Allometry and Evolution of Neurocranial Narrowness Across Nonhuman Anthropoid Primates

Area of Science:

• Primate Paleontology
• Comparative Anatomy
• Evolutionary Biology

Background:

• Small-bodied platyrrhines (New World monkeys) exhibit narrower neurocrania than larger-bodied relatives.
• Existing hypotheses suggest body size allometry explains this variation.
• Interspecific variation in catarrhines (Old World monkeys and apes) appears inconsistent with platyrrhine allometry.

Purpose of the Study:

• To investigate the relationship between body size and neurocranial narrowness across anthropoids.
• To determine if clade-specific allometries for neurocranial narrowness have evolved.
• To estimate ancestral neurocranial narrowness and its evolutionary polarity.

Main Methods:

• Generalized least-squares analysis (GLS) combined with phylogenetic analysis of covariance (pANCOVA) to identify inter-clade shifts and allometric scaling.
• Parsimony reconstruction methods for ancestral trait estimation.
• Linear measurements of extant and fossil anthropoid skulls.

Main Results:

• Neurocranial narrowness in callitrichines and female cebines was greater than predicted by body size, with a distinct allometric relationship.
• This divergence was linked to reduced neurocranial breadth in callitrichines, but not cebines.
• Most anthropoid clades did not show a correlation between neurocranial narrowness and body size.

Conclusions:

• A unified allometric pattern for neurocranial narrowness was not found across platyrrhines or anthropoids.
• Reduced neurocranial narrowness correlated with increased body size in catarrhines, atelids, and pitheciids.
• Larger body size may relax constraints on neurocranial shape, explaining narrowness in small-bodied platyrrhines; feeding and brain size may also influence variation.