Commonality in Pelvic Anatomy among Three Fossorial, Scratch-Digging, Mammalian Species

Abstract

This study tests whether three species of fossorial, scratch-digging mammals – Dasypus novemcinctus, Ictidomys tridecemlineatus, and Taxidea taxus – show commonality in pelvic anatomies inferred to be adaptive for this behavior and, correspondingly, divergence in these anatomies from nonfossorial species. The fossorial and nonfossorial species are in the same superorder or family; the comparative sets of species are D. novemcinctus with Bradypus variegatus and Choloepus hoffmanni, I. tridecemlineatus with Sciurus carolinensis, and Ta. taxus with Gulo gulo, Martes americana, and Neovison vison. Fossorial species are hypothesized to show higher prevalence of fusion of sacroiliac and sacroischial (if present) joints, reduction in size or loss of pelvic symphysis, greater number of sacral vertebrae, greater number of vertebrae in articulation with ilium and ischium, and longer ischium. Only males were used in the study; total sample size was 318 adults and 135 subadults. Results show commonality among fossorial species in greater number of sacral vertebrae and rigidity of the pelvis. One inconsistency with expectations is that the pelvic symphysis in fossorial species is as sturdy as in nonfossorial species, based on prevalence of symphyseal fusion and relative length and breadth of pelvic symphysis. Greater number of sacral vertebrae may be due to assimilation of a lumbar vertebra(−e) with the sacrum in fossorial species associated with their slow running speed. A rigid pelvis, due to fusion of pelvic joints and relatively longer or broader pelvic symphysis, effectively transfers forces between vertebral column and hind limb.