Flippers versus feet: comparative trends in aquatic and non-aquatic carnivores

1. It is commonly accepted that many adaptations characterize carnivores th at live in water. However, no comparative tests have ever shown systematic differences between aquatic and terrestrial carnivore species as a whole. W e examine numerous hypotheses that purport to distinguish aquatic and terre strial carnivores using 20 morphological, life history, physiological and e cological traits. 2. Using the method of independent contrasts with a complete species-level phylogeny of extant carnivores, we found few differences between aquatic an d terrestrial species. Compared to terrestrial sister taxa, aquatic carnivo res are streamlined (increased head and body length for a given body weight ), have larger brains, smaller litter sizes, shorter interbirth intervals, and shorter lifespans. 3. Some of these differences are important functionally. Larger brain size may be related to increased cognitive and sensory needs required for an amp hibious lifestyle; smaller litters are likely associated with increased neo natal survival amidst competition for suitable breeding sites and advantage s accruing to increased precociality. 4. We conclude that broad differentiation of carnivores into aquatic and te rrestrial ecotypes is not useful given that adaptive differences between th ese groups are limited and seemingly no more numerous than those that occur within each ecological group.