Bigger clutch sizes save offspring energy during nest escapes

Abstract

Hatchling turtles typically emerge from underground nests in groups, so the nest escape process may represent another example of animals sharing a task (in this case, digging out of a nest) to save on individual energy expenditure. Previous studies have reported the energetic cost of embryonic development across chelonian taxa, but none has quantified the extra amount of energy needed to escape the nest. Brisbane river turtle (Emydura macquarii signata) hatchlings were found to fuel this activity by using approximately 50 % of their residual yolk energy content. An open-flow respirometry system was used to quantify the effect of clutch size on an individual’s energetic cost while digging out of the nest. The energetic cost of nest escaping 15 cm upward in the fine moist sand was calculated to be between 0.34 and 2.32 kJ per individual depending upon the number of hatchlings digging together. The energetic cost 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 amongst clutch mates to reduce individual energy expenditure. The reduced energetic cost associated with large cohorts was chiefly caused by the shorter time taken to dig out of the nest by larger numbers of individuals. We conclude that synchronous digging activity of many individuals during nest escape evolved not only to facilitate quicker nest emergence but also reduce the energetic cost to individuals.