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.

Methods
Scans of murine vertebral columns (museum specimens sourced from the Field Museum of Natural History) were collected using a GE Phoenix v|tome|x µCT scanner. Spherical VOIs were chosen and segmented in ORS Dragonfly 2021. Trabecular bone morphological data were collected from medullary cavity VOIs using Dragonfly and BoneJ. See the publication Methods section for additional details.