The influence of individual size on clutch size and hatchling fitness traits in sea turtles

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Fitness theory attempts to explain how organisms survive by understanding not survival as such but the design that confers the highest survival. This study investigated the effects of phenotypic variation on the reproductive strategy (clutch and egg size) of loggerhead and leatherback sea turtles as well as the influence of size on hatchling fitness traits. We examined whether larger females produced larger hatchlings of higher fitness by testing two hypotheses; the optimal egg size theory, and the bigger is better hypothesis. The study investigated how maternal length influences reproductive output (clutch size, egg size), and how hatchling morphology affects their performance. Hatchling performance was analysed by measuring two correlates of fitness, crawling and swimming speeds, as these two traits positively influence their survival by minimising the time of exposure in highly predated areas. The reproductive strategy of both loggerhead and leatherback turtles nesting in South Africa was found to support the optimal egg size theory whereby larger turtles produce larger clutches but not larger eggs. Egg size was constant irrespective of maternal length. The low contribution of female body size to clutch size variability suggests that other factors are affecting female reproductive output during a nesting season. Hatchling fitness assessment trials demonstrated that hatchlings with longer carapaces and flippers lengths swam faster. This adds to the growing body of evidence suggesting that larger hatchlings are more likely to survive because they spend less time in high predation risks areas (i.e. beach and nearshore) and thus assumed fitter than smaller ones. These findings have implications for overall population growth/recovery if smaller hatchlings of lower fitness are produced and have decreased dispersal abilities and thus lower likelihood of survival.