Relating endocrinology, physiology and behaviour using species with alternative mating strategies

1. Recent reviews demonstrate that genetically determined alternative mating strategies (AMS) are widespread and typically consist of morphs that are recognized by morphological or colour traits. Despite well-established behavioural differences associated with each morph, and evidence that androgens are involved in the induction of morphs, few studies have examined whether morphs also vary in whole-organismal performance traits, which may affect dominance status, resource holding potential (RHP) or mate attraction. 2. Our survey revealed a link between androgens and physiological performance traits that are associated with territorial or courtship displays across vertebrate taxa, although the number of species in the sample is limited. Experimental elevation of testosterone alters muscular contractile properties, swimming performance, sprint speed and endurance in males. Whether morphs differ in physiological capacities is relatively unexplored, although recent studies have found that males with high dominance status also exhibit greater physiological capacities (locomotor performance, call duration). 3. Multiple studies support the hypothesis that elevated testosterone results in fitness trade-offs. Potential costs of testosterone include impaired immune function, higher parasite loads, greater energetic requirements and ultimately reduced survival. Long term studies of Uta stansburiana highlight the trade-offs among life-history traits induced by variation in testosterone. Circumstantial evidence suggests a role of testosterone in depressing immune function in species with AMS. 4. We argue that hypotheses regarding the role of trade-offs in shaping selection on functional modules, which are involved in sexual selection, are best developed by manipulative studies on discrete morphs. Our review highlights the need to measure multiple traits to provide additional insights into the roles of sexual selection and physiological epistasis in maintaining intraspecific variation in reproductive phenotypes. The integration of endocrine control of behaviour, physiology and performance is rarely attempted in most studies and may be facilitated by analyses that focus on estimating correlational selection.