Nest environment and hatchling fitness correlates in the sea turtles Caretta caretta and Natator depressus

Abstract

Marine turtles have a complex lifecycle and face threats in both marine and terrestrial environments. Nesting females lay a large number of eggs, very few of which produce hatchlings that survive to reach breeding age. As hatchlings cross the beach, they are exposed to predation, disorientation, dehydration and debris on the beach. When hatchlings enter the water they are exposed to aquatic predators. Hatchlings do not actively avoid predators, or defend themselves, so simply being able to move quickly through this environment would increase their chance of survival. A number of variables can affect hatchling locomotor performance, and this thesis examines three of these: incubation temperature, scute pattern and rookery location in two species of sea turtle, loggerhead (Caretta caretta) and flatback turtles (Natator depressus). The first part of this thesis focuses on scute pattern. Scutes cover the carapace of turtles and tortoises, and each species has a modal pattern. Deviations from this modal pattern are more common in hatchlings than in adult turtles, suggesting that hatchlings with non-modal scute patterns have higher mortality rates, but this hypothesis has not been tested previously. Hatchlings with modal scute patterns were larger and heavier than hatchlings with non-modal scute patterns in both species examined, however this size difference did not translate into a difference in terrestrial locomotor performance. However, N. depressus hatchlings with the modal scute pattern produced more thrust than hatchlings with non-modal scute patterns in the first 40 minutes of swimming, which may give them an advantage over hatchlings with non-modal scute patterns. The second part of this thesis focuses on incubation temperature. Marine turtle eggs successfully incubate within a narrow range of temperatures. Even within the viable developmental temperature range, it has been proposed that hatchlings from eggs incubated close to the thermal tolerance limits may have reduced fitness compared to hatchlings from eggs incubated at intermediate temperatures. In this study, C. caretta hatchlings from hot nests were less likely to emerge from the nest, were smaller, and also performed poorly during crawling and swimming trials compared with hatchlings incubated in moderate temperature nests. The third part of this thesis focuses on differences in hatchling morphology and locomotor performance between two N. depressus rookeries representing two separate ‘evolutionarily significant units’ (ESUs). Hatchlings at the two sites differed in size, but this size difference did not translate into a difference in either crawling speed or self-righting ability. This thesis contributes to our understanding of factors that influence marine turtle hatchling fitness, both environmental and morphological. This information can be used in the monitoring of endangered marine turtle populations.