Hanging around: the functional anatomy of primate tails

Abstract

The prehensile tail (PT) may have evolved twice in New World monkeys, suggesting it is an effective adaptive strategy for negotiating arboreal habitats. However, the mechanical differences between the PT and the nonprehensile tail (NPT) are poorly understood. Previous work by German (1982) showed that external measurements of caudal vertebrae distinguish PTs from NPTs, but only distally within the vertebral sequence, not proximally. In the present study, peripheral quantitative computed tomography was used to examine whether PT caudal vertebrae (Alouatta and Cebus) are stronger and more rigid in bending and torsion than homologous vertebrae in NPTs (Pithecia), and whether proximodistal location along the tail influences vertebral structure differently between tail types. Vertebral cross-sectional properties were regressed against vertebral length and equivalent structural properties of limb bones using reduced major axis regression (RMA). Caudal vertebral strengths and rigidities were found to scale isometrically in each taxon. Species-specific RMA elevations are different in the proximal region, where Alouatta is stronger and more rigid than both Cebus and Pithecia, and Cebus is stronger and more rigid than Pithecia. However, the two prehensile-tailed taxa do not have different RMA elevations in any other tail region, and both are stronger and more rigid that Pithecia. These results suggest that finer distinctions among prehensile-tailed taxa may be detectable if internal structural data are examined. This may, in turn, relate to caudal muscular differences among taxa.