Functional and Behavioral Consequences of Axial Stability in Primates and Other Mammals

Abstract

A stable thoracolumbar region found in some arboreal mammals and certain primates has been proposed as advantageous for maintaining stable back postures during antipronograde locomotion, which is any form of movement in which the forelimbs and/or hindlimbs are loaded “in tension.” However, little data exist testing the inferred link between osteological features cited as enhancing whole‐body stability and the frequency of antipronograde locomotion within an animal's locomotor repertoire. To fill this gap this study uses multivariate methods to compare vertebral morphology of primate and non‐primate mammals with varying levels of antipronograde locomotion within their locomotor repertoires. Overall patterns suggest that species that commonly use antipronograde locomotion exhibit thoracolumbar stability related to osteological features that reduce potential intervertebral spaces. As an additional test of the inferred link between axial rigidity and behavioral stability, this study contrasted the frequency of cantilevering and bridging of Caluromys philander and Loris tardigradus (species that commonly use antipronograde locomotion), to Monodelphis domestica and Cheirogaleus medius (species that rarely use antipronograde locomotion) on a raised horizontal pole and terminal branch experimental set‐up. We observed C. philander cantilevering and bridging significantly more often (55% of observed events) than M. domestica, which never cantilevered or crossed any arboreal gaps (P <0.001). No difference in cantilevering frequency was observed between L. tardigradus and C. medius, but the duration of cantilevering bouts were significantly greater in L. tardigradus (4.24 sec ± 2.11 vs. 2.35 sec ± 1.71; P < 0.01). These data suggest that bridging, cantilevering, and other antipronograde behaviors likely benefit from a stable trunk. However, disparate clustering of antipronograde species based on multivariate statistics suggests that no single adaptive suite characterizes the axial skeleton of these mammals. From this, we can infer that different functional solutions may have evolved to meet the similar mechanical challenges imposed by antipronograde positional behaviors.Support or Funding InformationThis research was support by the National Science Foundation's Graduate Research Fellowship Program and the Force and Motion Foundation.