Moore, M.C.*; Painter, D.; Woodley, S.K.; Lacy, E.L.; Jennings, D.H.; Wikelski, M.: Neuroendocrine mechanisms of alternative male reproductive tactics in marine iguanas

Alternative male phenotypes often differ as much in appearance and behavior as do males from females. These extreme examples of within-sex variation are good models to examine neuroendocrine bases of such variation. The relative plasticity hypothesis (RPH) generalizes the organization-activation model of sexual differentiation to within-sex variation. It proposes that species with fixed alternative male morphs will have developmental (organizational) mechanisms of differentiation whereas species with plastic alternative male morphs will use activational mechanisms. In this study, we examined marine iguanas (Amblyrhynchus cristatus), which have a complex system of plastic alternative male phenotypes: there are 3 distinct morphologies and 5 distinct social classes. Since males transform types as they age, the RPH predicts that there will be hormonal mechanisms operating at the time of transformation. Comparison of testosterone (T) levels in free-living males from the Genovesa Island population partially confirms this prediction. Males of the two nonterritorial classes had T levels lower than those of males in the two successful territorial classes. Furthermore, when subjected to 30 minutes of handling stress, nonterritorial males increased their T levels whereas successful territorial males did not. Unsuccessful territorial males were intermediate. These results contrast with those from species with fixed systems and suggest that activational neuroendocrine mechanisms are important in plastic alternative male phenotypes of marine iguanas.