Rethinking Brain Evolution in Insects

As surprising at it may seem, wasps, bees, and even ants have relatively large and complex brains. That allows these “social insects” to keep track of the intricate relationships between the thousands of individuals in their colony—or so researchers thought. A new study indicates that these insects didn’t grow big brains to cope with social living; they evolved them millions of years earlier when they were solitary parasites.

The link between brain size and social living was first noted in 1850, when scientists identified mushroom bodies in the insect brain. Aptly named because they’re shaped like mushrooms, the structures contain thousands of neurons responsible for processing and remembering smells and sights. Social insects tend to have larger mushroom bodies than solitary ones, leading researchers to believe that the transition from solitary to social living increased the size of these brain regions.

But Sarah Farris has found a different explanation. Instead of comparing social insects with solitary ones, Farris, a neurobiologist at West Virginia University in Morgantown, looked into the past. To get a sense of how the wasp brain evolved over time, she and taxonomist Susanne Schulmeister of the American Museum of Natural History in New York City compared the mushroom bodies of parasitic wasps with those of nonparasitic wasps, which represent the very oldest form of wasp. The parasitic wasps had consistently larger and more elaborate mushroom bodies than the nonparasites, the duo reports online today in the Proceedings of the Royal Society B. In particular, the caps, called calyces, of the parasitic mushroom bodies were twice the size of nonparasites.

Farris points out that parasitism evolved 90 million years before social insects appear, and so “insects had big mushroom bodies for quite a while before sociality arose.” This is the first evidence that parasitism, and not sociality, was the driver of insect mushroom body complexity, she says. That may be because well-developed mushroom bodies help parasitic wasps better locate the nests of the larvae they lay their eggs in.

Francis Ratnieks, an evolutionary biologist at the University of Sussex in the United Kingdom agrees with the study’s findings, but he thinks the researchers need to also look at the brains of social insects. It would be useful, for example, to compare the brains of social worker bees, which process vast quantities of visual information as they fly from flower to flower, with those of parasitic wasps. If bees have even larger mushroom bodies than parasitic wasps, he says, this would suggest that social insects have further improved on the brains that they inherited from their ancestors.

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