Wing bone stresses in free flying bats and the evolution of skeletal design for flight
Nature (1992) 359: 726-729
Sharon M. Swartz, Michael B. Bennett and David R. Carrier

The primary mechanical functions of limb bones are to resist deformation, and
hence provide stiff levers against which muscles can act, and to be sufficiently
strong to prevent breaking under static or dynamic loads which arise from normal
and accidental activities. If bones perform these functions with a minimum amount
of material, the energetic costs associated with building, maintaining and
transporting the skeleton will be minimized. Appropriate skeletal architecture for
minimizing mass while maximizing strength depends on forces imposed on
structural elements. In the evolutionary acquisition of flight in the bat lineage, the
forelimb skeleton must have come to experience locomotor-forces that differed
from those engendered by the terrestrial locomotion of non-flying bat relatives.
Here we successfully measure in vivo strain in the wing bones of flying mammals.
Our data demonstrate that torsion and shear are unique and crucial features of
skeletal biomechanics during flight, and suggest that the evolution of skeletal design
in bats and other flying vertebrates may be driven by the need to resist these loads.