Didelphis virginiana: torsion and mediolateral bending in mammalian locomotion

INTRODUCTION For most tetrapod vertebrates, limb bones play a crucial role in the support of the body and transmission of muscular and propulsive forces. The forces to which limb bones are exposed during terrestrial locomotion likely impose some of the highest loads that these structures experience (Biewener, 1990; Biewener, 1993). However, a growing body of data now indicates that substantial differences in loading mechanics (both loading regimes and magnitudes) are present among tetrapod lineages with different characteristic locomotor patterns. For example, early studies of mammals running with upright, parasagittal limb postures indicated that anteroposterior (AP) bending was generally the most important loading regime, and that the ratio of limb bone strength to load magnitude (i.e. safety factor) was generally between two and four (Rubin and Lanyon, 1982; Biewener et al., 1983; Biewener et al., 1988). In contrast, more recent data from amphibians and reptiles that use sprawling limb posture indicated prominent limb bone torsion in addition to bending, with limb bone safety factors of usually at least five and sometimes exceeding 10 (Blob and Biewener, 1999; Blob and Biewener, 2001; Butcher and Blob, 2008; Butcher et al., 2008; Sheffield and Blob, 2011; Sheffield et al., 2011). Yet, a view that such patterns have strict phylogenetic associations may not be appropriate. For example, significant torsional loading has been described for the hindlimb elements of running birds (Carrano, 1998; Main and Biewener, 2007) and laboratory rats (Keller and Spengler, 1989), species that move the limbs in essentially parasagittal planes, but hold the femur in a more crouched position than the upright stance typical of the cursorial mammals (e.g. horses and dogs) examined in most early studies (Rubin and Lanyon, 1982; Biewener et al., 1983). Limb posture, therefore, also appears to play a crucial role in the mechanics of limb bone loading. To evaluate how limb bone loading patterns have diversified across clades that use different characteristic postures and locomotor kinematics, we recently analyzed in vivo strains from the femora of the Virginia opossum, Didelphis virginiana (Kerr 1792), during running on a treadmill (Butcher et al., 2011). Examination of this species has expanded perspectives on the diversity of limb bone loading mechanics in several ways. First, as a running marsupial, opossums belong to a lineage that is phylogenetically intermediate between the mammals and reptiles that have received previous study (Meyer and Zardoya, 2003), and could provide insight into transitions in loading patterns between these groups. Second, opossums provide additional limb bone loading data from a mammalian species that uses a more crouched limb posture (Jenkins, The Journal of Experimental Biology 214, 3455-3466 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.060178