U. WASHINGTON (US) — Big-bodied wasps have larger brains and devote up to three times more of their brain tissue to coordinate social interaction, learning, and memory.
Further, the regions of the queen’s brain that manage those complex behaviors are larger than those of worker wasps.
“As the brain gets larger, there’s disproportionately greater investment in the size of brain tissue for higher-order cognitive abilities,” says Sean O’Donnell, professor of psychology at the University of Washington.
“As larger wasp brains evolve, natural selection favors investing most heavily in the brain regions involved in learning and memory.”
For smaller-brained species, cognitive power may be limited by their inability to invest in central brain regions.
“In many kinds of animals, it’s only with a larger brain—which is determined by body size—that more complex and flexible behaviors are achieved,” O’Donnell says.
The results appear in the journal Proceedings of the National Academy of Sciences.
O’Donnell and co-authors collected samples of 10 types of adult social wasps from four field sites in Costa Rica and Ecuador and found that the larger the wasp, the larger the overall brain size. But increase of brain size was not uniform across all brain regions.
The researchers dissected the wasp brains and measured the volume of two brain regions, focusing on the central processing region known as the mushroom bodies that, like the cerebral cortex in humans, handles elaborate cognitive functions such as learning, memory, and social interactions.
They also measured the peripheral processing regions—the optic lobes and the antennal lobes—that deal with vision and smell and are thought to perform more basic cognitive functions.
Across the 10 species, brain areas that process peripheral sensory information increased only slightly with overall brain size. But the wasps with larger bodies—and correspondingly larger-sized brains—had disproportionately larger central processing regions.
“These findings suggest that absolute brain size matters a lot, because it sets limits on central cognitive processing tissue,” O’Donnell says.
Surprisingly, in nine out of 10 wasp species, queens have larger central processors than worker wasps.
Queen social wasps don’t seem to perform complex tasks like food collection—instead, they are relatively inactive, staying in the nest to lay eggs while the workers go out to forage.
Queens’ greater brain power may be due to having to defend their social status, O’Donnell says. “Queens are constantly tested for their potency. They must be up for those social cognitive demands.”
The next step is to test the prediction that large-brained species will have enhanced cognitive abilities compared with smaller-brained species, which could have ecological payoffs for challenges like invading new habitats and expanding geographic range.
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