Meeting Abstract
Flight evolved independently in bats and birds, but in spite of this convergence on the same locomotion mode birds occupy a wider range of niches. Bird forelimb morphology is associated with a vast array of flight modes (e.g., soaring, diving, highly-maneuverable flight, flightlessness), and variation among birds’ hindlimbs reflects specializations for other locomotor functions (e.g., perching, grasping prey, swimming). In contrast with birds, almost all bats are limited to flapping flight and use their hindlimbs primarily to hang in their roosts. Given the differences between how birds and bats use their hindlimbs, we tested the hypothesis that bat humeri are stronger relative to their femora than birds’ and that birds have greater disparity in cross-sectional parameters. We assessed relative strength of the femur by calculating humeral/femoral ratios from measurements derived from the cross-sectional geometry of the bones: relative cortical bone area (RCA), polar section modulus (Zpol), second moment of area (Imax/Imin). Using phylogenetic comparative methods, we found that the birds (18 orders) and bats (7 families) sampled do not differ in any of their cross-sectional geometry metrics. Larger samples that include mammalian quadrupeds may shed light on whether and how this similarity relates to the evolution of flight. As expected, birds and bats do differ in the coefficient of variation for the humerus/femur RCA ratio (2.9 times higher in birds), and the humerus/femur Zpol ratio (18 times higher in birds). This is a clear validation of the wider range of humeral and femoral morphologies observed in birds relative to bats.
