Ethological and phenotypic divergence in insular fire salamanders: diurnal activity mediated by predation?

Abstract

Phenotypic divergence can occur through both local adaptations and phenotypic plasticity in response to particular environmental pressures. Holocene land-bridge islands harbor recent isolated populations that offer an excellent natural framework to study the microevolutionary processes involved in rapid or ongoing phenotypic shifts. Two insular populations (Ons and San Martino) of Salamandra salamandra in NW Iberian Peninsula exemplify these phenotypic shifts with a rapid evolutionary transition in reproductive mode and genetic differentiation since their isolation ca. 8000 years ago. We evaluate parallel phenotypic changes in body size and behavior. In particular, we (1) collected observational fieldwork data during diurnal and nocturnal transects in both islands for over a decade (2004–2016); (2) investigated the climatic conditions associated to fire salamanders’ activity; (3) compared physical condition between insular and mainland populations; and (4) used plasticine models and camera trapping to identify potential predators that might cause the observed diurnal activity in San Martino Island. Our 13-year-long study shows that salamanders from San Martino show mainly diurnal activity and are significantly smaller than salamanders from Ons and nearby continental populations. Overall, we present an exceptional case of behavior and phenotypic differentiation of an insular population of fire salamanders and suggest that this unusual behavior in the small population of San Martino might be triggered by a response to predator pressure exerted by a dense population of Rattus rattus.