My complementary allometric analyses of wing bone dimensions in several hundred bat species document that although bat wing bones appear slender, they do not show a general pattern of reduced diameters in correlation with increased bone length, and therefore the evolution of wings may have added an enormous amount of dense, massive structural tissue to the wing in order to increase flight surface area, potentially at great energetic cost (Swartz, 1996b). Moreover, the pattern of wing bone scaling changes along the wing's length in a manner that relates directly to the variation in bone loading that I have documented in the in vivo bone strain analyses; only the most energetically critical wing regions, the distal tips, have significantly reduced bone volumes (Swartz, 1996b). In studying patterns of structural shape I have considered bone wall thicknesses in addition to outer diameters; I have shown that only the bones subjected to torsional loads are thin-walled, and I have related this pattern of cortical reduction to the mechanics of wing twisting that follow from basic features of vertebrate flight shared among bats, birds and pterosaurs, providing a new explanation for the reduction of bone wall thickness in all flying vertebrates (Swartz, Bennett and Carrier, 1992; Swartz, 1996b).
