The modular nature of the trunk skeleton in anthropoid primates
Authors: Middleton Emily R 
Published: November 2023

The mammalian ribcage, spine, and pelvis share a variety of functional locomotor demands that are thought to result in strong covariation across the trunk skeleton and, hence, a unified response to selection. However, this hypothesis has never been formally tested, and preliminary results of research on primates suggest that torso shape variation in anthropoids is much greater than previously thought. This unexpected result suggests that the underlying patterns of covariation that characterize the relationships among bony elements of the trunk may also display a high degree of variation, likely in response to previously underappreciated selective pressures such as postural behavior, obstetrics, and ecogeography.

Here I explore integration across the ribcage, spine, and pelvis in four representative anthropoid taxa (Homo, n=572; Pan, n=109; Hylobates, n=81, and Macaca, n=105) using linear metrics and 3D geometric morphometric data. Using an application of Lande's multivariate response to selection equation, I calculated a set of evolvability statistics in the R statistical computing environment that characterize the evolutionary potential of taxa to respond to selection. Differences among taxa were assessed from the 95% confidence intervals of the evolvability indices, which were calculated from the standard error of each index derived via a bootstrapping technique.

Results indicate that patterns of integration within the trunk across taxa tend to be fairly conservative, whereas magnitudes of integration are far more variable. The non‐human primate taxa in general have stronger integration across the trunk than do humans, and apes tend to have more weakly integrated trunks than do macaques. All taxa are characterized by stronger integration within the caudal ribcage than within the cranial ribcage, but in non‐human primates the entire ribcage is integrated with other regions of the torso as more of a unit, whereas in humans the cranial and caudal regions of the ribcage are more dissociated. Finally, humans show unique and stronger relationships than do non‐human primates within the trunk relative to the bony birth canal, suggesting greater evolutionary constraints likely related to the protection of obstetric dimensions in humans.

These results suggest that the variation among primate taxa in terms of the magnitude of integration relates to differing selective pressures and may also contribute to observed variation in anthropoid torso shape. Future study of additional taxa will help to tease apart the effects of body size and phylogeny on patterns and magnitude of integration. Further exploring these issues in the trunk skeleton is especially interesting given the importance of changes to this anatomical region in the colonization of terrestrial habitats by early tetrapods and to the evolution of diverse niches and locomotor behaviors on land, sea, and air by a variety of vertebrate taxa.

Support or Funding Information

Grant support for data collection was provided by the Wenner‐Gren Foundation, the Leakey Foundation, and New York University. Additional support was provided by the American Association of Anatomists.