We may not be raring to go on a Monday morning, but humans are the Energizer Bunnies of the primate world. That’s the conclusion of a new study that, for the first time, measures precisely how many calories humans and apes burn each day. Compared with chimpanzees and other apes, our revved-up internal engines burn calories 27% faster, according to a paper in Nature this week. This higher metabolic rate equips us to quickly fuel energy-hungry brain cells, sustaining our bigger brains. And lest we run out of gas when food is short, the study also found that humans are fatter than other primates, giving us energy stores to draw on in lean times.

“The brilliant thing here is showing for the first time that we do have a higher metabolic rate, and we do use more energy,” says paleoanthropologist Leslie Aiello, president of the Wenner-Gren Foundation for Anthropological Research in New York City. “Humans during evolution have become more and more hypermetabolic,” says biological anthropologist Carel van Schaik of the University of Zurich in Switzerland. “We turned up the thermostat.”

For decades, researchers assumed that “there weren’t any differences in the rate at which different species burned calories,” says biological anthropologist Herman Pontzer of Hunter College in New York City, lead author of the new study. Comparing humans and other primates, they saw little difference in basal metabolic rate, which reflects the total calories used by our organs while we are at rest.

But in many ways, we’re not like other apes: Our brains are at least three times larger, and we produce more babies in shorter intervals—both of which consume more energy. “It has been an open question—how do we do all these expensive things?” Pontzer says.

For the past 2 decades, researchers looked for an answer in tradeoffs between the energy demands of different parts of the human body. For example, Aiello and her colleagues proposed that when our brains began to expand dramatically about 1.6 million years ago, our direct ancestor Homo erectus evolved a smaller gut that sucked up less energy (Science, 15 June 2007, p. 1560). Other teams suggested that humans reduced muscle mass to save energy; walked and ran more efficiently; or got extra calories faster by eating a higher quality diet, cooking food to cut down on the energy spent in digestion, and sharing food. Indeed, there seemed to be no shortage of human adaptations that conserve energy.

Then, in 2010, researchers began to measure apes’ total energy expenditures (TEEs) accurately for the first time, rather than just their resting rate. Orangutans delivered the first surprise: They have an unexpectedly low metabolic rate, Pontzer says.

So he and primatologist Stephen Ross of the Lincoln Park Zoo in Chicago, Illinois, set about measuring TEE directly in as many apes as possible at 14 zoos and two ape sanctuaries in the United States and Africa. They fed 27 chimps, eight bonobos, 10 gorillas, and 11 orangutans water labeled with certain isotopes of hydrogen and oxygen. Then they measured those two isotopes in the apes’ urine to see how the ratio changed over time. The ratio reveals how much carbon dioxide the animal had generated, which reflects how many calories it had burned. The technique is the “gold standard” for metabolic studies, and the researchers did a “terrific job” using it to compare the total calories burned daily by apes and humans, says biological anthropologist William Leonard of Northwestern University in Evanston, Illinois.

The team used the same method on 141 adults from five populations around the world. After taking body size into account, they found that humans averaged about 400 more calories per day than chimps and bonobos—635 calories more than gorillas and 820 calories more than orangutans. This meant that humans burned over 27% more energy per day on average than chimps.

Although ideally all data would come from animals in the wild, other studies have shown that TEE rates in captive and wild apes are about the same, regardless of activity levels, Van Schaik notes. To be safe, the study matched relatively sedentary humans with captive apes.

The team also measured body fat in people and other primates by analyzing isotopes in the urine, finding that humans had significantly more fat than even these zoo animals. “If you’re going to burn fuel faster, you better have a backup tank,” Pontzer says. Once early hominins had boosted their metabolism and grown bigger brains, he says, natural selection would have favored not only fatter individuals, but also smaller guts and other energy-saving adaptations, such as cooking and efficient walking.

“What is fantastic about this paper is that Herman and his colleagues have effectively integrated all of the earlier ideas into a unified theory for energy and the evolution of human characteristics,” Aiello says. Van Schaik agrees. “There has to be more energy going into our systems,” he says. “Now, [Pontzer] has measured it, and it all fits together.”

Next on Pontzer’s agenda is to try to figure out how and when human ancestors boosted their metabolisms above the levels of our ape ancestors, for example by analyzing rates of bone growth in fossils. That’s particularly intriguing to Aiello. “I’d really like to know,” she says, “when did fossil hominids get fat?”