In parallel, my lab has documented variation in bone material properties within the wing skeleton of single bat species. We have shown that the mineral content declines, sometimes precipitously, from the shoulder to the wingtip, with variation in ash content in a single bat in some cases more than encompassing the entire previously documented range of variation for all mammalian limb bones (Papadimitriou, Swartz, and Kunz, in press). This material variation implies an great range of mechanical properties, given the dependence of stiffness, failure and yield strength and strain, and energy absorbing ability on mineralization. Perhaps equally importantly, and in concert with the proximodistally changing geometry of wing bones, this patterning of decreasing mineral content and thus changing bone density along the wing has significant implications for flight energetics. I have developed a model to quantify the magnitude of the effect of changing geometry and density on wing mass moments of inertia, and have shown that the specializations of bat wing bones will decrease inertial power several-fold, significantly reducing the metabolic cost of flight (Swartz, 1996b).
My work on the functional design of the wing skeleton has highlighted the importance of the non-bony "skeleton" of the wings as well. In bats, the skin serves not only as a regulatory and protective organ, but as a primary locomotor structure, and as such, has a highly developed connective tissue structural framework. I have begun to investigate the mechanical properties of this membrane, and have shown that the skin of a single wing, much like the bony tissue, encompasses an extraordinary range of mechanical characteristics (Swartz et al., 1996). These characteristics vary in direct relation to the underlying gross architecture of the wing's collagen-elastin structural network, and differ both among wing regions and among taxa, with large-bodied, load-carrying taxa characterized by some of the strongest and stiffest skins among mammals. In keeping with energetic constraints on flight, we have also demonstrated that bat skin is several times thinner than predictions based on body size, and thus meets unusually extreme mechanical demands with a significantly reduced mass (Swartz et al., 1996). Together, these studies on the functional behavior, morphology, and mechanical properties of bat wings demonstrate that the wing must be viewed as comprising at least two distinct regions, each of which is unique among mammalian limbs in its mechanics and design, but in dramatically different ways.
