Phylogenetic and fossil evidence for a common body size attractor in marine mammals

Evolutionary transitions between terrestrial and aquatic habitats are rare and often have large effects on the evolutionary trajectory of the clade making the transition. Following a single transition from the marine realm to the terrestrial realm, tetrapods have subsequently re-evolved a marine lifestyle at least 30 separate times. At least six of these re-invasions of the water occurred within crown-group mammals and four [sirenians (Sirenia), whales (Cetacea), pinnipeds (Pinnipedia), and otters (Lutrinae)] clades are extant. Although marine mammals are widely known to be larger than their terrestrial sister groups, the extent to which the body size evolution of these clades reflects common constraints of a marine lifestyle remains little studied. •Mammals living in aquatic environments have higher optimal body sizes than their terrestrial counterparts. •Results suggest the existence of a body size attractor that has been discovered independently by three mammalian clades. •The fossil record suggests it often takes a long time for body size to increase, implying there is low pressure to get bigger. •Some groups may still be getting larger, although preliminary analyses suggest there may be an upper limit without key innovations (e.g. baleen whales). •The sustained small size of aquatic mustelids could indicate the presence of a second attractor at small size or competitive exclusion from the ~1 ton attractor. •Differences between phylogenetic and fossil analysis results may be indicative of poor method assumptions or low power