Which came first, social dominance or big brains? Wasps may tell

There’s new evidence supporting the idea that bigger brains are better. A study of a tropical wasp suggests that the brainpower required to be dominant drives brain capacity.

University of Washington researchers have found that key processing regions in the brains of both males and females of one wasp species not only increased in size with age but were also associated with being dominant. The study also showed different patterns of brain development in males and females. Certain subregions were larger in males and others were larger in females. This matched expectations based on males’ greater use of vision and females’ greater reliance on their antennae.

UW researchers Sean O’Donnell and Yamile Molina found increased brain growth in areas of the insects’ brains called the mushroom bodies, which vaguely resemble the cerebrum in humans and other vertebrates. A mushroom body sits atop each hemisphere of the wasp brain. The mushroom bodies process input from the eyes and antennae, and are involved in learning and memory.

The social paper wasp that was studied, Mischocyttarus mastigorphorus, is unusual because males are dominant over females, a rarity among social insects, said O’Donnell, a UW associate professor of psychology. Most social insect societies – bees, ants and wasps – are predominantly female, with males short-lived and subordinate.

O’Donnell and Molina, a UW doctoral student, focused on a part of the insects’ mushroom body, called the calyx, where neural connections are made. While the overall size of the calyces did not differ between the males and females, specific subregions were larger in each sex. Males rely on vision when they leave the nest for mating opportunities, and the part of the calyx that receives visual input was larger. In contrast, most female interaction takes place on the nest, where tactile and odor senses are important and the part of the calyx that received input from the antennae was bigger among the females.

“When you are dominant among insects you get more food,” O’Donnell said, “and in this case it gives males more energy to leave the nest and mate. The fact that the males are dominant and long-lived makes this species interesting from a neurobiological standpoint. We think they have pretty sophisticated cognition compared to males of other wasp species.”