Meeting Abstract
Within the great diversity of birds, many lineages have colonized aquatic environments. Birds that swim using their feet face opposing constraints for locomotion. Underwater, birds benefit from large feet that can produce stronger fluid forces. Plus asymmetrically shaped feet could produce valuable lift forces. But on land, large, asymmetrical feet make walking unstable. From these differing demands we expect foot size and shape to vary with the degree of aquatic specialization. To test this hypothesis, we photographed and analyzed 50 feet of 6 bird species ranging in swimming ability: from surface swimmers—mallards (Anas platyrhynchos), Canada geese (Branta canadensis)—to specialized diving birds—double crested cormorant (Phalacrocorax auritus), Western grebe (Aechmophorus occidentalis), red-throated loon (Gavia stellata), common loon (Gavia immer). We measured foot size using normalized foot area and digit lengths. To measure shape, we compared medial to lateral areas, digit length ratios, digit splay angles, and Elliptic Fourier shape Descriptors (EFDs). Surprisingly, we find that specialized diving birds do not a have larger relative foot area except for grebes (p<0.05). All swimmers demonstrate some foot asymmetry. In specialized divers digit IV is longer than digit II, the inverse of surface swimmers. Discriminant analysis of the EFDs shows that the foot shapes of grebes and red-throated loons are distinct. Although cormorants have four webbed toes, their foot shape overlaps with the three-webbed-toed birds. Our findings reveal patterns in foot shape underlying swimming specialization, with implications for understanding evolution in aquatic animals and with potential applications for robotics or industrial design.
