The Evolutionary Significance of Delayed Emergence from the Nest by Hatchling Turtles

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Abstract
Temperate zone turtles are generally believed to lay eggs in the spring or summer and emerge as hatchlings in the fall. The phenomenon of overwintering in the nest as hatchlings or advanced embryos has been reported anecdotally and has often been considered as unusual or characteristic only of northern U.S. populations. However, field data on almost 400 hatchlings of five species from a climatically mild environment in the southeastern U.S. reveal that overwintering typifies most species of aquatic turtles indigenous to the region. A thorough review and consolidation of published literature indicates that delayed emergence from the nest by turtles is more geographically and phylogenetically wide-spread than commonly believed. In the habitats studied, late fall temperatures were actually higher than those at times of emergence in early spring. Also, emergence times were not correlated with rainfall. Therefore, we reject conventional explanations of the 'cold temperature' and 'dry weather trap' hypotheses proposed by others to account for the emergence of hatchlings in the spring. We submit that delayed emergence is a strategy of those species in which high environmental variability and uncertainty exist for hatchlings that emerge immediately upon hatching. Thus, natural selection favors individuals awaiting an environmental cue (such as wintertime in temperate regions or heavy rainfall in the tropics) which indicates a high probability of ensuing favorable conditions. Individuals opting for immediate emergence are more likely to enter a high risk environment and are therefore usually selected against. Multiple clutches are implicated as a major cause of hatchling uncertainty about environmental timing of emergence.

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