The evolution of aquatic mammals toward a nearly universal large size? Evidence from phylogenetics and fossils

Most mammal species live on land, but the largest mammals live in the oceans. Aquatic and terrestrial habitats clearly impose differing selective pressures on body size. However, the quantitative study of body size evolution in mammals and other major animal clades typically focuses on either terrestrial or marine clades independently, thus failing to capture the dynamics of size evolution associated with the transition between land and water. Consequently, the extent to which the rate, magnitude, and outcome of size change associated with habitat transitions are shared among clades remains unknown, leaving open the question of whether the apparently common phenomenon of size increase associated with the acquisition of an aquatic lifestyle reflects idiosyncratic responses of individual clades versus a common response to universal constraints. • 3 out of 4 mammal groups living in aquatic environments have larger optimal body sizes than their terrestrial counterparts. • Results suggest the existence of a body size attractor (~500 kg) that has been discovered independently by these three aquatic clades, coupled with shared relatively rapid selection toward, and limited deviation from, this attractor (not shown here). • Some groups may still be getting larger, although analyses suggest there may be an upper limit without help from key innovations (e.g. baleen). • The sustained small size of aquatic mustelids could indicate the presence of a second attractor at a smaller size or competitive exclusion from the 500 kg attractor. • Analyses of the fossil record find indistinguishable optima (with large error), but produce different model support