The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


Functional morphology of the primate hallucal metatarsal (Mt1) and implications for inferring hallucal grasping capability in fossil primates

KATHERINE E. GOODENBERGER1, CALEY M. ORR2, DOUG M. BOYER3, RACHEL L. JACOBS4, JOHN C. FEMIANI5 and BIREN A. PATEL6.

1Post-Baccalaureate Premedical Program, Washington University in St. Louis, 2Anatomy, Midwestern University - Downers Grove, 3Evolutionary Anthropology, Duke University, 4Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 5Engineering, Arizona State University, 6Cell and Neurobiology, University of Southern California

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Primates are unusual in having a highly abducted, opposable hallux. It has been suggested that ‘powerful’ hallucal grasping evolved along the primate stem lineage, facilitating locomotion on vertical substrates and access to terminal branch resources in early primates and extant strepsirrhines. A reduction in hallucal grasping capability is believed to characterize anthropoids and may be related to use of relatively larger, horizontal substrates. Researchers have asserted that variation in hallucal grasping capability, as it relates to substrate size and orientation, is reflected in hallucal metatarsal (Mt1) morphology. This study examined the relationship between Mt1 morphology (torsion, relative peroneal process length and thickness, abduction angle, and proximal articular surface curvatures) and substrate data taken from the literature across 80 taxa (78 primates, one tupaiid, one dermopteran). Phylogenetic ANOVAs were used to examine differences in Mt1 morphology among six substrate categories and PGLS regressions were used to assess the relationship between Mt1 morphology and proportion of time spent on ‘relatively small’ branches (defined as a function of body mass). Vertical clingers and leapers have significantly longer peroneal processes and more curved articular surfaces than all other groups. Mt1 torsion in non-graspers is significantly lower than in all grasping groups, and higher Mt1 torsion is associated with relatively more time spent on small branches. These results indicate that a more detailed reconstruction of the sequence of behavioral changes leading from stem-primates to the euprimate ancestor should be possible using the Mt1 when fossils and extant taxa are considered in an explicit phylogenetic framework.

Funding for this study was provided by the Wenner-Gren Foundation (CMO), National Science Foundation (BSC 1125507) (DMB), and The Leakey Foundation (BAP)

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