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. Morphological differences between the cerebral hemispheres are well established in Hominoids and other large primate species, but regarded as subtler, or inconsistent, in other primates. We demonstrate that the left hemisphere is reproducibly larger than the right hemisphere in one of the smallest primates, the marmoset. Among 208 adult marmosets the left hemisphere was on average 2.31% larger than the right (range: 0.22%- 4.37%). This asymmetry develops during early postnatal life, persists throughout adulthood, and is primarily due to differences in the volume of cortex associated with social cognition, including in the lateral and inferior temporal lobe areas and ventral premotor areas. These results challenge the notion that the morphological asymmetry between the hemispheres is linked to the evolution of large brains and progressive reduction in the relative connectivity between hemispheres.