BODY SIZE PATTERNS OF VERY LARGE MAMMALS. DOES WATER MATTER?

Recent studies examining the similarity of body size across the mammalian hierarchy have demonstrated extreme conservatism among most nonvolant terrestrial taxa, with broad sense heritability estimates exceeding 0.9. Interestingly, however, the pattern does not hold for the very smallest mammal species (<18g), perhaps because increasingly severe allometric constraints on life history and ecological parameters limit the range of possible adaptations when diversifying. Thus, mammals near the lower body size boundary may adapt to novel ecological conditions by modifying size. On land, the maximum size of mammals has hovered between 10-15 tons over much of the Cenozoic, suggesting that large mammals are at a size where structural, biomechanical or other constraints influence their evolutionary diversification. Moreover, the ten-fold difference in maximum size between aquatic and terrestrial habitats suggests that the aquatic environment either selects for, or allows the evolution of yet larger size. Here, we investigate the body size similarity of large mammals across the taxonomic hierarchy, and across geographic space, using data that include the complete late Quaternary mammalian fauna for five continents (North and South America, Eurasia, Africa and Australia), and all aquatic forms. Thus, we include the largest animal that has ever existed, the blue whale (Balaenoptera musculus) at 190,000 kg, as well as numerous extinct large bodied members of Proboscidea, Gomphotheriidae and Megatheriidae. Contrary to our initial belief, there is no support for the contention that the very largest mammals are facing constraints that influence patterns of body size diversification. The forces shaping mammalian morphospace do not act differently on animals of very large size, nor do they act differently on mammals occupying aquatic environments. Regardless of habitat or body size, mammalian body size is extremely heritable among congenerics and at all levels of the taxonomic hierarchy. Moreover, the greater range of variation in body size found in some genera is not related to phylogeny, age, or body mass. However, species rich genera and families (> 10 species) show greater variation in body mass.