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

Investigating the relationship between endocranial volume and cranial shape in Alouatta


1CUNY/NYCEP at the Department of Vertebrate Paleontology, American Museum of Natural History, New York, NY, 2Department of Biomedical Sciences, Grand Valley State University, Grand Rapids, MI

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Cranial shape in Alouatta is unique among platyrrhines and is assumed to be derived for atelids due to selective pressures from their folivorous diet, enlarged hyoid, and low encephalization, all of which are part of their energy-minimizing lifestyle. Protopithecus, Paralouatta, and Antillothrix have been linked to Alouatta based on shared small brain size and shape of the basi-occipital region, despite distinct morphology in other aspects of the skull and dentition. Tests of the “spatial packing” hypothesis suggest that relative brain size impacts cranial base angle as well as facial size. We test two hypotheses: (1) there are other aspects of cranial shape related to encephalization; and (2) it is these cranial traits that unite the “Alouatta-like” fossils.

To test these hypotheses, 3D landmarks were collected on laser scans of over 50 individuals representing six species of Alouatta. Endocranial and hyoid volumes were measured for the same individuals. Regressions were performed on the landmark data against size, sex, endocranial volume, encephalization quotient, hyoid volume, latitude/longitude, and shearing index in order to correlate aspects of cranial shape with other biological factors. Results indicate that encephalization quotient represents a relatively small proportion of the shape variability in Alouatta, but there are aspects of cranial variability that are uniquely correlated with this index, particularly: outline and width of the nuchal plane, posterior and lateral shape of the neurocranium, and bi-zygomatic breadth. Thus, the similarities amongst Alouatta and the fossils, many of which are in these regions, are potentially a product of low encephalization.

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