In a study comparing brain size to body size, Drexel University researchers found that although body size limits brain volume it does not have the same impact on brain region volume.
Sean O'Donnell, professor in the College of Arts and Sciences, and his team studied a wide variety of Costa Rican paper wasps and found that the wasp brains followed Haller's Rule when it came to overall brain size, but not all brains were equal in structure. The team’s findings were published in Biological Journal of the Linnean Society earlier this week.
"Brains are not homogeneous masses: They are divided up into subregions that perform different functions," O'Donnell said. "And we found that not all of these subregions evolve to the same proportional size." Overall brain size does evolve in step with body size, but the usual pattern within a lineage of animals is for the smallest species to have the largest brains relative to their body size.
Humans are an exception to Haller's Rule. Our total brain size is much larger than what our body size would indicate among primates. So how did our large brains evolve? O'Donnell and his team hoped the paper wasps would begin to provide some clues.
"These wasps have a tremendous size range among species—the largest species was over 25 times the size of the smallest," O'Donnell said. "And, importantly, their brains are divided into distinct regions that perform different brain functions, like processing visual versus chemical (smell and taste) inputs."
The team found that the wasp’s brains seemed to follow Haller's Rule, however, there was a caveat to Haller's Rule. When their body sizes decreased, the proportional size of their brains did increase. However, some of the specific, complex regions of their brain did not. "In the smaller species, overall brain size was staying nearly constant, as smaller bodies evolved, but some brain regions were shrinking rapidly," O'Donnell explained.
In total, O'Donnell and his team examined 94 wasps across 19 species native to Costa Rica. Their findings on the shrinking complex regions of the brain held—except for one species. "Our findings show that studies of brain-body size relationships should not assume all parts of the brain are equal," O'Donnell said. "And we hope we're paving the way to more insight into how bigger, more complex animal brains evolved."
Image: Apoica pallens around their hive. Image courtesy of Sean O'Donnell.