Testosterone in Females: Mediator of Adaptive Traits, Constraint on Sexual Dimorphism, or Both?

Abstract

When selection on males and females differs, the sexes may diverge in phenotype. Hormones serve as a proximate regulator of sex differences by mediating sex-biased trait expression. To integrate these perspectives, we consider how suites of traits mediated by the same hormone in both sexes might respond to selection. In male birds, plasma testosterone (T) varies seasonally and among species according to mating system. When elevated experimentally, it is known to enhance some components of fitness and to decrease others. We report that female T also varies seasonally and co-varies with male T. Female T is higher in relation to male T in sexually monomorphic species and is higher absolutely in females of species with socially monogamous mating systems, which suggests adaptation. We also consider the effect of experimentally elevated T on females and whether traits are sensitive to altered T. We hypothesize that sensitive traits could become subject to selection after a natural change in T and that traits with opposing fitness consequences in males and females could constrain dimorphism. Results from birds, including the dark-eyed junco (Junco hyemalis), reveal many sensitive traits, some of which appear costly and may help to account for observed levels of sexual dimorphism.

Optimal annual routines: behaviour in the context of physiology and ecology

“…For example, there may be constraints resulting from the multiple effects of hormones (e.g. Ketterson & Nolan 2000;Ketterson et al 2005). There may also be phylogenetic constraints (e.g.…”

Hormone-mediated maternal effects in birds:mechanismsmatter but what do we know of them?

“…Ignoring, for simplicity, the slightly lower specific mass of yolk, yolk hormone concentrations in pg mg K1 are converted into ng ml K1 . Among species, female plasma testosterone concentration during egg formation is 0.45 ng ml K1 on average (range 0.03-12.4; Ketterson et al 2005), while yolk testosterone concentration is 18 ng ml K1 on average (range between 0.5 and 86 ng ml K1 ; Schwabl et al 2007), which is approximately 40-fold higher. Thus, the yolk lipid/lipoprotein matrix (see above) seems to facilitate high concentrations of testosterone compared with blood.…”

The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage

“…The major role played by this mechanism has been Sexual dimorphism and sex-linkage DJ Fairbairn and DA Roff well established by studies of the developmental genetics and ontogeny of SD in species with chromosomal sex determination (Daly and Wilson, 1983;Short and Balaban, 1994;Mittwoch, 1996;Mealey, 2000;Badyaev, 2002;Skelly and John-Alder, 2002;Cox et al, 2005;Emlen et al, 2005;Ketterson et al, 2005). The discovery of similar developmental cascades and hormonal mechanisms in organisms with environmental sex determination (Pieau et al, 1994), including those that are sequentially hermaphroditic (Shapiro, 1994), provides unassailable evidence that sex-linkage is not required for the evolution of even extreme SD.…”