Small skeletons show size-specific scaling: an exploration of allometry in the mammalian lumbar spine

Studies of vertebrate bone biomechanics often focus on skeletal
adaptations at upper extremes of body mass, ignoring the importance of
skeletal adaptations at lower extremes. Yet mammals are ancestrally small
and most modern species have masses under 5 kg, so the evolution of
morphology and function at small size should be prioritized for
understanding how mammals make a living. We examined allometric scaling of
lumbar vertebrae in the small-bodied Philippine endemic rodents known as
cloud rats, which vary in mass across two orders of magnitude
(15.5g-2700g). External vertebral dimensions scale with isometry or
positive allometry, likely relating to body size and nuances in
quadrupedal posture. In contrast to most mammalian trabecular bone
studies, bone volume fraction and trabecular thickness scale with positive
allometry and isometry, respectively. It is physiologically impossible for
these trends to continue to the upper extremes of mammalian body size, and
we demonstrate a fundamental difference in trabecular bone allometry
between large- and small-bodied mammals. These findings have important
implications for the biomechanical capabilities of mammalian bone at small
body size, for the selective pressures that govern skeletal evolution in
small mammals, and for the way we define “small” and “large” in the
context of vertebrate skeletons.