Oct. 31 (UPI) -- New research suggests when it comes to the energy of demands of the brain, humans aren't as special as previously thought.
On average, the human brain comprises just 2 percent of a person's body weight, but demands 25 percent of a person's daily energy budget. In other words, roughly a quarter of all the calories humans consume each day are used to keep the brain operating at full capacity.
Until now, scientists assumed this ratio was unique to humans. But new research suggests the brains of other animals require similar amounts of energy.
"We don't have a uniquely expensive brain," Doug Boyer, assistant professor of evolutionary anthropology at Duke University, said in a news release. "This challenges a major dogma in human evolution studies."
Blood vessels carry energy to the brain in the form of glucose, a type of sugar. To gauge how much energy is consumed by the brains of other animals, scientists collected a variety of anatomical measurements.
Researchers measured the size of the bony canals that encircle cranial arteries in several species, including mice, rats, squirrels, rabbits, monkeys and humans. Scientists compared the data with estimates of each species' brain energy demands as well as measurements of each species' internal skull volume.
Their analysis -- detailed in the Journal of Human Evolution -- showed larger canals were associated with brains with greater energy demands.
Scientists used their findings to estimate the brain glucose uptake of 15 species for which brain energy costs are unknown, including lemurs, monkeys and treeshrews.
The new analysis confirmed humans use proportionally more energy than rodents, Old World monkeys and great apes to keep their brain functioning. But several species use similar proportions of energy to power their brains. Pen-tailed treeshrews, ring-tailed lemurs and pygmy marmosets all used nearly a quarter of their daily energy budget to keep their brain functioning.
"This shouldn't come as too much of a surprise," Boyer said. "The metabolic cost of a structure like the brain is mainly dependent on how big it is, and many animals have bigger brain-to-body mass ratios than humans."
The findings suggest the ability to grow an increasingly energy-expensive brain developed well before the emergence of humans.