According a new study, researchers from Drexel University, they observed that a 19 species of wasps found that body size may lead to variation in the complex parts of their brains. In comparison, the brain size to body size shows that just because you've evolved to have a big brain. This study has published in the Biological Journal of the Linnean Society.
Overall brain size does evolve in step with body size, but the usual pattern within a lineage of animals called "Haller's rule”, is for the smallest species to have the largest brains relative to their body size. "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 wasps brains seemed to follow Haller's Rule. If you're a little paper wasp, your brain is going to be about the same size as any of your cousin species. But that doesn't mean that the individual regions of your brain will match up to your bigger cousins. There appears to be a bit of a caveat to Haller's Rule, at least when it comes to the wasps. 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. The wasps' "mushroom bodies" (a collection of neurological fibers that are involved in learning, memory and sensory integration) and antennal lobes (which process chemical information) significantly decreased in proportional size in smaller-bodied wasps.
"We predict other insect species that made major environmental transitions — from diurnal to nocturnal, or from above to below-ground — to show brain-structure deviations from body size expectations for their lineage," O'Donnell said. This is the one of the first times specific brain structures are being looked at in this way. The researchers were not sure about the actual effect the different-sized brain lobes might have on the wasps.
And it's also unclear whether such changes are specific to these wasps or could be part of the evolution across the spectrum of social insects or other types of animals. O'Donnell concludes that their findings shows that study of brain-body size relationships should not assume all parts of the brain are equal. They are hoping to concentrate more in bigger, complex animal brains which are evolved.