ENERGETICS BOTH PROMOTE AND LIMIT AQUATIC MAMMAL GIGANTISM

Four extant mammalian lineages have invaded and diversified in the water: Sirenia, Cetacea, Pinnipedia, and Lutrinae. Most of these aquatic clades are larger-bodied, on average, than their closest land-dwelling relatives, but the extent to which potential ecological, biomechanical, and physiological controls contributed to this pattern remains quantitatively untested. Here we use previously published data on the body masses of 3859 living and 2999 fossil mammal species to examine the evolutionary trajectories of body size in aquatic mammals through both comparative phylogenetic analysis and direct interrogation of the fossil record. Both methods indicate that the evolution of an aquatic lifestyle is driving three of the four extant aquatic mammal clades toward a size attractor at approximately 500 kilograms. Based on the existence of this body size attractor and relatively rapid selection toward, and limited deviation from, this attractor, we rule out most potential drivers of size increase. Using observed constraints on food intake and energy demand, we show that these independent body size increases and the shared aquatic optimum size are consistent with control by differences in the energetic intake and cost functions between the terrestrial and aquatic realms. We propose that thermoregulatory costs constrain minimum size whereas limitations on feeding efficiency constrain maximum size. The optimum size occurs at an intermediate size where thermoregulatory costs are low but feeding efficiency remains high. Rather than being released from size pressures, aquatic mammals are driven and confined to larger body sizes by the strict demands of the aquatic medium.