Juni, a adult female, and her 11 month old daughter, Jane. Juni is eating fruit of Ardisea sp. and Jane is watching/learning and eating some herself Bornean Orangutan Wurmbii Sub-species (Pongo pygmaeus wurmbii) Tuanan Field Site Tuanan Orangutan Research Project Mawas Conservation Area Central Kalimantan Province Island of Borneo Indonesia. Tim Laman

It is incredibly hard to observe an orangutan nursing its baby. Even without dedicated nursing rooms, these already reclusive animals manage to find privacy in trees and at night to nurse their young, making it difficult for biologists to tell how long and how often orangutan mothers breastfeed their offspring.

But now there’s another way for biologists to understand the nursing behaviors of orangutans: their teeth. In a paper published Wednesday in Science Advances researchers found that orangutans breastfeed for even longer than the six to seven year nursing period observed in the wild, breastfeeding for over eight years in some cases, using the milk cyclically to supplement a young orangutan’s diet of fruits and insects.

“We’ve found the first evidence for a seasonal pattern of increasing and decreasing milk consumption over several years, which is unexpected as primates are conventionally believed to simply increase milk consumption after birth, reach a plateau, and then decrease consumption as they transition onto solid foods and then stop suckling,” says lead study author Tanya Smith. “This kind of behavioral flexibility in orangutans may be unique, but we’ll need to investigate other wild primates’ nursing histories to know.”

Smith, who works at the Australian Research Centre for Human Evolution at Griffith University, studies how teeth form in orangutans and other primates. She and her colleagues analyzed orangutan teeth (molars in particular) from museum collections, looking for chemical signs of breastfeeding.

When a mammal is breastfeeding, some of the calcium that goes into her milk comes from her skeleton. Calcium is very similar, chemically, to another element found in bodies—barium.

When an infant consumes the calcium-laden milk from its mother, “Some of the barium is along for the ride,” says Christine Austin, who did much of the chemical analysis in the study. “Barium is a bone-seeking element that ends up in the skeleton and teeth,” she explains.

In orangutans and humans, teeth build up slowly over time, forming layers like trees that can be used to gather information about an individual’s diet or environment.

Orangutans don’t really get a lot of barium in their diet when they switch to solid foods (in their case, fruit and insects). So by looking at barium in their teeth, the researchers can tell when an orangutan was breastfeeding, and when they were eating solid foods.

In the four individuals in this study, the orangutans primarily relied on their mother’s milk when they were very young. Once they got older, they went through cycles where they would breastfeed more, and cycles where they were breastfeeding less, a situation that the researchers think might have been due to changing food availability in the area where the orangutans lived. The idea is that when food was less plentiful, the orangutans relied on their mothers more for sustenance. In some cases, the individuals were breastfeeding for over eight years, according to the barium signals.

This research opens up a number of avenues for future studies. Austin and colleagues have been working for several years on how signals in teeth can indicate breastfeeding in other primates, including humans and even neanderthals.

“We love teeth. It’s amazing the amount of information you can glean from them,” Austin says. In addition to information about breastfeeding, she explains, teeth can also record lead exposure and other stresses. Austin and colleagues are using data from long-term studies to identify potential factors that could show up in teeth.

There’s also a lot that can still be learned about orangutans as well.

“We’re planning to look at other primates who lived in seasonal environments to determine if this nursing behavior is part of the unique suite of features that characterize orangutans,” Smith says. “I’m also hopeful that people will consider this information in their conservation efforts, as prolonged development makes orangutans especially vulnerable to extinction.”