Some primates have evolved big brains because their extra brainpower helps them live and reproduce longer, an advantage that outweighs the demands of extra years of growth and development they spend reaching adulthood, anthropologists from Duke University and the University of Zurich have concluded in a new study.
The four investigators compared key benchmarks in the development of 28 different primate species, ranging from humans living free of modern trappings in South American jungles to lemurs living in wild settings in Madagascar.
"This research focused specifically on the balance between the costs and benefits of growing a large brain," said Nancy Barrickman, a graduate student in Duke's Department of Biological Anthropology and Anatomy, who is first and corresponding author of a report now posted online for a future print edition of the Journal of Human Evolution.
"Growth rates are much slower in large-brained organisms, and that causes a delay in reproduction," Barrickman said. "If individuals wait too long to reach maturity then they run the risk of dying before they've had the chance to reproduce. So there must be some benefit to large brain size at the same time these costs are incurred.
"Is larger brain size causing life histories to become extended and slowed down? We think so," Barrickman added. "That obviously fits in very well with humans, who take forever to grow up and live a really long time. So we have the opportunity to have lots of offspring over that long period."
Barrickman drew these conclusions working with Carel van Schaik, a Duke adjunct professor on her doctoral studies committee who directs the University of Zurich's Anthropological Institute and Museum. Other coauthors include Duke graduate student Meredith Bastian, and Karin Isler, a collaborator of van Schaik's in Switzerland.
"Our main finding is that brain size is a far better predictor of the duration of immaturity than body size, at least among primates," said van Schaik. "This study is also useful because it allows us to understand why humans develop so slowly and live so long -- we have no other choice!"
Other studies have linked primate brain size to life span and other factors, but those results have been contradictory, according to the new report. Previous studies were "polluted" by mixing data on captive and wild animals, van Schaik said. "Because development and survival are highly responsive to conditions, this variability made it impossible to do clean comparisons."
Their study was supported by the scientific research society Sigma Xi, the American Museum of Natural History and the Ruggles Gates Fund for Biological Anthropology in the United Kingdom.
Barrickman and her colleagues focused on primates living in the wild because "animals tend to grow up faster in captivity," she said. In the case of humans, they studied the Ache, a tropical forest culture in eastern Paraguay.
"Their food is exclusively wild food they forage from the forest," she said of the Ache. "And they don't have other things like modern birth control methods that you'd find in an industrial population like ours. My argument is that we're basically captive primates by comparison."
After analyzing available data on life history benchmarks such as length of pregnancy, years from birth to maturity, pre- and post-natal brain development and lifespan, the researchers found that humans and other big-brained species such as chimpanzees share certain survival traits.
It takes longer to grow a bigger brain, thus leaving immature offspring in need of extra care for longer periods. But larger brains also provide adult caretakers with "more complex foraging techniques, predator avoidance and social skills," the researchers wrote.
Greater skill allows adults to live longer, which in turn gives them longer reproductive lives. Humans have added to this adaptive advantage by using their cognitive and social skills to work together in providing shelter and nourishment for the young, they said.
Additionally, human females can live well beyond their reproductive years. And the contributions of non-reproducing grandmothers may further enhance their own children's reproductive effort and decrease infant mortality, Barrickman said. That's because grandmas offer extra assistance in child rearing and food gathering.
Studies of some primitive societies, such as the Hadza in East Africa, show that "grandchildren are more likely to survive if they have a grandmother present," she said.
Some studies suggest that starting life with a brain that is still developing itself confers some survival advantages to offspring, according to Barrickman. Extended interactions with mothers and their surroundings can help "wire their brain" as it grows, she said.
"They wind up with very plastic brains that can adjust to whatever environmental stimulations come at them," she said.
The Energy of Brain-Making (see video above)
Scientists already know that many primates have much bigger brains than other animals of the same body size. That realization has left researchers asking "Why?," since growing additional brain matter requires expending lots of extra energy. They are also left wondering whether the brainiest primates expend their growth energy differently than the less endowed.
To help answer those questions, Barrickman measured respective head and body growth rates of three different lemur species at the Duke Lemur Center in 2006.
Barrickman found the biggest brained lemur -- the aye-aye -- grew brain tissue faster than the smallest-brained of the three species, the ring-tailed lemur. Meanwhile, the intermediately endowed sifaka exhibited brain growth rates in between the others. She also found that the demands of brain growth did not impose an energy limit on body-growth rate.
As a follow-up, Barrickman and undergraduate pre-med student Maggie Lin have been placing mothers of all three species and their babies in special chambers that can measure their respective metabolic rates -- a measure of energy use -- when they are at rest. The researchers gauge how much oxygen flows into the chamber, and how much comes out, the difference being the amount of oxygen the animals burn.
The real targets are the rapidly growing infants. But since the little ones don't like being tested alone, the researchers put mother and baby in the chamber together for one test, then the mother alone for a second, and use the difference to figure out the baby's rate.
They are investigating a hypothesis proposed by Lin: that the babies must increase their resting metabolic rates as their brains grow bigger.
"When you're resting, you're not spending energy on anything like locomotion," Barrickman explained. "So if the resting metabolic rate of a big-brained species rises above the rate of small-brained species, the brainy ones must be dumping that excess energy into something else other than the growth and maintenance of ordinarily-sized organs. We thinking they're dumping energy into the growth of their big brains."
Lin, a senior, said it was her idea to focus on the metabolism of growing babies. "They're fun to watch, too," she added. She joined Barrickman after responding to an e-mail ad for an undergraduate assistant. "I'm interested in brains, so I jumped on the opportunity," Lin said.
During one visit, the two were testing a two-month-old aye-aye named Mr. Eko that was born and reared under the Lemur Center's successful breeding program. Aye-ayes are nocturnal lemurs with bat-like ears that make them vaguely resemble the "Star Wars" character Yoda.
Aye-ayes are thought to need extra brainpower to master the skill of tap-foraging, a complex task that involves listening for, locating and digging out insect larvae embedded inside tree trunks or branches -- all in the dark.
"First, they use their really long middle finger, which has a long claw on it, to tap on the wood and listen for hollow spaces," Barrickman said. "Once they find those spaces, they'll carve out the wood using their incisors, and then use that long finger to scoop out the larvae.
"Experienced adults will carve just one tiny hole. They know exactly how to find the best spot to break through the wood to get the most insects. The young infants and juveniles are just picking randomly and carving holes and missing all the time.
"It appears to be something they have to learn." (Also see Video above).