The relationship between steroid hormones and seasonal reproductive events in free-living female Pygmy Rattlesnakes, Sistrurus miliarius

Abstract

Steroid hormones regulate reproductive investment and play a critical role in coordinating reproductive responses to both short and long-term environmental change. Laboratory studies typically examine the mechanistic relationships among steroids and reproductive processes under controlled conditions and may fail to identify any seasonal patterns in regulatory mechanisms exhibited by free-living organisms. In snakes, field studies describing the seasonal relationships among steroids and male reproduction are prevalent. However, comparatively less work has been conducted on free-living female snakes. To bridge this gap in knowledge, we described the seasonal patterns of estradiol (E2), corticosterone (cort), and plasma constituents (total protein, albumin, phosphorus, and glucose) in relation to the seasonal events of the reproductive cycle in free-living female pygmy rattlesnakes (Sistrurus miliarius). We predicted to observe elevated E2 and plasma constituents during the spring vitellogenenic season, and that E2 would also be elevated during the fall mating season. Additionally, we hypothesized that cort acts to mobilize resources to meet the energetic challenges experienced during reproduction. We, therefore, predicted that elevated cort would be associated with the vitellogenic season and summer pregnancy, and that cort would be positively associated with elevated E2 and plasma constituents. In S. miliarius, plasma estradiol was significantly elevated during both the fall mating and spring vitellogenic seasons. Elevated E2 was associated with significantly higher plasma total protein, albumin, and phosphorous in both seasons, but unrelated to plasma glucose. Corticosterone was not elevated in individuals with elevated E2. Additionally, cort was not related to plasma total protein, albumin, or phosphorus, but was positively related to plasma glucose. Corticosterone was elevated during late pregnancy compared to early pregnancy and postpartum, and also elevated in pregnant compared to non-reproductive females sampled in the field. Our results indicate that the mechanistic links between steroids and reproductive processes may be seasonally uncoupled and highlight the need for the continued study of the hormonal coordination of reproduction in free-living vertebrates with diverse life histories.