Mechanisms that produce and maintain a foraging dichotomy in adult loggerhead turtles (Caretta caretta): Which tactic has higher fitness, oceanic or neritic?

Abstract

Habitat use decisions by mature individuals may be subject to early experiences. In the early part of their lifecycle, loggerhead sea turtles (Caretta caretta) forage on planktonic prey in oceanic waters (>200 m depth). It is generally accepted that most adults move to neritic waters (<200 m depth) later and forage on benthic prey; however, some adults within some populations continue to forage on planktonic prey in oceanic waters. Although this foraging dichotomy is suggested to be maintained environmentally, it is necessary to prove the unequal fitness between the two types of foragers by accurately quantifying fitness. In this study, the survival rates during the period from aboveground emergence to first reproduction were calculated in three scenarios with varying annual survival rates and ages at first reproduction for both types of foragers, based on a hypothetical mechanism that leads to foraging dichotomy, and the results were incorporated into fitness comparisons. Except in the case of (1) high annual survival rates and the small differences in age at first reproduction and (2) young age at first reproduction, oceanic foragers generally had higher fitness than neritic foragers, providing support for the environmental maintenance of the foraging dichotomy. The higher fitness of oceanic foragers might also suggest that the foraging dichotomy is maintained by status-dependent selection in relation to growth conditions during early oceanic life.