Abstract
Exploring a trade‐off between quantity and quality of offspring allows differences in the fitness between alternative life histories to be accurately evaluated. We addressed the mechanism that maintains alternative life histories (small oceanic planktivores vs. large neritic benthivores) observed in a loggerhead sea turtle (Caretta caretta) population, which has been suggested to be environmental, based on the lack of genetic structure and a large difference in reproductive output. We examined whether maternal foraging habitat affects offspring quality, by measuring the morphology, emergence success, and righting response of hatchlings following incubation in a common open sand area over the whole nesting season at Yakushima Island, Japan, and by recording early growth and survival of offspring that were reared in a common environment at a Japanese aquarium. Furthermore, we tested whether sea turtles adjust egg size in response to temporal shifts of the incubation environment. There were no significant differences in any hatchling traits between oceanic and neritic foragers (which were classified by stable isotope ratios), except for clutches laid during the warmest period of the nesting season. There were also no significant differences in the growth and survival of offspring originating from the two foragers. The size of eggs from both foragers significantly increased as the season progressed, even though the rookery had heavy rainfall, negating the need to counteract heat‐related reduction in hatchling morphology. In comparison, the sizes of adult body and clutches from both foragers did not vary significantly. The results further support our previous suggestions that the size‐related foraging dichotomy exhibited by adult sea turtles does not have a genetic basis, but derives from phenotypic plasticity. Adjustment in reproductive investment may be associated with: (1) predation avoidance, (2) founder effect, and/or (3) annual variation in egg size.
Highlights
Intrapopulation variation in habitat use and its resultant alternative life histories are common in mobile animals (Bolnick et al, 2003)
In this study, we examined whether maternal foraging habitat affects hatchling quality, which was measured as the morphology, emergence success, and righting response of hatchlings following the incubation of clutches in a common open sand area under a wide range of ambient temperatures
The current study showed that differences in the use of foraging habitats by adult female sea turtles do not affect the quality of TA B L E 1 Comparisons of body size and egg and hatchling characteristics between oceanic and neritic foraging loggerhead turtles (Caretta caretta) nesting at Yakushima Island, Japan, during 2013–2016 (Hatase et al, 2014, 2015; this study)
Summary
Intrapopulation variation in habitat use and its resultant alternative life histories are common in mobile animals (Bolnick et al, 2003). Alternative life histories observed in a sea turtle population were suggested to be environmentally maintained, based on the genetic similarity at mitochondrial DNA sequences and microsatellite loci (Watanabe et al, 2011), and a large difference in offspring number produced between two alternative phenotypes (Hatase, Omuta, & Tsukamoto, 2013). To counteract the heat-related reduction in hatchling morphology, sea turtles that nest on temperate rookeries might invest more resources in individual eggs during the warmer period of the nesting season, because larger hatchlings generally hatch out from larger eggs (Pinckney, 1990; Wallace et al, 2007) We tested this hypothesis by examining seasonal/annual variations in egg size, in relation to variations in the sizes of adult body and clutches. We discuss how alternative life histories are maintained in a sea turtle population, and how sea turtles cope with environmental changes
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