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

Abstract

Sea turtles deposit their eggs in beaches at a depth of about 50 cm and leave them to incubate for approximately 60 d. After that time, hatchlings emerge from the sand at night, both to avoid predation and to prevent overheating. Three hypotheses have been proposed regarding the thermal cues involved 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. This study evaluated the cue(s) that loggerhead sea turtle Caretta caretta hatchlings use to emerge into a safe thermal environment. We collected thermal data at 5 depths in the sand from 150 loggerhead nests that had been relocated in Broward County, Florida. Hatchlings had a significant effect on sand 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 which hatchlings did not emerge. The threshold temperature at 0 cm was 32.4°C, which is similar to upper thermal thresholds of swimming activity for loggerhead and green turtle Chelonia mydas hatchlings measured in other studies. Most hatchlings emerged after a reversal in the thermal gradients between sand depths, but no particular temperature differential existed between those depths that cued emergence. A delay occurred after both threshold and reversal cues before hatchlings emerged, which may protect hatchlings under rapidly changing environmental conditions. Most hatchlings emerged sooner after the threshold temperature had been reached than after the temperature gradient had been reversed, but the 2 cues cannot be completely distinguished without further research. Rate of temperature decline was not a cue because most hatchlings emerged when rates were close to zero. We conclude that the critical threshold temperature is the most probable cue and that the most probable mechanism for controlling time of emergence is physiological - that is, a thermal inhibition of coordinated muscle movement so that emergence from the sand is only possible below a critical temperature threshold. Emergence data from in situ nests support these thermal patterns in relocated nests. These results have important implications for management of beach habitat for nesting sea turtles. Human impacts, such as beach renourishment and beach-front development, can change the thermal characteristics of the nesting environment, and therefore affect temporal emergence patterns of hatchlings.