THE EVOLUTION OF AQUATIC MAMMALS TOWARD A UNIVERSAL LARGE SIZE? EVIDENCE FROM PHYLOGENETICS AND FOSSILS

Crown mammals have invaded the water at least six times. Of these six invasions, four clades are extant: Sirenia, Cetacea, Pinnipedia, and Lutrinae. Three of these aquatic clades are substantially larger-bodied than their closest land-dwelling relatives, but the extent to which this pattern reflects common constraints of an aquatic lifestyle that favor larger body size versus a simple coincidence remains to be tested quantitatively. Here we use previously published and publicly available data on the body masses of 3832 living and 3005 fossil mammal species in addition to the most recent mammal supertree and up-to-date fossil ranges to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and direct interrogation of the fossil record. Both of these methods confirm that the evolution of an aquatic lifestyle has driven three of the four extant aquatic mammal clades toward a size attractor at approximately 500 kilograms. Our results indicate that these aquatic mammals have independently evolved toward larger body sizes due to the existence of this body size attractor and relatively rapid selection toward, and limited deviation from, this attractor. We believe that this optimum represents the culmination of selective forces resulting from thermoregulation, skeletal support, habitat area, locomotion energetics, and protein abundance differences between terrestrial and aquatic ecosystems, all of which are theorized to promote larger body sizes in aquatic systems. However, selection against extreme body size persists even in the aquatic realm, causing optimal body size convergence within these three groups. Furthermore, this balance of selective forces has caused increased rates of evolution towards this optimum and prevented deviation from it without the evolution of key innovations.