Experimental analysis of an early life-history stage: direct or indirect selection on body size of hatchling turtles?

1. Linking phenotype, performance and fitness has proven difficult to document despite the tremendous influence of this concept on microevolutionary research. One of the major issues involves identifying the target of selection when phenotype and performance are correlated. 2. In a replicated manipulative field experiment, we evaluated this model by testing the hypothesis that predation on hatchling turtles during the critical period of migration from their terrestrial nests to their future aquatic home is random with respect to phenotype after controlling for performance. We decoupled migration performance from body size by exposing hatchling red-eared slider turtles Trachemys scripta elegans of all body sizes to environmental conditions and potential predators for four different time durations. 3. Body size (measured at initiation of the experiment either as carapace length or as mass) of turtles that were not recovered alive was slightly smaller than that of turtles that survived the experiment. More significantly, however, the number of turtles that survived decreased with increasing exposure duration, particularly after 60 h and was unrelated to meteorological conditions. 4. Our experiment thus suggests that selective predation largely acts indirectly on body size of hatchling turtles during this important life stage, consistent with the random predation hypothesis. As enhanced migration performance minimizes exposure of larger hatchlings to predators under natural conditions upon leaving the nests, our experimental results provides strong support for the model linking phenotype, performance and fitness.