Progesterone induces bright orange throat coloration in female Petrosaurus mearnsi

Female Petrosaurus mearnsi develop orange throat coloration when gravid. This coloration was induced in the nonbreeding season (late fall) by exogenous progesterone delivered by subcutaneous implantation in Silastic capsules or intramuscular injection. Color change was first apparent after five days of administration. The time course of brightening was similar for the two methods. Color change was first apparent after five days, and was detectable in most females receiving implants after six days. For both routes of administration, maximum brightening usually occurred within three days of the time that brightening was first detected. Following removal of implants or termination of daily injections, the throat coloration of most females had begun to fade within 25 to 30 days. However, two injected females did not fade at all after sixty days and none faded completely to pretreatment coloration. This suggests that endogenous steroid production by early winter is insufficient to induce brightening, but it might maintain some degree of artificially induced brightening. Because progesterone has also been shown to induce orange female coloration in a distantly related phrynosomatid species and two species of crotaphytids, we hypothesize that induction of bright female secondary sexual coloration by progesterone may be a widespread trait in iguanian lizards.