By Jennifer Rosenberg

The human brain accounts for roughly 2 percent of total body weight, but uses approximately 20 percent of our body’s energy. Relative to our body size, human brains are much larger than those of other mammals, and take a lot of energy to keep humans going.  The evolution of our unusually large “social brain” is thought to have given us advanced intelligence and enabled the formation of large complex societies.

Research from Boston University now suggests that this brain metabolic functioning may not be unique to humans.

Researchers studied pavement ants and found that their brains account for about 5 percent of body weight while consuming roughly one-quarter of total body energy—almost the same energetic relationship seen in humans and other mammals. When ants were malnourished, their bodies dramatically reduced energy use while their brains continued operating at nearly full capacity. The study results were published in Proceedings of the National Academy of Sciences (PNAS).

The findings suggest that protecting the brain during times of stress may be a deeply conserved feature of evolution rather than a uniquely human adaptation. The research demonstrates that ant brains, like human brains, require a disproportionate amount of energy for workers to perform their social behaviors.

To learn more about this work, first author Dr. Zachary Coto, a member of BU’s Traniello Lab, answered questions about this fascinating study looking at brain size and energy demands.

Why are you interested in brain evolution?

Understanding brain evolution in animals that live in complex societies has been a key research area for decades and has generated much controversy. How does social life influence brain size and the energetic cost of its functions? Our research integrates sociobiology and evolutionary neurobiology to focus on how social life influences brain evolution. Questions of brain size evolution must be addressed with integrated studies of the energetic cost of brain tissue, which is metabolically expensive.

Why study ant brains and what were you hoping to learn?

We have a limited understanding of how brains of any size and of any animal generate complex social behavior. Humans and ants provide an interesting comparison that is particularly compelling because ant brains are so small and both ants and humans are so social. Human brains are unusually large relative to our body size, disproportionately larger than the brains of other mammals, and require a disproportionate amount of energy to maintain. To explain this, researchers have proposed “selfish brain” theory—the idea that when energy becomes scarce, the brain claims priority over other organs to preserve critical functions. Although previous studies hinted that this occurs in humans and other mammals by measuring size, directly measuring both brain metabolism and whole-body metabolism has proven difficult. We can measure all of these variables in ants.

Our research addresses fundamental questions about brain evolution by recording metabolic rates of ant brains by measuring whole-body metabolism. We wanted to determine whether the energy allocated to the brain is maintained while other systems are compromised during stress. When resources become scarce, worker ants may not regulate their physiology to survive low-energy conditions, but instead simply die.

What are your key research findings?

Several key findings indicate remarkably similar patterns of brain energy usage between ants and humans, despite the significant differences in body size, brain size, and physiology. We found that worker ant brains are about 5% of their body weight, but use approximately 25% of body energy, consistent with the mammalian pattern. Additionally, we  found that worker ant brain metabolic rates did not significantly decrease under physiological stress, despite declines in whole-body metabolic rates. The proportion of energy allocated to the brain nearly doubled, consistent with the pattern of energy prioritization observed in human brains.

When workers faced severe physiological limits, like starvation, we found that within only a few days of dying, they still maintained their brain physiology. We paired these brain metabolic measurements with behavioral tasks to determine if they could still process information relevant to significant social behaviors such as nestmate identification and colony defense, and found that workers performed comparably with those that had not been starved. These results suggest that during starvation, workers maintain their brain physiology at the expense of other physiologically demanding systems, and they maintain adaptive social behaviors.

Furthermore, we scaled human and mammalian data to predict the brain energy use of an ant-sized mammal. The model accurately matched our ant species, showing that shrinking a brain doesn’t change its basic energy requirements. Our findings show that even miniature brains are expensive to run, and even in social insects that put the colony above the individual, prioritizing energy for brain processing remains essential.

How does this research correlate with humans?

The question of the uniqueness of our species is of enduring interest. Tool use, social learning, personality, consciousness, and even numerical ability and facial recognition have all been found in other species, including insects. Our “big brains” shape our individuality and creativity, allow us to navigate complex, dynamic and nuanced social relationships, and are critical to the cultural sophistication that defines us as human. Our finding that ants allocate no less energy to operate their brains than humans is surprising. Our brains are complex relative to those of ants, but nevertheless share significant neurophysiological properties and thus offer insights into understanding the evolutionary neurobiology of the brain.