DAVID GREENE, HOST:
Scientists have identified a group of mothers who breast feed their offspring for eight years or more. These moms are wild orangutans. As NPR's Jon Hamilton reports, their willingness to nurse for so long might be a survival strategy.
JON HAMILTON, BYLINE: Researchers already knew that orangutans in the wild nurse for many years but it's been hard to know just how long. Christine Austin is a researcher at the Icahn School of Medicine at Mount Sinai in New York.
CHRISTINE AUSTIN: They're very difficult to study because, you know, they live in the trees.
HAMILTON: Orangutans lead reclusive lives in remote places like Borneo and Sumatra. Austin is part of a team that hoped to learn more about orangutan nursing without spending years in the field. Austin says their idea was to study the animal's teeth.
AUSTIN: Teeth are like a biological hard drive that's recording what's happening in your body each day.
HAMILTON: Austin says teeth have growth rings like a tree, and levels of barium in each ring indicate when an orangutan was consuming milk. So Austin and the team examined teeth from four wild orangutans who died decades ago. And in the journal Science Advances, they report a clear pattern.
AUSTIN: For the first 12 to 18 months, the orangutans are exclusively nursing.
HAMILTON: Then they started eating other things, mostly fruit, but Austin says the animals didn't stop nursing entirely.
AUSTIN: During periods where there was low fruit availability, the offspring would rely more heavily on their mother's milk to get the nutrition they need for growth and that this pattern could last up until 8 to 9 years of age, which is very long.
HAMILTON: So long that the orangutans were nursing almost until puberty. That's much longer than our own species. Shara Bailey of New York University says around the world, human babies rarely breastfeed beyond age 3.
SHARA BAILEY: That's what makes humans weird. And it's certainly one of the reasons why our population is so successful as a species.
HAMILTON: Bailey says switching babies to other foods allowed human females to have more offspring than other primates. The question now is, when did our human ancestors first start weaning babies early? Bailey says the answer may come from the teeth of our extinct relatives.
BAILEY: The potential is there to look now at Neanderthals, Homo erectus, Homo habilis, Australopithecines, you know, and we can actually get an idea of when this very weird thing that characterizes humans occurred.
HAMILTON: There's a catch, though. In order to be analyzed, a tooth has to be sliced up, and museums are reluctant to damage their prized fossils. Jon Hamilton, NPR News.
(SOUNDBITE OF KINACK'S "SHINE")
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.