Neurocranial narrowness across nonhuman anthropoid primates

Body size allometry has been proposed to explain neurocranial shape
variation across platyrrhine monkeys, including the extreme neurocranial
narrowness (NCN) of small-bodied taxa. However, catarrhine primates do not
align with the proposed platyrrhine allometry, suggesting that body size
alone may not explain anthropoid NCN variation. We measured NCN in
platyrrhine and catarrhine anthropoids, and identified inter-clade
differences in degree and allometric scaling of NCN using GLS pANCOVA, a
method combining generalized least squares analysis and phylogenetic
analysis of covariance. We further performed character history analysis
using extant anthropoid data and fossil evidence to estimate neurocranial
shape in anthropoid ancestors. We found no unified allometry of NCN across
platyrrhines or across anthropoids as a whole, and there was no
significant correlation between body mass and NCN across platyrrhines or
catarrhines. At the subfamily level, callitrichine and possibly cebine
platyrrhines differ from other anthropoids in degree and allometric
scaling of NCN. Character history analysis suggests that higher NCN is the
ancestral platyrrhine condition, and broader neurocranial shape evolved
independently in catarrhines and larger-bodied platyrrhines. Our results
support the hypothesis that body size does not explain anthropoid NCN
variation, and provide a foundation for future research into potential
correlates of anthropoid NCN.