Small Skeletons Show Size-Specific Scaling

The authors analyzed mammalian skeletal scaling using two systems. First, they looked at a group of Philippine rodents known as the cloud rats, which range across several orders of magnitude in body size, with body mass ranging from 15 grams to about 6 pounds. For this group, they analyzed how body mass affects the external shape of vertebrae and the internal shape of vertebral trabecular bone (the spongy bone that fills vertebrae, the ends of long bones, and some parts of the skull). The cloud rats are all tree-living herbivores, and this uniform ecology controls several confounding variables, making it easier to isolate the effects of body size. For the other approach, they analyzed trabecular bone scaling in a larger group of mammals that included all the cloud rats plus an additional data set covering a much wider range of body masses, from shrew-sized to elephant-sized. They found that small mammals and large mammals have different scaling patterns in their skeletons. For example, in mammals under around 3.75 lb. in mass, the thickness of their internal trabecular bone components increases “with geometric similarity”—which basically means they are the same size relative to the whole bone no matter how much the animal weighs. But above that weight, that rate slows down, and the trabecular elements start to become thinner and thinner relative to the size of the whole bone. This seems backwards, noted Dr. Smith in an e-mail,because trabecular thickness is correlated with the strength of the bone it’s in—which raises the question of why bigger, heavier animals developed this trait that probably makes their bones less strong relative to their mass. This is a question that has received some attention in the literature, and other workers have tested hypotheses about how overall bone cross sectional area and limb posture change in extremely large vertebrates. But the more interesting follow-up question is what this mean for the biomechanics of tiny skeletons. Like, is this why mice can fall off a shelf and run away without having suffered any apparent damage? Are their bodies “overbuilt” for the types of stresses they undergo on a daily basis? Is there some other selective pressure that has resulted in them having relatively thick, dense trabecular bone?Based on results obtained thus far, there are likely to be biomechanical consequences of the relative robusticity of these tiny skeletons. Now, Stephanie is leading an NSF-funded follow-up project (with Ken and a colleague from Bucknell) that considers how body size relates to actual functional performance of tiny mammal bones.
April 26. 2024