Highly Consistent Anatomical Asymmetry in a Small Primate Brain: Left is Always Larger in the Marmoset Monkey

Although lateralisation of brain function is relatively common in animals, this is rarely accompanied by overt anatomical asymmetries. In primates, morphological differences between the cerebral hemispheres are well established in Hominidae but have been regarded as subtler or inconsistent in other species. Here we demonstrate that the left hemisphere is reproducibly larger than the right hemisphere in one of the smallest primates, the marmoset. This asymmetry develops postnatally, persists throughout adult life, and is found in individuals from genetically isolated colonies. Voxel-based morphometry reveals that the larger left hemisphere is primarily due to differences in the volume of cortical areas linked to social cognition. This result challenges the notion that the development and evolution of marked morphological asymmetry between the hemispheres is linked to the evolution of large brains.