Interspecific allometry of bone dimensions: A review of the theoretical models

A fascinating problem in biological scaling is the variation of long-bone length (or diameter) Y with body mass M in mammals, birds, and other vertebrates. It turns out that Y and M   are related by a power law, namely Y=Y0MbY=Y0Mb, where Y0Y0 is a constant and b is the so-called allometric exponent. The origin of these power laws is still unclear because, in general, biological laws do not follow from physical ones in a simple manner.Here we make a historical review of this subject, summarizing the main experimental papers as well as discussing the main theoretical proposals. Long-bone allometry seems to be determined by the need to resist the particular loads applied to each bone in each taxon. Experimental measurements of in vivo stresses have found that mammalian long bones are subjected mainly to compression and bending, while avian wing-bones and reptilian limb-bones suffer a high degree of torsion. A recent model, based on the hypothesis that mammalian long-bone allometry is determined by compressive and bending loads, was able elucidate several aspects of mammalian limb-bone scaling. However, an explanation for avian and reptilian long-bone allometry is still missing.