Sex‐dependent implications of primary productivity and conspecific density on geographical body size variation in a newt: disentangling local, large scale and genetic factors

AIM: Geographical body size variation has interested biologists since the 19th century. However, there is no consensus regarding the mechanisms involved, especially in amphibians. At the intraspecific level, different approximations and hypotheses have addressed this problem, but no study has integrated information at different spatio‐temporal ecological and (phylo)genetic scales whilst considering differences between sexes. We investigated the mechanisms involved in geographical body size variation in the Iberian newt (Lissotriton boscai), accounting for the main hypotheses potentially explaining variation in this phenotypic trait in amphibians. LOCATION: Iberian Peninsula. METHODS: We used a novel multivariate technique (partial least squares regression) that accounts for interdependence amongst variables whilst allowing for the inclusion of multiple local and macroscale predictors. We considered intraspecific genetic differentiation including information from the mitochondrial and nuclear genomes in populations representative of the species’ distribution. We addressed causal hypotheses of heat conservation, temperature‐related development, water and food availability, seasonality, competition and predation. RESULTS: Annual primary productivity and intraspecific competition were of greatest relevance to explain body size variation in females, whilst seasonality of primary productivity was more important in males. Differences between sexes are interpreted based on previous experiments on the behavioural ecology of this species. MAIN CONCLUSIONS: This study shows that sex‐dependent implications of competition for food drive geographical body size variation in an urodele amphibian, supporting the density‐resource hypothesis. Our results suggest that food availability, conspecific density and competition are important in explaining intraspecific body size variation in amphibians. This study highlights how selecting model species with an experimental background can help interpret geographical variation in phenotypic traits. We emphasize the value of simultaneously considering alternative hypotheses of phenotypic variation at different ecological and (phylo)genetic spatio‐temporal scales in revealing potentially hidden relationships.