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The endocrine mechanisms causing variation and plasticity in life history traits (e.g., development time, mass at birth/hatching, rate of postnatal growth, age or size at sexual maturity, litter or clutch size, annual survival, and lifespan) or fitness (annual or lifetime reproductive success) have recently garnered considerable interest. We review three issues facing studies that quantify relationships between endocrine traits and life histories or measures of fitness and describe possible solutions using insights from evolutionary ecology. We focus in particular on the steroid hormones glucocorticoids that are involved in the vertebrate neuroendocrine stress response. First, context-dependent associations between endocrine traits and life histories or fitness are widespread, and therefore, it is important to quantify how intrinsic or extrinsic factors modify these relationships. Second, studies in evolutionary endocrinology may aspire to quantify patterns of natural selection on endocrine traits, but this may not tell us how they influence fitness. Studies that also identify the actual targets of selection that the endocrine traits are influencing will be very useful. Third, environmental or intrinsic factors can cause co-variance between endocrine traits and life histories or fitness. This is problematic for interpreting the potential evolutionary consequences of selection on endocrine traits, but it can also produce divergent answers for relationships between endocrine traits and life histories or fitness depending upon whether the data are analyzed in an among- or within-year framework. Future long-term studies following uniquely marked individuals over their lifetime (longitudinal individual-based approach) in combination with experimental manipulations of the endocrine traits or environmental factors influencing both endocrine traits and life histories or fitness may help to produce new insights in evolutionary endocrinology despite these issues. This is an ambitious endeavor, and we briefly review some of the key issues facing such long-term studies and experimental manipulations of endocrine traits.

Relationships between Endocrine Traits and Life Histories in Wild Animals: Insights, Problems, and Potential Pitfalls