Hormonal responses to environmental change can be generalized or specialized.

In highly fluctuating environments, a specialized hormonal response may maximize fitness.

Food, density, and predators, but not temperature, affect selection on female life history traits.

Female squirrels exhibit glucocorticoid responses to all four environmental factors.

Squirrels mount a generalized, rather than specific, response to environmental change.

Evolutionary endocrinology aims to understand how natural selection shapes endocrine systems and the degree to which endocrine systems themselves can induce phenotypic responses to environmental changes. Such responses may be specialized in that they reflect past selection for responsiveness only to those ecological factors that ultimately influence natural selection. Alternatively, endocrine responses may be broad and generalized, allowing organisms to cope with a variety of environmental changes simultaneously. Here, we empirically tested whether the endocrine response of female North American red squirrels (Tamiasciurus hudsonicus) was specialized or generalized. We first quantified the direction and magnitude of natural selection acting on three female life history traits (parturition date, litter size, offspring postnatal growth rate) during 32 years of fluctuations in four potential ecological agents of selection (food availability, conspecific density, predator abundance, and temperature). Only three of the four variables (food, density, and predators) affected patterns of natural selection on female life history traits. We then quantified fecal glucocorticoid metabolites (FGMs) across 7 years and found that all four environmental variables, regardless of their effects on patterns of selection, were associated with glucocorticoid production. Our results provide support for a generalized, rather than specific, glucocorticoid response to environmental change that can integrate across multiple co-occurring environmental stressors.

Glucocorticoids

Life history

Natural selection

Reproduction

Stress

All data and code used for this study is available at the figshare repository: https://figshare.com/s/0bef9fcf4e963da9712d.

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