Allometry and Evolution of Neurocranial Narrowness Across Nonhuman Anthropoid Primates

Among anthropoid primates, small-bodied platyrrhines exhibit striking mediolateral narrowness of the neurocranium relative to larger-bodied platyrrhines. Body size allometry has been hypothesized to explain variation in neurocranial narrowness. However, interspecific catarrhine variation appears inconsistent with the proposed platyrrhine allometry. This study tested the relationship between body size and neurocranial narrowness across anthropoids to identify whether clade-specific allometries have evolved. We further estimated neurocranial narrowness in anthropoid ancestors to determine the trait's evolutionary polarity.

Inter-clade shifts in degree and allometric scaling of neurocranial narrowness were identified with GLS pANCOVA, which combines generalized least-squares analysis (GLS) and phylogenetic analysis of covariance (pANCOVA). Ancestral trait estimation was performed using parsimony reconstruction methods, based on linear measurements of extant and fossil anthropoid skulls.

In callitrichine and female cebine platyrrhines, neurocranial narrowness was significantly greater than predicted by body size and displayed a different allometric relationship than in other anthropoids. This shift is correlated with decreased neurocranial breadth in callitrichines, but not cebines. Neurocranial narrowness was not correlated with body size in most anthropoid clades.

We did not identify a unified neurocranial narrowness allometry across platyrrhines or across anthropoids collectively. Nonetheless, reduced narrowness occurred parallel to the evolution of increased body size in catarrhines, atelids, and pitheciids, suggesting that larger body size may mitigate constraints on neurocranial shape responsible for narrow neurocrania in small-bodied fossil anthropoids, which have been retained only in small-bodied platyrrhines. Feeding adaptations and brain size also provide potential explanations for neurocranial narrowness variation in some clades.