Early swimming activity of hatchling flatback sea turtles Natator depressus: a test of the ‘predation risk’ hypothesis

Abstract

Marine turtle hatchlings emerge from nests on oceanic beaches, crawl to the surf zone and migrate offshore. Predators in shallow water can take many hatchlings, but once the turtles reach deeper water, both encounters with predators and mortality rates probably decline. Behavioral stud- ies have demonstrated that hatchlings show changes in swimming activity as they migrate offshore. During the first 24 h of migration, most species swim continuously (the 'frenzy'), thereby minimizing their time in shallow waters; however, swimming activity later declines in duration and vigor, espe- cially at night (the 'postfrenzy'). One interpretation of these differences is that hatchling migratory behavior evolved in response to the threat of predators (the 'predation risk' hypothesis). To further test this hypothesis, we quantified the daily swimming activity shown by the flatback Natator depres- sus, the only marine turtle that lacks an oceanic phase in its development. Instead, the hatchlings remain in relatively shallow Australian (continental shelf) waters where they may frequently encounter predators. We speculated that because of these encounters, flatback hatchlings might have evolved activity patterns that show little, if any, decline during migration. Over 4 d of laboratory observations, flatback activity at night declined by <13%. Over the same time period, nocturnal activity declined by 60 to 95% (depending on species) in other marine turtles with an oceanic phase in their life history. Our data therefore support the hypothesis that predation played an important role in shaping the evolution of hatchling migratory behavior, although they do not provide direct evi- dence of a cause and effect relationship.