Allometric Patterning in the Limb Skeleton of Bats: Implications for the Mechanics and Energetics of Powered Flight
Journal of Morphology (1997) 234: 277-294
Sharon M. Swartz

Allometric analysis was employed to compare linear dimensions of forelimb and
hindlimb bones (humeri, radii, third and fifth metacarpals, third and fifth manual
phalanges, femora, and tibiae) of 227 species of bats and 105 species of
nonvolant mammals of varying degrees of phylogenetic affinity to bats. After
accounting for body size, all forelimb bones are longer in bats than in nonvolant
species, with the exception of humeri and radii of a few highly arboreal primates.
Hindlimb bones are generally, but not uniformly, shorter in bats than in other
mammals. For the humerus, radius, and metacarpals, midshaft diameters are
greater in bats than in their comparably sized relatives. Proximal phalangeal
midshaft diameters are statistically indistinguishable from those of other mammals,
and distal phalanges show significantly reduced outer diameters. The pattern of
relative reduction in wing bone diameters along the wing's proximodistal axis
parallels the reduction in bone mineralization along the same axis, and a similar
pattern of change in cortical thickness from the smallest wall thicknesses among
mammals in the humerus and radius to the greatest wall thicknesses among
mammals in the phalanges. The combination of altered cross-sectional geometry
and mineralization appears significantly to reduce the mass moment of inertia of the
bat wing relative to a theoretical condition in which elongated bones preserve
primitive mammalian mineralization levels and patterns of scaling of long bone
diameters. This intercorrelated suite of skeletal specializations may significantly
reduce the inertial power of flight, contributing significant energetic savings to the
total energy budgets of the only flying mammals.