Effective management of sea turtle populations in the future will require an understanding of exogenous and endogenous factors that may influence aspects of their reproduction, especially reproductive output. Therefore, in this thesis I aim to combine physiological and ecological data to investigate potential relationships between interrenal gland function, lipid mobilisation and reproductive cycles in female green sea turtles (Chelonia mydas).
I use data collected from three foraging areas and from four nesting seasons at Heron Island (including one season in which approximately 99% of clutches laid on the island were recorded). My first objective was to collect information that would expand current understanding of reproductive output in female C. mydas. Reproductive data from this species have been collected from several populations for many decades, however some gaps still remain in the literature. For example, little is known about how many clutches they lay for a season, factors that may influence seasonal reproductive output and intervals between breeding seasons. The data discussed in this thesis indicate that a female's seasonal reproductive output (how many clutches she lays in a season) is related to both her breeding history, and when in the season she arrives at the nesting beach. Remigrant turtles were larger than first time breeders, arrived earlier in the season and laid more clutches of eggs. Females in this study bred every five to eight years, each season laying five clutches of eggs at 13-day intervals.
Second, I combined physiological data derived from blood samples collected at a variety of stages in the female reproductive cycle with ecological data to investigate the relationships between plasma hormones and reproductive condition. Similar to previous findings, vitellogenesis began approximately eight to ten months prior to the nesting season. Plasma constituents varied with reproductive state, and vitellogenic females in this study showed seasonally increased plasma hormones (corticosterone (CORT) testosterone (T+DHT), epinephrine (EPI)) and triglyceride, with levels peaking during courtship (T+DHT and triglyceride) or in the early nesting season (CORT and EPI). Moreover, in vitro experiments indicated that CORT and EPI induce lipolysis from sea turtle adipose tissue. This suggests that seasonal increases in these two hormones may be involved in regulating lipolysis and transport of lipid into developing follicles.
Although, both CORT and T+DHT levels following each clutch showed significant variation, they were both typically lowest following the last clutch for the season. Because CORT levels were low at the end of the season when a female is presumably at her lowest body condition, I suggest that CORT plays a relatively minor role in energy maintenance in nesting turtles. Rather, it seems likely that CORT plays a functional role in preparing the ovary or oviduct for ovulation. Energy balance could be a major factor regulating the length of the breeding season. Females exposed to prolonged periods of unsuccessful nesting (> 3 days) had lower levels of plasma triglycerides, and females with high numbers of atretic ovarian follicles had lower levels of total lipid in their adipose tissue. Moreover, my data suggest that the end of the nesting season was characterised by a decline in plasma steroids (CORT and T+DHT) and plasma triglycerides and a gradual increase in plasma proteins.
Data from numerous green turtle nesting populations indicate significant variation in the numbers of females breeding each year, and most authors concur that this variation has a nutritional base. I collected plasma samples from non-vitellogenic females at three different foraging areas across multiple years. Analysis of levels of plasma triglyceride, protein and cholesterol suggests that the physiological condition of female sea turtles varies annually and between locations. More specifically, females residing at Heron Reefs showed lower plasma triglyceride and higher cholesterol than females at Moreton Bay and Shoalwater Bay. Moreover at both Heron Reefs and Moreton Bay, levels of plasma triglyceride were highest in the El niño year (1997). Together, these data strengthen claims that, breeding rates are strongly tied to proximal conditions in the foraging environment.
The results from this study illustrate the complex physiological system, and pathways that maximise reproductive output in sea turtles. Although their specific roles are not clear, it is evident that the interrenal hormones CORT and EPI probably play an integral role in lipid mobilisation and maximisation of reproductive output. My findings indicate that the importance of further research on reproductive cycles and physiological systems in order to facilitate future management of sea turtles in their constantly changing environment.