify innovative areas for future research and generate novel predictions for empirical testing. Our Review introduces and defines endocrine flexibility, reviews existing studies, makes suggestions for future empirical work, and recommends mathematical modeling approaches to complement empirical work and significantly advance our understanding.
The need for repeated samples from individuals can make empirical studies of endocrine flexibility logistically challenging, but methods based in mathematical modeling can provide insights that circumvent these challenges. The ability to quickly and appropriately modify phenotype is predicted to be favored by selection, especially in unpredictable environments. Repeated measures of hormone levels in individuals provide additional insight into individual variation in endocrine flexibility – that is, how individuals modulate hormone levels in response to the environment. There is growing interest in studying hormones beyond single ‘snapshot’ measurements, as recognition that individual variation in the endocrine response to environmental change may underlie many rapid, coordinated phenotypic changes.
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