Does predator swamping promote synchronous emergence of turtle hatchlings among nests?

The spatial and temporal synchrony observed for many behaviors is often viewed as deriving from selection to swamp predators, ensuring that at least some individual prey survive. However, this adaptive explanation has rarely been put to experimental test. We conducted 2 field experiments to evaluate the importance of alternative mechanisms (predator swamping and prey switching) to explain synchronous emergence among nests by red-eared slider turtle hatchlings (Trachemys scripta elegans) that initiates the critical migration from nests to water. In the 1998 experiment, we released 1400 hatchlings simultaneously and allowed another 1400 hatchlings to emerge independently from artificial nests. Hatchlings emerging from nests were just as likely to be recaptured as those in the mass release. Moreover, survivorship was highest among nests whose hatchlings began emerging early compared with hatchlings that emerged from nests later during migration. In the 2006 experiment, we released 2 groups of 100 hatchlings 8 days apart and a third group of 300 hatchlings 13 days later. We recaptured more than twice as many hatchlings from the first group as from the similar sized second group and from the larger third group. If the predator swamping mechanism had been important, we should have seen improved survivorship for the hatchlings in the mass release in the first experiment and for hatchlings in the third group in the second experiment. Overall, the results reject predator swamping as an explanation. Our findings instead suggest that synchronous emergence of turtle hatchlings from nests across a population maximizes individual survival by minimizing exposure to prey-switching predators.