In 2012, a Time magazine cover showing a three-year-old kid breastfeeding caused a ruckus. Well, that photo would have been just fine in the orangutan world: young orangutans keep nursing for eight years or more — longer than any other mammal, according to a new study.

Researchers analyzed the teeth of four wild orangutans for an element absorbed from breast milk, barium. The presence of barium suggested that breastfeeding continues in cycles for at least eight years, helping young orangutans get their nutrition even when other food sources like fruits are scarce. The findings, published today in the journal Science Advances, deepen our understanding of these elusive primates — and could help scientists in their efforts to protect them from extinction.

Orangutans are the world's largest tree-climbing mammals. The big apes live in forests in Indonesia and Malaysia — unpredictable environments with limited nutritional resources. Because they evolved in this environment — never really knowing when the next meal is — “their whole life history is kind of slowed down,” says Cheryl Knott, an associate professor of anthropology at Boston University. Their metabolism is slower than that of other apes, they reproduce later in life, and they nurse their babies for longer.

Scientists, in fact, have long known from observations in the field that young orangutans nurse for years after birth. But getting accurate data is hard, because suckling often happens on remote tree tops or even during the night, says study co-author Christine Austin, a postdoctoral fellow at the Icahn School of Medicine at Mount Sinai. “These are animals that are very difficult to study,” she says. So the team behind today’s research decided to look at milk intake by studying the teeth of wild orangutans.

Teeth “are basically a biological hard drive.”

Teeth “are basically a biological hard drive that’s reporting every day what’s going on in your body,” Austin says. Literally every day of childhood is recorded in teeth — and that record can be analyzed to understand what the person (or monkey) was eating or how healthy he was. That allows “biological anthropologists an unprecedented window into the past,” lead author Tanya Smith, at the Australian Research Center for Human Evolution at Griffith University, writes in an email to The Verge.

So the researchers got ahold of the teeth belonging to four wild orangutans that were shot by collectors during expeditions, and stored at the Humboldt Museum in Berlin, the State Anthropological Collection in Munich, and the Harvard University Museum of Comparative Zoology. (At least two of the orangutans were shot more than 100 years ago, Smith says. “This was a normal way for Western countries to obtain zoological material for research collections,” she says. But today, orangutans and all great apes are protected, and the practice is forbidden.)

The researchers cut the teeth open and fired a laser at them to vaporize a small bit of tooth material. That material was then analyzed to calculate the chemical components, including barium, which is present in breast milk. By calculating how much barium was present through the tooth “history,” the researchers were able to determine the breastfeeding behavior of the four specimens.

For example, they saw that after the first year, barium levels generally decreased. That’s because the orangutans were feeding exclusively on milk for the first year of their lives, but then began eating also other things, like leaves and fruits, relying less on breast milk. They also saw that breastfeeding happened in cycles — sometimes the babies were suckling more, sometimes they were sucking less. That’s probably because whenever other food sources were scarce, the babies drank more breast milk, and vice versa. But breastfeeding continued into the eighth and ninth year of life.

That’s longer than other primates. Chimps, for example, nurse until about five years old, Knott says. For humans, the time frame varies, but the American Academy of Pediatrics recommends that babies be exclusively breastfed for about the first six months of life. Breastfeeding can then continue for one year, or as long as the mother and baby want, the guidelines say.

The findings corroborate what we already suspected from field observations, Knott says, but also add more information on the cyclical nature of breastfeeding in orangutans. The information could help scientists in their efforts to protect orangutans from extinction. The animals are endangered because they’re very slow to reproduce — females wait until they’re 10 or 15 to reproduce, and they give birth once every five years at most. Sometimes they wait as long as 10 years between babies. Better understanding how breastfeeding works, and how the environment plays a role in it, is key to protect the animals, Knott says.

And there’s no time to waste. A century ago, according to the WWF, there were around four times as many orangutans in the world as there are today.