Scute variation in loggerhead turtle hatchlings: genetic and environmental influences

Scute anomalies in turtles are commonly interpreted as developmental responses to environmental stressors. However, previous studies indicate that these traits may also reflect underlying genetic variation. The present study investigates the genetic and environmental influences on carapacial scute variation in captive-bred loggerhead turtle ( Caretta caretta ) hatchlings with known parental lineage, using a long-term dataset. By integrating paternity assignments from eight microsatellite loci with incubation duration as a proxy for thermal exposure, the effects of maternal and paternal identity, incubation duration, and bilateral asymmetry on scute count variation were evaluated. Comparisons among half-siblings revealed parental influences on marginal, costal, and nuchal scute counts, whereas successive full-sibling clutches showed that incubation duration alters anomaly frequency. Generalized linear mixed models indicated a threshold-like expression pattern in scute count variation, with parental identity and incubation duration significantly predicting scute variation. Scute anomalies were not associated with hatchling body size, suggesting that scute count is not simply a function of overall growth. Together, these results support a threshold model in which scute anomalies represent polymorphic traits shaped by probabilistic inheritance and thermal conditions. This framework complements the traditional classification of scute anomalies by considering them as naturally occurring polymorphisms, rather than entirely pathological conditions. Recognizing scute variation as a natural polymorphism has important relevance to sea turtle conservation, particularly under climate change, as rising incubation temperatures are likely to influence trait expression through interacting with underlying genetic and developmental variation.