Developmental Plasticity in the Ossification of the Proximal Femur of Heterocephalus glaber (Bathyergidae,Rodentia)

The proximal femoral morphology in rodents of different body sizes, locomotor modes, and from the three main rodent lineages (Sciuromorpha, Myomorpha, and Hystricomorpha) exhibit a separated condition of the femoral head and greater trochanter. We assessed the femoral ossification of eight species of all six genera of a subterranean lineage of mammals, the African mole-rats (Bathyergidae), including the naked mole-rat (Heterocephalus glaber). Here we report a surprising level of intraspecific variation in the ossification of the proximal femur of H. glaber, which presents both separated and coalesced conditions, regardless of sex and reproductive status. The other bathyergids, including chisel-tooth and scratch-diggers exhibit a separated condition, similar to the typical rodent condition. Because the coalesced condition is uncommon among rodents, our data suggests that the presence of two femoral morphologies in H. glaber represent developmental plasticity in this species. Such a dual condition may result from a constricted femoral head and greater trochanter morphology and slow skeletal growth rates, which could be also influenced by differential loading histories, such as magnitude and orientation of forces acting on the limb during ontogeny. This is the best documented case of intraspecific variation for this trait amongst non-human vertebrates, and its investigation is important to understanding the mechanisms of skeletal development and phenotypic plasticity in mammals.