28.2 Friday, Jan. 4 Regional alterations in bone thickness and density helped bats acquire active flight COOPER, LISA N*; SEARS, KAREN; SIMMONS, NANCY; NEOMED, Ohio; University of Illinois, Urbana-Champaign; American Museum of Natural History, New York firstname.lastname@example.org
The origin and diversification of bats are intimately linked with flight, therefore bats face a host of locomotor challenges not encountered by terrestrial mammals. For decades biologists have presented the need for elongated wing bones as one of the primary selective pressures shaping bat locomotor morphology and behavior. Only recently, biologists have reported that bat wing bones display decreased cross-sectional geometries and densities relative to terrestrial mammals. These architectural novelties likely increase bone compliance to accommodate the high bending strains that result from powered flight. This study investigates this fundamental issue of chiropteran bone function by testing hypotheses that relate evolutionary bone architecture with in vivo bone function. High resolution micro-CT scans of a taxonomically diverse sample of approximately thirty extant bats showed that, compared to terrestrial rodents, bats displayed thinner cortices forelimb and vertebral elements, and the mandible was only 80-85% as dense. Hindlimb elements showed a surprising range of thicknesses that were correlated mostly with locomotor patterns. Preliminary character state reconstructions, using mice as an outgroup, showed that the evolution of forelimb cortical bone thickness displayed little homoplasy, and the megabat Cynopterus displayed unusually thick cortices. Taken together, these data based on scans of the appendicular and axial skeletons of bats and mice showed that lightening of the chiropteran skeleton is localized to mostly the wing bones and is therefore associated with the acquisition of active flight, rather than a systemic lightening of the entire skeleton.