When should babies stop breastfeeding? Given the explosive response to a 2012 TIME cover of a woman breastfeeding a three-year-old child, that question is a controversial one at best. But when it comes to primates, orangutans, it seems, have all other species beat, reports Austa Somvichian-Clausen for National Geographic. They breastfeed their young for up to eight years.
Studying orangutan nursing habits, however, is far from easy. Orangutans like to climb and nurse at night, clasping their babies close, making it almost impossible to make direct observations of how everything works in the wild—or how long they nurse.
But it turns out there’s a clue to when orangutans wean: their babies’ teeth. The teeth and other bones of orangutans that drink mothers’ milk contain the element barium. So researchers decided to analyze the barium levels in orangutan teeth in the hopes it would serve as a kind of backdoor to their nursing habits.
They detail their findings in a paper in the journal Science Advances. The team studied four immature orangutans that were shot in the wild and their bones housed in zoological museums.
Since teeth grow outwards, kind of like trees, they’re like time capsules of different growth periods. Using a mass spectrometer, the team analyzed the layers of each of the orangutan’s teeth, measuring barium as an indicator of times during which they drank a lot of their mothers’ milk.
They found that though barium levels fell after a year—when the babies presumably started eating solid foods—it was still present years later. Though the barium levels varied among the animals, they showed much longer periods of breastfeeding than previously expected. One of the orangutans was shot when it was nearly nine years old, and still had barium in the outer layers of its teeth.
That’s significant for two reasons. Not only does it mean that orangutans nurse longer than any known primate, but it could reveal important information about how breastfeeding evolved in humans. The team hypothesizes that during food shortages, orangutans breastfed their older kids instead. In turn, that could help other researchers figure out times of environmental variation for early humans or even when they started living longer and needing less breast milk.
Barium has already revealed clues about prehistoric humans: In 2013, researchers found that one Neanderthal breastfed continuously for about seven months and stopped intermittent feeding by 1.2 years of age. Perhaps information about how orangutans breastfeed—and how that milk benefits their young—could one day influence how long their human cousins nurse their babies.