Performance and fitness. Hormones are known to coordinate the expression of numerous aspects of morphology, behavior, physiology and performance. What is less well known is how variation in performance links to fitness and to generate selection on underlying traits and their agents of endocrine coordination. Work by Henry John-Alder, Greg Haenel, Linda Smith and myself on the eastern fence lizard shows that body size, exercise endurance, and home range area are all weakly linked to variation in male reproductive success (Fig. 2). Given that the hormone testosterone is known to influence male aggression, exercise endurance, movement, and home range area (figure here), is seems likely that variation in circulating testosterone is also subject to sexual selection.

How do hormones mediate performance to influence an organism’s fitness?

A major goal of evolutionary biology is to link an organism’s phenotype (morphology, physiology, behavior) with it’s resultant fitness (survival , mating success, fecundity). Many “lower order” phenotypic traits are linked to fitness as a result of their interactions, which produce “higher order” measures of organismal performance. Performance is often defined as an organism’s ability to conduct some ecologically relevant task. As such, performance integrates morphology, physiology, and behavior (Fig. 1). Because hormones are natural biological agents of coordination, they are appealing candidate mechanisms for the coordinated regulation of the various underlying traits that determine performance and, by extension, fitness.

Related Papers

Cox, R.M., D.S. Stenquist*, J.P. Henningsen, and R. Calsbeek. 2009. Manipulating testosterone to assess links between behavior, morphology and performance in the brown anole, Anolis sagrei. Physiological and Biochemical Zoology 82: 686-698.

John-Alder, H.B., R.M. Cox, G. Haenel, and L. Smith. 2009. Hormones, performance and fitness: natural history and endocrine experiments on a lizard (Sceloporus undulatus). Integrative and Comparative Biology 49:393-407.

Hormones and fitness. Despite the effects of testosterone on performance and of performance on reproductive success, circulating levels of plasma testosterone are unrelated to reproductive success in male eastern fence lizards (Fig. 3a). However, male reproductive success is strongly related to variation in circulating corticosterone (Fig. 3b). This could reflect the fact that increased reproductive success is achieved via the corticosterone-induced mobilization of energy stores to fuel increased home-range patrolling and mate searching. Further experiments are required to determine the causal effects of testosterone on male performance and fitness.

Figure 1. Morphological, physiological, and behavioral components of the phenotype interact to determine organismal performance. These interactions are shaped by the environmental context in which they occur, which also determines the ecological relevance of performance with respect to fitness. Variation in fitness among individuals gives rise to selection on performance and the phenotypic traits subsumed within it. Hormones coordinate the expression of phenotypic traits and thereby influence performance and fitness.

Figure 2. Weak sexual selection on (A) body size, (B) endurance, and (C) home range area of male eastern fence lizards. Left panels show mean (+SEM) levels of each trait for males that sired at least one clutch (blue) and those that did not sire any clutches (orange), as determined by DNA fingerprinting. Right panels show total number of progeny sired as a function of each measure. One extreme outlier (circled) sired 65 offspring and had a disproportionate effect on these analyses. Bottom statistics are derived from analyses in which this outlier was excluded. Modified from John-Alder et al. (2009).

Figure 3. Relationships between reproductive success and circulating levels of two steroid hormones, testosterone (T) and corticosterone (B), measured during the breeding season for male eastern fence lizards. Symbols and notation as in Figure 2, above. Modified from John-Alder et al. (2009).