Foraging area, not trophic position, is linked to head size variation in adult female loggerhead turtles

Variation in trophic morphology among individuals within a population may suggest intrapopulation variation in diet and resource use. In such situations, individuals may occupy more specialized roles within a generalist population. Among loggerhead turtles (Caretta caretta), adults exhibit variation in head size, a likely predictor of bite-force generation and therefore feeding performance. We investigated several factors that may affect variation in head size for female loggerheads nesting on Wassaw Island, Georgia, USA. We first quantified the amount of variation in head size (width, length and depth) that was explained by differences in body size. We then investigated whether variation in absolute head size and variation in head size not explained by body size (relative head size) were related to foraging area, trophic position or both using stable isotope analysis of carbon and nitrogen. We found that greater than 50% of the variation in head size was not explained by body size and that foraging area contributed at least partially to differences in head size, while trophic position was not correlated with differences in head size. These results suggest that there may be some trophic specialization within foraging areas, such that turtles foraging in distinct areas maintain different relative head sizes as a result of differences in diet and resource availability. However, the variation in head size that does exist within a foraging area does not appear to be related to specialization at different trophic positions. Understanding factors that affect variation in trophic morphology can provide valuable insight into the trophic interactions of a species and reveal mechanisms that reduce intraspecific competition.