Plasma Steroid Interactions during High-Density Green Turtle Nesting and Associated Disturbance

Abstract

Raine Island in the Northern Great Barrier Reef constitutes an extremely high-density green turtle (Chelonia mydas) rookery. On this island, competitive interactions for nesting space and subsequent disturbance of individual nesting are widespread. High-density nesting often delays successful oviposition by one or more nights. There is little information on how hormones in female reptiles interact during competitive reproductive events in such high-density nesting populations. In this three-part study we investigated the interactions between density (within and between rookery/ies), nesting success and failure, and plasma steroid profiles in green turtles. First, we compared levels of plasma corticosterone (B) and combined testosterone + 5α-dihydrotestosterone (T+DHT) in turtles during five stages of oviposition in both a high-nesting-density sector (1 turtle/m2) and a low-nesting-density sector (0.1 turtle/m2). Second, we investigated the relationship between increasing delays (0, 1, 2, 3, and 6 days) in successful oviposition and the plasma steroids B and T + DHT. Third, we assessed a comparative measure of steroid hormone levels of females at low-density sites on Raine Island (high-density rookery) and Number Seven Sandbank (low-density rookery). Despite a significant trend suggesting high-density nesting turtles elaborated more plasma B than turtles in low-density sectors, the magnitude of this increase was small. We suggest that this increase may be an artifact of increased metabolic demand and hence catabolism of energy substrates associated with high-density nesting. Plasma T + DHT remained stable in response to density-dependent effects associated with nesting. Furthermore, prolonging successful oviposition because of multiple nightly disturbance failed to elicit any change in either plasma B or T + DHT. These data suggest that green turtles may be exhibiting adrenal desensitization to prevent both physical and behavioral disturbances interfering with reproduction. We suspect that down-regulating the acute adrenocortical response may represent an adaptive trade-off mechanism for optomizing current reproductive success at the potential expense of survivorship.