Abstract 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.

中文翻译：

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个体大小对海龟离合器大小和幼体健康性状的影响

摘要 适应度理论试图通过理解赋予最高生存率的设计而不是生存本身来解释生物体如何生存。本研究调查了表型变异对红海龟和棱皮海龟繁殖策略（离合和卵大小）的影响以及大小对幼龟健康性状的影响。我们通过测试两个假设检查了较大的雌性是否会产生更大的适应度更高的幼体；最佳鸡蛋大小理论，越大越好的假设。该研究调查了母体身长如何影响生殖输出（离合器大小、鸡蛋大小），以及孵化形态如何影响其性能。通过测量健康、爬行和游泳速度的两个相关因素来分析孵化性能，因为这两个特征通过最大限度地减少在高度早熟地区的暴露时间对它们的生存产生积极影响。发现在南非筑巢的红海龟和棱皮龟的繁殖策略支持最佳蛋大小理论，即较大的海龟产生更大的窝但不会产生更大的蛋。无论母体长度如何，卵子大小都是恒定的。雌性体型对离合器大小变异性的低贡献表明其他因素正在影响筑巢季节的雌性繁殖量。幼体健康评估试验表明，背甲和鳍状肢较长的幼体游得更快。这增加了越来越多的证据表明较大的幼体更有可能存活下来，因为它们在高捕食风险区域花费的时间较少（即 海滩和近岸），因此假设比较小的更健康。如果生产出适应度较低的较小幼龟并且传播能力降低从而生存可能性降低，则这些发现对总体种群增长/恢复有影响。