Influence of androgens on differentiation of secondary sex characters in tree lizards, Urosaurus ornatus.

Vertebrates species vary in the degree to which the sexes differ in their expression of secondary sex characters, which can be expressed in one sex but not the other, fully expressed in both sexes, or expressed to different degrees in the two sexes. Sex steroid hormones contribute to the development of sex differences, either through action early in life (organization), following sexual maturation (activation), or both. However, relatively little is known about the contributions of sex steroid hormones to species-level variation in sexual dimorphism. We began to address this by assessing in tree lizards, Urosaurus ornatus, the effects of testosterone (T) and dihydrotestosterone (DHT) on expression of the male-typical traits: femoral pore secretions, accessory sex tissues, patches of ventrolateral blue, intensity and hue of throat color, and body size. We administered long-lasting hormone implants of these androgens to intact or ovariectomized adult females, intact hatchling females, and intact hatchling males to determine the relative contribution of organizational and activational influences of androgens on sexual differentiation of these traits. Waxy femoral pore secretions and full intensity of the orange background color of the throat fan (dewlap) required only activation and both androgens were equally effective. Both androgens caused hypertrophy of accessory tissues associated with the hemipenes, but only in hatchling males. Full expression of ventrolateral blue patches required organization by DHT. Androgens had complex organizational actions on growth. In the doses used in the experiment, DHT inhibited body-length growth but not mass growth of hatchling males. Only T inhibited the growth (length and mass) of hatchling females. Because earlier work found that castration also reduced the growth of hatchling males, the organizational effects of androgen on growth might be dose-related. Results for hormonal control of dewlap type were complex. In this population dewlap color is solid orange in females, but polymorphic in males with 50% expressing solid orange (O) and 50% orange-blue (orange with a central blue patch, OB). These color morphs represent permanent male alternatives: OB males are territorial and more aggressive than O males who do not defend territories. Results of the hormone manipulations are most consistent with the interpretation that dewlap morph type requires both organization and activation by androgen. T appears to be more important in organizing morph differences, whereas DHT appears to be more important in activating expression of underlying morph differences. Females only expressed male-typical OB dewlaps when given DHT, and only about 50% did so.