Elizabeth Hunt Burrett, a mother from Australia, experienced a moment with an orangutan while breast-feeding her son at Melbourne Zoo in Australia last year. As she tells it, the orangutan came over to watch, locked eyes with her and gave an affirming nod. “It was the most beautiful thing,” she wrote in a widely circulated Facebook post. While it may be impossible to know exactly what this orangutan was thinking, it’s true that the critically endangered apes are exceptionally dedicated mothers. They give birth to one baby at a time, raising each for six to nine years, until it’s time to rear another. Mother and young sleep and spend most of their time with only each other. And young orangutans nurse longer than any other mammal — sometimes into their ninth year of life, according to a study published in Science Advances on May 17.
Recreating the nursing history
Because observing wild orangutans can be difficult, the authors recreated the nursing history of four orangutans by analysing barium, an element absorbed from maternal milk, in teeth taken from museum collections. In doing so, the scientists also discovered a possible clue why the apes nurse for so long: The teeth showed cycles in barium, which might correspond to environmental fluctuations in food. The Southeast Asian rain forests that orangutans call home are challenging environments, with unpredictable booms and busts in fruit, the animals’ most important food, said Tanya Smith, an associate professor in the Australian Research Centre for Human Evolution at Griffith University, Australia and an author of the paper.
“When times are tough, adult orangutans will fall back on things like bark or hard seeds,” Tanya said. “But offspring may not have the ability to eat some of these foods, or the knowledge to find them on their own, so they’re maybe falling back on mothers’ milk during periods of scarcity.” These are the first findings tying nursing to food scarcity in a primate, Tanya added. Previously, researchers estimated that orangutans wean off maternal milk between six to eight years old, but they could never be sure because field surveys of the animals are tricky: Offspring often suckle inconspicuously, high up in trees or at night, and even when suckling is observed, it’s hard to know whether the animals are consuming milk or just comfort nursing, with no milk transfer.
To get around this, Christine Austin and Manish Arora, researchers at the Icahn School of Medicine at Mount Sinai, USA, found a proxy for milk intake in orangutan teeth. Teeth are great for studying change over time because you can date them by growth rings, much like trees, Manish said. When producing milk, mothers incorporate ingredients from their own skeleton, including calcium and trace amounts of the chemically similar barium. These elements get written into the skeletons and teeth of their offspring. Barium makes a good marker for nursing, because it’s readily absorbed from maternal milk but not so much from other foods, Christine said.
The researchers studied nine teeth, belonging to two Sumatran and two Bornean orangutans, taking microscopic samples at hundreds of points in time per tooth, then quantifying the amount of barium in each sample. In all four orangutans, barium increased through the first year of life. It dipped between 12 and 18 months old, indicating that the apes started supplementing milk with solid food.
Vulnerable to extinction
Thereafter, barium fluctuated on a roughly annual basis, presumably in response to solid food availability. Scientists typically think of weaning as a ‘linear progression’ from dependence on maternal milk to independence, said Laurie Reitsema, an assistant professor of anthropology at the University of Georgia, USA who was not involved in the research. This study suggests that “it’s actually a series of pulses throughout in the orangutan’s case,” she said.
One reason food varies drastically for orangutans is because many live in forests that experience so-called mast fruiting, in which a large number of trees fruit at once, independent of the seasonal cycle. “These events occur every two to six years, and are kind of unpredictable,” said Cheryl Knott, an associate professor of anthropology at Boston University who was not involved in this study. An extended nursing period relates to why orangutans are headed for extinction at current rates of habitat loss and poaching, Tanya said. This study “points to an even more vulnerable ape than we might have originally understood,” she said. “That they need so long to be able to transition from one infant to the next explains why they’re struggling.”