Environmentally cued hatching allows embryos to alter the time of hatching in relation to environment through phenotypic plasticity. Spatially variable temperatures within shallow nests of many freshwater turtles cause asynchronous development of embryos within clutches, yet neonates still hatch synchronously either by hatching early or via metabolic compensation. Metabolic compensation and changes in circadian rhythms presumably enable embryos to adjust their developmental rates to catch up to more advanced embryos within the nest. Hatchlings of the North American freshwater turtle Chrysemys picta usually overwinter within the nest and emerge the following spring, but still hatch synchronously via hatching early. Here, we used rates of oxygen consumption and heart rate profiles to investigate the metabolic rates of clutches of C. picta developing in conditions that result in asynchronous development to determine if compensatory changes in metabolism occur during incubation. Embryos hatched synchronously and displayed circadian rhythms throughout incubation, but exhibited no evidence of metabolic compensation. Phenotypic traits of hatchlings, including body size and righting performance, were also not affected by asynchronous development. We conclude that less developed embryos of C. picta hatch synchronously with their clutch-mates by hatching early, which does not appear to inflict a fitness cost to individuals. The ultimate mechanism for synchronous hatching in C. picta could be for hatchlings to ensure an optimal overwintering position within the center of the nest. Consequently, immediate fitness costs will not hinder hatchling survival. The geographic location, as well as environmental and genetic factors unique to populations, can all influence hatching behavior in turtles through phenotypic plasticity. Hence, synchronous hatching is an adaptive bet-hedging strategy in turtles, but the mechanisms to achieve it are diverse.
Keywords: Circadian rhythms; convergent evolution; embryonic development; environmentally cued hatching (ECH); metabolic compensation; turtle.
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