Meeting Abstract
Within the great diversity of birds, numerous lineages have colonized aquatic environments. Birds that swim using their feet face opposing constraints for locomotion on land versus through water. On land, birds require powerful muscles to produce large ground reaction forces and must position their feet for body stability. On the surface or underwater, the production of hydrodynamic forces does not solely rely on muscle power, but also on foot shape and velocity. A swimming animal’s limb orientation is not constrained by terrestrial stability, but contributes to body drag. Due to these differing conditions, we expect hindlimb musculoskeletal morphology to vary with the degree of aquatic specialization. To examine this, we have dissected the hindlimbs of birds ranging from completely terrestrial to highly aquatic: Helmeted guinea fowl (Numida meleagris), Mallard (Anas platyrhychos), Canada goose (Branta canadensis), Mute Swan (Cygnus olor), Double-crested cormorant (Phalacrocorax auritus), Western grebe (Aechmophorus occidentalis), Red-throated loon (Gavia stellata), and Common loon (Gavia immer). We find that specialized swimming and running birds have equally muscular hindlimbs compared to surface swimming waterfowl (7% vs. 4% body mass). The distribution of hindlimb muscle mass shifts distally with increased swimming ability: from 40% distal in guinea fowl and mallards to 70% in grebes and loons. Most specialized swimmers have exceptionally large gastrocnemius muscles and digital flexors, with long fibers and a low degree of pennation. Cormorants, however, do not show the same trend. Since, many of these groups have evolved foot-propelled swimming independently, these observed trends in hindlimb morphology might represent different anatomical strategies for swimming in foot-propelled birds.
