Functional interpretations of the radiographic anatomy of the femora of Myotis lucifugus, Pipistrellus subflavus, and Blarina brevicauda.

Radiographs of right femora from 29 shrews (Blarina brevicauda) and 51 bats (27 Myotis lucifugus, 14 Pipistrellus subflavus) were analyzed to determine if bone in these small mammals conformed to Bassett's ('68) revision of Wolff's Law. Five external and four cortical dimensions were made on enlargements of radiographs of each femur. Comparative descriptions of the spongiosa are given. External dimensions appear to be determined genetically, and, in bats, are closely related to functional demands. Shrew femora retain a primitive mammalian morphology. No apparent relationship exists between animal weight and mid-diaphyseal cortical thickness. Although differences in cortical thickness in bats suggest a possible relation between bone and pressure, no relationship is apparent in shrews; further, the comparative magnitude of these dimensions is similar in all three species, indicating genetic control mechanisms. Computation of maximal loading suggest that loading is so slight, that the inherent strength of bone tissue is adequate to resist mechanical deformation. However, there is evidence or cortical bone response to physiological demands. Inspection of the spongiosa also fails to provide evidence of conformity to Wolff's Law. Instead, the trabeculae appear to be related to pysiologic factors, animal age, and inherited disposition patterns. Thus, there is no evidence that bone in these diminutive mammals responds to mechanical forces, and the applicability of Wolff's Law is not indicated. It is hypothesized that, as the mechanical forces are so minimal, intrinsic tissue strength is sufficient to resist mechanical deformation, and femoral anatomy in these species is dictated by genetic and inherent physiologic conditions.