The energetics of nest escaping by turtle hatchlings

Abstract

Turtles lay their eggs on land in underground nests and embryos take about two months to incubate to hatching. During this time, turtle embryos grow in size and consume much of the yolk that was deposited in the egg when it was laid. During the hatching process, and sometimes continuing for a period of 24-48 h after hatching, the residual yolk is absorbed into the abdominal cavity. The residual yolk acts as an energy reserve that powers the nest escape process and hatchling growth and development until feeding begins, which can be days or weeks after nest escape. Previous studies have reported the energetic cost of embryonic development across Chelonian taxa, but none has quantified the energy needed to escape the nest after hatching. This study used open-flow respirometry and bomb calorimetric analysis to measure metabolic expenditure of turtle hatchlings during simulated nest escape. Digging activity was monitored simultaneously with metabolic rate to determine the pattern of digging effort, energy expenditure, and the total energetic cost per individual during nest escape. Two species of chelonian were used in this study; the Brisbane river turtle (Emydura macquarii signata) and green turtle (Chelonia mydas). The first part of the study quantified the energy expended by hatchlings during nest escape. The Brisbane river turtle and green turtle hatchlings were found to fuel this activity by using approximately 5-36 % and 11 – 68% respectively of their residual yolk reserve energy. This energy expenditure was then put into an energy budget context by comparing it to the energy in a freshly laid egg, the energy used during embryonic development, and the energy contained within the residual yolk of newly hatched hatchlings. The second part of the study examined the phenomenon of ‘social facilitation’, a term that has been coined to describe how interactions between nest mates facilitate the nest escape process. The hypothesis behind this concept, that escaping the nest in a larger group is more advantageous than in a smaller group, was tested by splitting a clutch into different sized groups and measuring the time taken to dig a set distance upwards, and calculating the per-individual metabolic cost of nest escape. Both the time taken to escape the nest and the energetic cost of nest escape decreased as the number of individuals digging together increased and thus supports the ‘social facilitation’ hypothesis which suggests hatchlings cooperate to share the workload of digging out of the nest among clutch mates to reduce individual energy expenditure. Lastly, because freshwater turtle can construct their nest in a wide range of soil types, the time taken and the energy required by the same number of hatchlings to dig through two different soil types was quantified. Brisbane river turtle hatchlings digging through fine sand escaped faster and spent less energy than hatchlings through coarse sand. Moreover, larger group size provided a clear energetic advantage while digging out of the nest in both soil types tested.