Are pinnipeds functionally different from fissiped carnivores? The importance of phylogenetic comparative analyses

It is widely assumed that adaptations to an aquatic lifestyle are so profou nd as to produce only obvious differences between pinnipeds and the remaini ng, largely terrestrial carnivore species ("fissipeds"). Thus, comparative studies of the order Carnivora routinely examine these groups independently . This approach is invalid for two reasons. First, fissipeds are a paraphyl etic assemblage, which raises the general issue of when it is appropriate t o ignore monophyly as a criterion for inclusion in comparative studies. Sec ond, the claim that most functional characters (beyond a few undoubled char acteristic features) are different in pinnipeds and fissipeds has never bee n quantitatively examined, nor with phylogenetic comparative methods. We te st for possible differences between these two groups in relation to 20 morp hological, life-history, physiological, and ecological variables. Compariso ns employed the method of independent contra!:ts based on a complete and da ted species-level phylogeny of the extant Carnivora. Pinnipeds differ from fissipeds only through evolutionary grade shifts in a limited number of lif e-history traits: litter weight (vs. gestation length), birth weight, and a ge of eyes opening (both vs. size). Otherwise, pinnipeds display the same r ate of evolution as phylogenetically equivalent fissiped taxa for all varia bles. Overall functional differences between pinnipeds and fissipeds appear to have been overstated and may be no greater than those among major fissi ped groups. Recognition of this fact should lead to a more complete underst anding of carnivore biology as a whole through more unified comparative tes ts. Comparative studies that do not include monophyletic groups for phyioge netically based comparative tests should be reconsidered.