Abstract
The pelvis plays an active role in weight bearing and countering the ground reaction forces incurred by the hindlimbs thus making it a critical component of the locomotor skeleton. Accordingly, this anatomical region is theoretically ideal for inferring locomotor behavior from both external skeletal morphology and trabecular microarchitecture, with the latter possibly offering nuanced insights into the mechanical loading environment given its increased plasticity and higher turnover rate. However, trabecular microarchitecture is also known to be influenced by a variety of factors including body size, sex, age, genetic regulation, diet and activity level, that collectively hinder the ability to generate consistent functional inferences. In this study, a comparative sample of mammals (42 species spanning four orders) of varying sizes, yet comparable locomotor repertoires, were evaluated to determine the effects of body size, phylogeny and locomotion on hipbone trabecular microarchitecture. This study found a weak functional signal detected in differences in bone volume fraction and the degree of anisotropy across certain pre-assigned locomotor categories, while confirming previously recognized allometric scaling trends reported for other mammalian samples based on the femur. Within primates, a more anisotropic pattern was observed for quadrupedal species attributed to their repetitive loading regimes and stereotypical limb excursions, while isotropic values were revealed for taxa utilizing more varied arboreal repertoires. Humans, despite a frequent and predictable loading environment associated with their use of bipedalism, showed relatively isotropic values. This study highlights the confounding factors that influence trabecular microarchitecture and consequently limit its utility as a method for investigating locomotor adaptation.
Highlights
The observation that bone responds to the mechanical loading through altering its structural composition is an adaptive phenomenon commonly referred to as "Wolff’s Law" (Wolff, 1892)
There are statistically supported mean differences recovered via pairwise t-tests with applied Holm-Bonferroni correction for arboreal quadrupeds (AQ) versus knuckle-walkers/terrestrial quadrupeds (KW/TQ), saltators and arboreal quadrupeds that utilize climbing behaviors (AQ, climbers)
This is observed for vertical clingers and leapers (VCL) versus arboreal quadrupeds that climb (AQ, climbers) and saltators (Table 2)
Summary
The observation that bone responds to the mechanical loading through altering its structural composition is an adaptive phenomenon commonly referred to as "Wolff’s Law" (Wolff, 1892). This biological reaction to mechanical strain is the conceptual basis for studies that use trabecular microarchitecture to infer the frequency and directionality of loading. Some attempts to infer locomotor function from trabecular bone, even within controlled experimental settings, have proven unsuccessful (e.g., Carlson et al, 2008; Swartz et al, 1998) or have yielded inconsistent results that limit their overall interpretability (see review in Kivell, 2016)
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