Evolutionary Change in the Endocrinology of Behavioral Receptivity: Divergent Roles for Progesterone and Prolactin within the Genus Phodopus

Siberian (Phodopus sungorus) and Djungarian (P. campbelli) hamsters are phenotypically similar and were long considered subspecies. Progesterone (P4) and prolactin (PRL) changes (determined by repeated sampling of individuals) during the behavioral receptivity of both an ovulatory cycle and a postpartum mating, as well as the hormonal requirements for behavioral receptivity, were determined. Changes in P. sungorus were similar to well-described hormonal changes in rats, mice, and golden hamsters, suggesting that previously described differences between P. campbelli and those species had evolved recently. Specifically, 1) P4 facilitated behavioral receptivity at low priming doses of estradiol in P. sungorus but was not needed in P. campbelli; 2) in P. sungorus, P4 increases were synchronous across females and of similar amplitude during each estrus, whereas in P. campbelli, P4 increases were less synchronous across females and were reduced in amplitude postpartum; and 3) PRL profiles were similar (high average PRL levels, few high surges detected) in each species on Day 18, but on proestrus, cyclic P. sungorus had elevated PRL levels and frequent surges while cyclic P. campbelli had lower PRL levels and rare surges. As the endocrinology of P. campbelli also differs from known laboratory rodents in other ways, additional within-genus divergence is predicted.