Embryonic aestivation and emergence behaviour in the pig-nosed turtle, Carettochelys insculpta

Two related aspects of hatchling emergence were studied in a population of pig-nosed turtles (Carettochelys insculpta) in northern Australia. Using em ergence phenology data, nest temperatures, historical weather data, and a d evelopmental model, we tested the hypothesis that delayed hatching occurs i n C. insculpta, and that such a delay would allow hatchlings to time their emergence to match the onset of the wet season. Carettochelys insculpta hat chlings emerged, on average, 17 days after dates predicted from a developme ntal model. Combined with observations of hatchlings remaining in eggs unti l emergence, these results confirmed delayed hatching in nature. This delay was synchronized with initial river rises associated with the onset of wet -season rains, and is consistent with published criteria for embryonic aest ivation. On a diel scale, we generated predictions of two potentially compe ting models of nocturnal emergence in hatchling turtles based on the knowle dge that air temperatures decrease with season during the emergence period. A test of these predictions for C. insculpta produced ambiguous results. H owever, further analysis indicated that C. insculpta, and probably other no cturnally emerging turtle species, respond to a decline in diel temperature rather than to an absolute temperature. The former would ensure nocturnal emergence, while the latter is experienced during the day as well as at nig ht. Nocturnal emergence may be associated with nesting in open microhabitat s.