Pecohub The energy required for brains of different sizes to function could play a role in how different species evolved over time.
While the energetic costs of brains have long been deemed important in evolution, direct measurements of the rate at which the brains of different species consume energy, also known as brain metabolic rates (MRs), remain scarce.
Researchers at Arizona State University and Boston University recently studied a group of ant species called harvester ants, looking at the proportion of resting energy they devote to their brains. Their observations, presented in a paper published in Proceedings of the Royal Society B: Biological Sciences, suggest that species with smaller worker ants allocate more of their resting energy budget to their brains than those with larger workers.
The researchers set out to test three main experimental hypotheses. They first tried to determine whether more socially complex ant species exhibit higher brain MRs, or in other words, whether their brains use more energy per unit of time than those of less socially complex species.
In addition, they wanted to test whether species with smaller bodies use more energy per unit of brain tissue. Finally, they investigated the possibility that the brains of species with evolutionary lineages that branched out more recently use less energy per unit of tissue, which may suggest that they evolved to be more metabolically “frugal.”
To test these hypotheses, the authors measured the mass of the brains and bodies of worker ants of different species. They also examined brains dissected from the ants, specifically looking at indicators of oxygen consumption.
They used these indicators to estimate the brains’ rates of metabolic energy expenditure, or MRs. Finally, the researchers measured the ants’ whole-body resting metabolism.
Their analyses compared 11 species belonging to the Pogonomyrmex genus, a group of closely related harvester ants originating from North and South America.
The team found no association between the size of ant colonies, which served as an indicator of social complexity, and brain MR. However, the body size of worker ants appeared to be linked to their relative brain size, energy use per unit of brain tissue and the proportion of resting energy expenditure devoted to the brain.
“Worker body size and colony size, a proxy for social complexity, vary significantly among Pogonomyrmex species and were positively correlated,” wrote Craig D. Perl and colleagues in their paper. “Species with larger colonies did not have larger brains or higher brain MRs. Species with smaller workers had relatively larger brains and higher brain MSMR. Thirty-eight percent of resting MR was attributable to brain metabolism in species with the smallest workers, compared to 6% in species with the largest workers.”
While worker ants from some of the species considered are smaller, the researchers found that these smaller workers had larger brains relative to their body size than larger workers from other species. The smaller workers also appeared to use more energy per unit of brain tissue, with their brains accounting for a larger proportion of their whole-body resting energy expenditure than those of species with larger workers.
“We suggest that smaller worker body sizes may require higher brain mass-specific energetic costs to achieve comparable performance by absolutely smaller brains,” wrote the authors. “More derived species had strikingly lower mass-specific brain metabolic costs, suggesting that increases in worker body size have selected for neurometabolic frugality through reductions in brain MSMR.”
The recent work by Perl and his collaborators could contribute to the understanding of brain evolution, particularly of how body size and energy demands shape brain size and metabolism. In the future, other research teams could build on the team’s findings and perform similar analyses that include more ant species or entirely different animal groups.
