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Ecological diversification into new environments presents new mechanical challenges for locomotion. An extreme example of this is the transition from a terrestrial to an aquatic lifestyle. Here, we examine the implications of life in a neutrally buoyant environment on adaptations of the axial skeleton to evolutionary increases in body size. On land, mammals must use their thoracolumbar vertebral column for body support against gravity and thus exhibit increasing stabilization of the trunk as body size increases. Conversely, in water, the role of the axial skeleton in body support is reduced, and, in aquatic mammals, the vertebral column functions primarily in locomotion. Therefore, we hypothesize that the allometric stabilization associated with increasing body size in terrestrial mammals will be minimized in secondarily aquatic mammals. We test this by comparing the scaling exponent (slope) of vertebral measures from 57 terrestrial species (23 felids, 34 bovids) to 23 semi-aquatic species (pinnipeds), using phylogenetically corrected regressions. Terrestrial taxa meet predictions of allometric stabilization, with posterior vertebral column (lumbar region) shortening, increased vertebral height compared to width, and shorter, more disc-shaped centra. In contrast, pinniped vertebral proportions (e.g. length, width, height) scale with isometry, and in some cases, centra even become more spool-shaped with increasing size, suggesting increased flexibility. Our results demonstrate that evolution of a secondarily aquatic lifestyle has modified the mechanical constraints associated with evolutionary increases in body size, relative to terrestrial taxa. [ABSTRACT FROM AUTHOR]

Pinnipeds (seals, sea lions and walruses) are secondarily marine carnivorans that exhibit a wide range of feeding and reproductive specializations. Extant pinnipeds are split into three families: Phocidae (seals), Otariidae (sea lions) and Odobenidae (walruses). Morphometric analyses were used to examine cranial morphology in otariid and phocid pinnipeds. Phocids are more ecologically and taxonomically diverse than otariids, and this study quantitatively assessed the effects of life history, phylogeny and ecology on cranial morphology in these closely related clades of aquatic carnivorans. Fifty-three to 58 three-dimensional landmarks were gathered from 138 specimens, representing 31 of the 33 extant species of otariids and phocids. Principal components analysis was used to identify major axes of variation, and principal component scores were compared with phylogenetic distances and ecological variables to test for significant correlates of skull morphology. Results showed that phocids exhibit a much greater diversity of adult skull morphology than otariids. Shape differences within adult otariids were dominated by males of only one species, Otaria flavescens. In contrast, several species of phocids deviated markedly from the mean phocid morphology. These atypical morphologies were consistently associated with specializations of either feeding or mating strategies. Ontogenetic shape changes are greater, relative to interspecific differences, in otariids than in phocids, and shape dimorphism was observed in only one otariid and two phocid species. Unexpectedly, neither otariids nor phocids showed strong correlations between phylogenetic relationship and cranial morphology. Both clades show strong correlations between cranial shape and some life history and some environmental variables, but phocids show stronger correlations with life-history variables, perhaps reflecting the broad range of reproductive strategies observed in phocids. [ABSTRACT FROM AUTHOR]