Variable temporoinsular cortex neuroanatomy in primates suggests a bottleneck effect in eastern gorillas

We describe an atypical neuroanatomical feature present in several primate species that involves a fusion between the temporal lobe (often including Heschl's gyrus in great apes) and the posterior dorsal insula, such that a portion of insular cortex forms an isolated pocket medial to the Sylvian fissure. We assessed the frequency of this fusion in 56 primate species (including apes, Old World monkeys, New World monkeys, and strepsirrhines) by using either magnetic resonance images or histological sections. A fusion between temporal cortex and posterior insula was present in 22 species (seven apes, two Old World monkeys, four New World monkeys, and nine strepsirrhines). The temporoinsular fusion was observed in most eastern gorilla (Gorilla beringei beringei and G. b. graueri) specimens (62% and 100% of cases, respectively) but was seen less frequently in other great apes and was never found in humans. We further explored the histology of this fusion in eastern gorillas by examining the cyto- and myeloarchitecture within this region and observed that the degree to which deep cortical layers and white matter are incorporated into the fusion varies among individuals within a species. We suggest that fusion between temporal and insular cortex is an example of a relatively rare neuroanatomical feature that has become more common in eastern gorillas, possibly as the result of a population bottleneck effect. Characterizing the phylogenetic distribution of this morphology highlights a derived feature of these great apes.