Why bigger brains developed

December, 2010

More support for the theory that bigger brains were a response to living in social groups comes from a wide-ranging comparison of 511 mammalian species, but a comparison of wasp brains over time points to the importance of parasitism.

A comparison of the brain and body size of over 500 species of living and fossilised mammals has found that the brains of monkeys grew the most over 60 million years, followed by horses, dolphins, camels and dogs. Those with relatively bigger brains tend to live in stable social groups. The brains of more solitary mammals, such as cats, deer and rhino, grew much more slowly during the same period.

On the other hand, a new study comparing wasp brains over time has revealed that the mushroom bodies (neural clusters responsible for processing and remembering smells and sights) of parasitic wasps are consistently larger and more complex than those of nonparasitic wasps, which represent the very oldest form of wasp.

Previously, findings that social insects tend to have larger mushroom bodies than solitary ones have lead researchers to believe that the transition from solitary to social living was behind the larger brain regions. These new findings suggest that it is parasitism (which evolved 90 million years before social insects appear) that is behind the growth in size. That may be because well-developed mushroom bodies help parasitic wasps better locate hosts for their larvae.

Of course, this doesn’t rule out the possibility that sociality lead to another boost in size and complexity, and indeed the researchers suggest that these neurological developments may have been a crucial precursor for central place foraging. This behavior is widespread in this group of insects (the Aculeata), requires extensive spatial learning, and may have contributed to the various developments of social behavior. A comparison of the brains of social worker bees and those of parasitic wasps would be helpful.

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