Convergent digestive adaptation to resource limitation in an insular lizard across a microgeographic archipelago

Island systems provide valuable natural laboratories for investigating how ecological constraints drive physiological evolution. Although dietary shifts are known to influence morphological divergence in insular lizards, their consequences for digestive physiology remain insufficiently understood. Using 10 wild populations of the Skyros wall lizard (Podarcis gaigeae) across the Skyros Archipelago, we tested whether fine-scale variation in habitat and prey availability predicts divergence in digestive performance. We quantified arthropod abundance, gastrointestinal morphology, gut passage time, digestive enzyme activity and apparent digestive efficiency (ADE) under standardized laboratory conditions. Islet populations consistently exhibited slower gut passage, longer gastrointestinal tracts, higher protease and lipase activities, and greater ADE relative to populations on the main island of Skyros, indicating enhanced digestive performance in resource-limited environments. AIC_C-based model selection showed that prey availability and habitat type, rather than geographic distance or genetic relatedness, best explained patterns of digestive divergence among populations. These results demonstrate repeated and independent adaptive responses to resource scarcity and highlight digestive plasticity as a key mechanism facilitating persistence in insular habitats.