Washington, April 17 : Anthropologists from Duke University and the University of Zurich have found that the extra brainpower some primates derive from big brains actually helps them live and reproduce longer.
The researchers see this advantage as outweighing the fact that growth rates are much slower in organisms with large brains, which is also a cause of delay in reaching the age of adulthood reproduction.
During the study, a quartet of researchers 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, the first author of the study report published in the online edition of the Journal of Human Evolution.
"Growth rates are much slower in large-brained organisms, and that causes a delay in reproduction. 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," Barrickman added.
Barrickman further said: "Is larger brain size causing life histories to become extended and slowed down? We think so. 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."
Carel van Schaik, a Duke adjunct professor and a collaborator on the study, said: "Our main finding is that brain size is a far better predictor of the duration of immaturity than body size, at least among primates. 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!"
The new study contradicts previous studies that linked primate brain size to life span and other factors, and which involved 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," the researcher added.
The new study- supported by the scientific research society Sigma Xi, the American Museum of Natural History and the Ruggles Gates Fund for Biological Anthropology in the UK-focussed on primates living in the wild because "animals tend to grow up faster in captivity," said Barrickman.
In the case of humans, the researchers studied the Ache, a tropical forest culture in eastern Paraguay.
"Their food is exclusively wild food they forage from the forest. 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," she said of the Ache.
The researchers analysed available data on life history benchmarks such as length of pregnancy, years from birth to maturity, brain development before and after birth, and lifespan. The analysis showed that humans and other big-brained species like chimpanzees share certain survival traits.
In their study report, the researchers wrote that it takes longer to grow a bigger brain, thus leaving immature offspring in need of extra care for longer periods. However, they add, larger brains also provide adult caretakers with "more complex foraging techniques, predator avoidance and social skills."
The researchers say that greater skill provides adults with longer lives as well as 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 add.
Barrickman says that human females can live well beyond their reproductive years, and that the contributions of non-reproducing grandmothers may further enhance their own children's reproductive effort and decrease infant mortality.
She points out that studies on some primitive societies like the Hadza in East Africa show that "grandchildren are more likely to survive if they have a grandmother present."
She says that extended interactions with mothers, a result of slow growth of large brains, can help "wire their brain" as it grows.
"They wind up with very plastic brains that can adjust to whatever environmental stimulations come at them," she said.