Response of the Axial Skeleton to Bipedal Loading Behaviors in an Experimental Animal Model

Abstract

Many derived aspects of modern human axial skeletal morphology reflect our reliance on obligate bipedal locomotion. Insight into the adaptive significance of features, particularly in the spine, has been gained through experimental studies that induce bipedal standing or walking in quadrupedal mammals. Using an experimental animal model (Rattus norvegicus), the present study builds on earlier work by incorporating additional metrics of the cranium, employing quantitative methods established in the paleoanthropological literature, and exploring how variation in mechanical loading regimes impacts axial anatomy. Rats were assigned to one of five experimental groups, including "fully loaded bipedal walking," "partially loaded bipedal walking," "standing bipedally," "quadrupedal walking," and "no exercise control," and engaged in the behavior over 12-weeks. From μCT data obtained at the beginning and end of the experiment, we measured foramen magnum position and orientation, lumbar vertebral body wedging, cranial surface area of the lumbar and first sacral vertebral bodies, and sacral mediolateral width. Results demonstrate that bipedal rodents generally have more anteriorly positioned foramina magna, more dorsally wedged lumbar vertebrae, greater articular surface areas of lumbar and first sacral vertebral bodies, and sacra that exhibit greater mediolateral widths, compared to quadrupedal rodents. We further document variation among bipedal loading behavior groups (e.g., bipedal standing vs. walking). Our experimental animal model reveals how loading behaviors and adaptations may be specifically linked, and implicates a potential role for developmental plasticity in the evolutionary acquisition of bipedal adaptations in the hominin lineage. Anat Rec, 303:150-166, 2020.

The impact of bipedal mechanical loading history on longitudinal long bone growth

“…Previous work using this rat model, which tested how bipedal loading shapes axial anatomy, found significant differences in both absolute and percentage changes in vertebral wedging and percentage changes in sacral articular surface areas, consistent with expectations for adaptation to bipedal behaviors [30]. Therefore, this model has demonstrated utility for exploring how bipedal loading may influence other aspects of bone growth.…”

Phylogenetic and functional signal in prezygapophyseal articular facet shape of the first post‐transitional vertebra in anthropoids

“…Due to its functional role during primate locomotion and posture, vertebral column morphology has been studied in relation to various primate positional behaviors (Benton, 1967; Erikson, 1963; Johnson & Shapiro, 1998; Latimer & Ward, 1993; Linden et al, 2019; Meyer et al, 2018; Nalley & Grider‐Potter, 2015, 2017; Pal & Routal, 1987; Rose, 1975; Russo et al, 2020; Schultz, 1960; Shapiro, 1993, 1995, 2007; Shapiro et al, 2001; Shapiro & Simons, 2002; Slijper, 1946). One example of this is lumbar vertebral column length, which is cranio‐caudally greater in cercopithecoids compared to hominoids, both through increased craniocaudal vertebral body length and an increase in lumbar vertebrae number.…”

Extreme Anatomy: Gear for the Pioneer

“…Russo, Marsh, and Foster (this issue) follow a long tradition (e.g., Riesenfeld, ) in using an experimental rodent model to assess skeletal correlates of bipedalism. Rats were morphometrically studied before and after 12 weeks of bipedal behavior protocols (e.g., standing, walking) or control (no exercise).…”