Metabolic Rates and Thermal Thresholds of Embryonic Flatback Turtles (Natator depressus) from the North West Shelf of Australia

Nest microclimates influence embryonic development and survival in many lineages, including reptiles with temperature-dependent sex determination. These microclimates are dependent on physical drivers and biological processes, such as embryonic metabolism, that generate heat. The flatback turtle (Natator depressus) has among the largest hatchlings of the seven extant sea turtle species, making it an excellent candidate for quantifying the contribution of embryonic metabolism to the nest microclimate. Consequently, we measured embryonic metabolic rates, development rates, and the relationship between temperature and sex determination for a N. depressus population nesting at Cemetery Beach in Western Australia, a mainland beach characterized by high sand temperatures. Total oxygen consumed at 29.5 degrees C during an average 52-d incubation period was 2,622 mL, total carbon dioxide produced was 1,886 mL, and estimated embryonic heat production reached 38 mW at 90% of development. Adjustment of metabolic rates to 32 degrees C and 34 degrees C increased peak heat production by 18% and 27%, respectively. The pivotal temperature (T-PIV) producing an equal sex ratio was 30.3 degrees C, mixed sexes were produced between 29.3 degrees C and 31.2 degrees C, and only females were produced above 31.2 degrees C. The T-PIV was similar (within 0.2 degrees C) to that of an island rookery within the same genetic stock (North West Shelf), but the peak development rate (2.5% d(-1)) was estimated to be achieved at a temperature ~2.5 degrees C higher (34.7 degrees C) than the island rookery. Our results add to a growing consensus that thermal thresholds vary among sea turtle populations, even within the same genetic stock. Furthermore, we show that metabolic heat will have an appreciable impact on the nest microclimate, which has implications for embryonic survival and fitness under a future climate with warmer sand temperatures.