Meeting Abstract
P3.166 Monday, Jan. 6 15:30 Musculoskeletal specializations of foot-propelled swimming birds CLIFTON, GC*; CARR, JA; Concord Field Station, Harvard U., Bedford, MA; CFS, Harvard U., Bedford, MA glenna.clifton@gmail.com
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 so that the body is stable. 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, though 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), American coots (Fulica americana), Mallards (Anas platyrhychos), Double-crested cormorants (Phalacrocorax auritus), Western grebes (Aechmophorus occidentalis), and Red-throated loons (Gavia stellata). Unlike those of guinea fowl and coots, the legs of loons and grebes are almost completely incorporated into the body musculature, allowing the insertions of several hip muscles to attach more distally along the tibiotarsus. Furthermore, the distribution of muscle mass shifts more distally with increased swimming ability, ranging from 70% proximal in guinea fowl to 50% in cormorants to 30% in grebes. Specialized swimmers have exceptionally enlarged gastrocnemius muscles and digital flexors, all with a low degree of pennation. Since, many of these groups have evolved foot-propelled swimming independently, these observed trends in hindlimb morphology may represent key adaptations to swimming prowess in foot-propelled birds.
