Effects of the thermal environment on the temporal pattern of emergence ofhatchling loggerhead turtles Caretta caretta

Sea turtles deposit their eggs in beaches at a depth of about 50 cm and lea ve them to incubate for approximately 60 d. After that time, hatchlings eme rge from the sand at night, both to avoid predation and to prevent overheat ing. Three hypotheses have been proposed regarding the thermal cues involve d in emergence. Hatchlings emerge (1) after sand temperature falls below a critical threshold, (2) after a negative thermal gradient in the sand above the nest has been created, making upper sand cooler than sand below, or (3 ) in response to a rapid decrease in the temperature of the nest column. Th is study evaluated the cue(s) that loggerhead sea turtle Caretta caretta ha tchlings use to emerge into a safe thermal environment. We collected therma l data at 5 depths in the sand from 150 loggerhead nests that had been relo cated in Broward County, Florida. Hatchlings had a significant effect on sa nd temperatures due to metabolic heat. Therefore, to evaluate thermal cues, temperatures should be measured in nest columns and not adjacent to nests, as in previous studies. A critical threshold temperature existed, above wh ich hatchlings did not emerge. The threshold temperature at 0 cm was 32.4 d egrees C, which is similar to upper thermal thresholds of swimming activity for loggerhead and green turtle Chelonia mydas hatchlings measured in othe r studies. Most hatchlings emerged after a reversal in the thermal gradient s between sand depths, but no particular temperature differential existed b etween those depths that cued emergence. A delay occurred after both thresh old and reversal cues before hatchlings emerged, which may protect hatchlin gs under rapidly changing environmental conditions. Most hatchlings emerged sooner after the threshold temperature had been reached than after the tem perature gradient had been reversed, but the 2 cues cannot be completely di stinguished without further research. Rate of temperature decline was not a cue because most hatchlings emerged when rates were close to zero. We conc lude that the critical threshold temperature is the most probable cue and t hat the most probable mechanism for controlling time of emergence is physio logical - that is, a thermal inhibition of coordinated muscle movement so t hat emergence from the sand is only possible below a critical temperature t hreshold. Emergence data from in situ nests support these thermal patterns in relocated nests. These results have important implications for managemen t of beach habitat for nesting sea turtles. Human impacts, such as beach re nourishment and beach-front development, can change the thermal characteris tics of the nesting environment, and therefore affect temporal emergence pa tterns of hatchlings.