Hormonal regulation of whole-animal performance: Implications for selection

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One of the fundamental questions in evolutionary biology is how natural selection and sexual selection mold phenotypes, and the resulting functional capacities of those phenotypes. Strong selection within animal populations is well-documented (Endler 1986; Brodie et al. 1995; Kingsolver et al. 2001) and recent reviews point toward ‘wholeanimal performance’ as being central to understanding these patterns (Irschick et al. 2007, 2008). The last 20 years have witnessed an explosion of studies describing the evolutionary significance of performance traits such as running, biting, and swimming, yet the proximate mechanisms underlying such traits remain unclear for many animal species (Arnold 1983; Pough 1989; Garland and Losos 1994; Irschick et al. 2008). Moreover, as pointed out by recent authors (Blows 2007), fitness frequently is determined not by any one variable, but by a suite of interacting traits, including morphology, physiology, performance, and behavior. While many studies have focused on the musculoskeletal and energetic mechanisms underlying variation in performance of animals (Garland and Losos 1994; Biewener 2003), the hormonal regulation of performance traits has received less attention, despite the fact that hormones are known to exert profound effects on multiple aspects of morphology, physiology, and behavior, all of which affect most performance traits (Adkins-Reagan 2005). A discussion of how hormones affect performance is important because it could shed light on how hormonal variation ultimately affects fitness. Understanding the role of hormones in influencing fitness is important because of criticisms that performance traits may not be the true targets of selection, but might be correlated, via the integrating effects of hormones, with other traits that are the ‘‘true’’ target of selection (reviewed by Garland et al. 1990; Husak and Fox 2008). This debate stems from a poor understanding of how hormones mediate performance traits, and how such traits in turn affect fitness. Interestingly, despite a general lack of discussion in the literature on animals, there exists an extensive literature concerned with how steroid hormones affect human performance (see Husak and Irschick 2009). Whereas humans attempt to enhance performance by using steroids for purposes related to sports (e.g., winning races and hitting more home-runs), performance traits in non human animals often make the difference between catching dinner or becoming dinner (Irschick et al. 2008). Therefore, by studying the effects of hormones on performance, we may gain general insights into the factors that influence organismal fitness, a burgeoning area of discussion in evolutionary biology (Kingsovler et al. 2001; Hereford et al. 2004). A consideration of how hormones affect variation in morphology and performance opens up broader questions for how such traits evolve. For example, one important issue concerns whether traits such as morphology, performance, and behavior evolve more or less independently, or are tightly bound together in coadapted complexes. This issue has been debated at the level of hormonal control (Hau 2007; Ketterson et al. 2009) and has centered on two alternative hypotheses, namely evolutionary integration versus independence of receptors, target tissues,