Orangutans nurse their young for up to eight years or longer, a new study finds—a record for primates. As National Geographic notes, it’s difficult to study orangutans in the wild since they’re so often out of sight in trees, but it’s important for conservationists to know when juveniles become independent. Orangutans reproduce for the first time around age 15 and are estimated to live up to 50 years in the wild. They reproduce slowly, the New York Times reports, with mothers raising one baby at a time for six to nine years before birthing another. To determine nursing length, researchers analyzed the levels of barium (a trace element that orangutans absorb from their mother’s milk) in the teeth of four young orangutans whose bones had been kept in museums.
The team found that one orangutan weaned at 8.1 years old; another was still nursing when it died at 8.8 years old. Before this research, scientists estimated orangutans might nurse for six to eight years, but it was difficult to know since field surveys are so challenging. Because the growth patterns of teeth “resemble tree rings” and can be easily dated, per a press release, they also revealed other interesting aspects of the nursing relationship: During periods of time when fruit was known to be abundant, barium levels were lower, and vice versa, indicating that when food was scarce, young orangutans nursed more. Researchers believe the fact that orangutans’ food supply is unpredictable is part of the reason they nurse so long—captive primates with a steady supply of food tend to mature at a faster rate than wild primates. (Apes may be able to “read minds.”)