Reproductive strategies of the most geographically isolated Trachylepis support predictions of the island syndrome

The island syndrome hypothesis predicts that insular organisms exhibit phenotypic differences from their mainland counterparts, particularly in reproductive strategies. Island populations are expected to have shorter breeding seasons, smaller clutches, larger offspring, and lower reproductive frequency. Although these patterns are well-documented in birds and mammals, evidence from island squamates is conflicting. The skink genus Trachylepis, which includes many island endemics in the Atlantic and Indian oceans, offers a model for testing these predictions in squamates. However, robust reproductive data are scarce for most insular Trachylepis. We characterized the reproductive phenology and output of the Noronha skink (Trachylepis atlantica), an endemic species to the remote Fernando de Noronha Archipelago (northeastern Brazil) and the westernmost dispersion of the genus in the Atlantic. We analyzed macroscopic and microscopic data from free-ranging, museum, and zoo-housed individuals, and compared our findings with available data for other Trachylepis species. Our results support the predictions of the island syndrome. Both sexes show a seasonal breeding pattern concentrated in the dry season, lasting approximately seven months—substantially shorter than in most tropical mainland congeners. Free-ranging females produce the smallest clutches (typically two eggs) and the largest eggs relative to body size in the genus. Female reproduction occurs at least biennially, which is lower than in most congeners. These reproductive patterns primarily reflect evolutionary responses to an insular environment historically characterized by low predation pressure and high intraspecific competition. These patterns are further shaped by resource seasonality and phylogenetic constraints, illustrating the multifactorial nature of reproductive evolution in insular reptiles. However, these same reproductive traits may also reduce the species’ resilience to increasing threats that are already impacting the population, including invasive predators and environmental change. Conservation actions are therefore needed to protect this evolutionary unique insular lineage.