Experimentally induced sexual behavior in male gray treefrogs activates the HPG but not the HPI axis

While many studies have established how hormones modulate behavior, experiments that manipulate a behavior and quantify its hormonal consequences are rarer. Frog sexual behavior presents an opportunity to control a discrete behavior and observe its secretory consequences. Male frogs clasp females (amplexus) prior to gamete release. In the wild, amplexed males have endogenously higher gonadal and interrenal steroids. Here, we used Cope's gray treefrogs (Hyla chrysoscelis) to experimentally test the hypothesis that the behavioral act of amplexus causes these increased circulating hormones. We quantified levels of testosterone, estradiol, and corticosterone after 90 min in three treatment groups: unpaired males that were given access to a female and allowed to enter amplexus ("amplexus induced"), males in naturally amplexed pairs that were separated from their mate ("amplexus terminated"), and unpaired males that remained unpaired ("control"). Testosterone and estradiol were elevated in the amplexus induced group relative to males in the amplexus terminated and control groups. We did not observe any differences in corticosterone levels across treatment groups. These results suggest the behavioral act of amplexus activates the hypothalamic-pituitary-gonadal axis, but not the hypothalamic-pituitary-interrenal axis. This study provides experimental evidence of a discrete behavior rapidly initiating hormonal changes.