Meeting Abstract
Biologists have long recognized that circulating steroid levels vary significantly both among different species and within individuals of the same species. This observation often leads to the assumption that steroid levels themselves are “hypervariable traits,” which can easily evolve in response to a variety of factors. This idea, however, is seldom explored in a rigorous manner, especially when it comes to charting out the evolutionary trajectory of a circulating steroid across the vertebrate tree of life. Using data derived from HormoneBase, a data set that collates steroid hormone levels of hundreds of fish, reptile, amphibian, mammal, and bird species, we model how changes in selection regimes across the vertebrate phylogeny describe the evolutionary patterning of circulating androgen and corticosterone in both males and females. In males, we find that mammals, birds, and most fishes have shifted to new evolutionary regimes, and exhibit lower levels of testosterone or 11-ketotestosterone than the inferred ancestral vertebrate evolutionary regime (to which crocodiles, turtles, squamates, and amphibians belong). However, in females, we infer no shifts in optimal androgen levels across the entire phylogenetic landscape. For glucocorticoid levels, both males and females experienced distinct and parallel regime shifts, in which all vertebrates, except for fishes, are inferred to have shifted to exhibit lower levels of these hormones, with a subsequent shift to higher levels in most mammals. Overall, these data show episodes of major taxonomic divergence in regime changes that set the trajectory of circulating steroid level evolution.
