Our brainpower is not as exceptional as we think, say scientists who found that even the world's smallest monkeys devote as much of their body energy to their brains as humans do. For years, scientists assumed that humans devote a larger share of their daily calories to their brains than other animals.
Although the human brain makes up only two percent of body weight, it consumes more than 25 percent of our baseline energy budget. The study, published in the Journal of Human Evolution, compared the relative brain costs of 22 animal species. It found that, when it comes to brainpower, humans are not as exceptional as we like to think.
"We don't have a uniquely expensive brain. This challenges a major dogma in human evolution studies," said Doug Boyer, assistant professor at Duke University in the US. Researchers decided to see how humans stack up in terms of brain energy uptake. Since energy travels to the brain via blood vessels, which deliver a form of sugar called glucose, researchers measured the cross-sectional area of the bony canals that enclose the cranial arteries. By coupling these measurements with previous estimates of brain glucose uptake and internal skull volume as an indicator of brain size, they examined seven species, including mice, rats, squirrels, rabbits, monkeys, and humans. Researchers showed that larger canals enclose arteries that deliver more blood, and thus glucose, to the brain.
They calculated brain glucose uptake for an additional 15 species for which brain costs were unknown, including lemurs, monkeys, and tree shrews, primate relatives from Southeast Asia. Researchers found that humans allot proportionally more energy to their brains than rodents, Old World monkeys, and great apes such as orangutans and chimpanzees. Relative to resting metabolic rate - the total amount of calories an animal burns each day just to keep breathing, digesting and staying warm - the human brain demands more than twice as many calories as the chimpanzee brain, and at least three to five times more calories than the brains of squirrels, mice and rabbits. In terms of relative brain cost, there appears to be little difference between a human and a pen-tailed treeshrew, researchers said. Even the ring-tailed lemur and the tiny quarter-pound pygmy marmoset, the world's smallest monkey, devote as much of their body energy to their brains as we do.
The results suggest that the ability to grow a relatively more expensive brain evolved not at the dawn of humans, but millions of years before when our primate ancestors and their close relatives split from the branch of the mammal family tree that includes rodents and rabbits, Arianna Harrington, a graduate student at Duke University. What the data can not show is whether energetically expensive brains evolved first, and then predisposed some groups of animals to greater mental powers as a byproduct, or whether preexisting cognitive challenges favored individuals that devoted more energy to the brain, the researchers said.