Bergmann's rule in the oceans? Temperature strongly correlates with global interspecific patterns of body size in marine mammals

AimBergmann's rule remains unexplored in marine mammals. We first examine at a global extent whether these organisms show the same interspecific pattern reported for terrestrial mammals and then evaluate the influence of current environmental conditions and human impacts on the observed patterns. LocationGlobal. MethodsWe used range maps to document interspecific body size gradients and examined six environmental and human-based hypotheses. We analysed the data using a comparative cross-species method and a spatially explicit assemblage approach at three different grain sizes (200 km x 200 km, 400 km x 400 km and 800 km x 800 km). The associations between hypothesis-linked predictors and body size were analysed through simple and multiple regressions that controlled for both spatial and phylogenetic autocorrelation. ResultsWe detected clear global latitudinal body size gradients, following a Bergmannian pattern (i.e. increasing size polewards). Consistently across methodological approaches (cross-species and assemblage analyses) and grain sizes, sea surface temperature is the best predictor. Spatially, the temperature-size relationship is stronger in the Southern than in the Northern Hemisphere. Pinniped body sizes are critically constrained by temperature world-wide whereas cetacean size clines show a weaker, albeit dominant, association with temperature. Main conclusionsAs in terrestrial mammals, our findings show that ambient temperature better explains interspecific body size patterns in cetaceans, and especially pinnipeds, world-wide. Large-bodied species are favoured in colder environments, in accordance with Bergmann's rule and the heat conservation hypothesis. However, our analyses also reveal a relevant role for salinity and primary productivity in migratory cetacean species. The large body sizes of baleen whales are essential for migration, for survival during fasting periods and minimizing the effects of temperature variation. This finding highlights the importance of spatially and phylogenetically explicit deconstructive approaches, considering alternative hypotheses to the traditional physiological mechanism, to gain a better understanding of Bergmann's rule.