Weaning has been tricky to observe in the wild, so researchers turned to lab tests
The supermoms of the mammal world are big, shy redheads. Studying growth layers in orangutan teeth shows that mothers can nurse their youngsters for eight-plus years, a record for wild mammals.
Teeth from a museum specimen of a young Bornean orangutan (Pongo pygmaeus) don’t show signs of weaning until 8.1 years of age. And a Sumatran orangutan (P. abelii) was still nursing during the few months before it was killed at 8.8 years, researchers report May 17 in Science Advances.
Tests also show that youngsters periodically start to taper off their dependence on their mother’s milk and then, perhaps if solid food grows scarce, go back to what looks like an all-mom diet. Such on-again, off-again nursing cycles aren’t known in other wild mammals, says study coauthor Tanya Smith, an evolutionary anthropologist at Griffith University in Nathan, Australia.
Marks of milk drinking
Two images of a cross section of a first molar from a 4.5-year-old Bornean orangutan are shown. At left, numbers indicate days from birth (dotted line, starting with 0) when particular spots formed. At right, colors indicate concentrations of barium, which increase (shading toward red) when the youngster depended more on mother’s milk. A greenish swath at the top indicates nursing as an infant that gave way to blue as solid food became part of the diet. Yellow and red streaks indicate repeated times when the youngster again depended mostly on milk for nutrition.
Concentrations charted below (nonlinear) show a gradual increase during infant nursing and then peaks for the periodic returns to more intense suckling.
Before this work, weaning information for orangutans was sparse. Field biologists’ best efforts to track weaning in Bornean orangutans with known birthdays had pegged 7.5 years as the longest probable nursing time, Smith says. The only other weaning report in the wild for a Bornean youngster of known age was 5.75 years. Smith knows of no such reports for Sumatra’s orangutans.
Orangutans in their native forests don’t make weaning easy to detect, says Serge Wich of Liverpool John Moores University in England, who was not involved in the new study. He started watching the apes in 1993, and points out that “lactating happens very high up in trees, so we are always under a bit of an awkward angle to observe. Also, they’re quite furry.” Determining whether an infant is suckling or just cuddling is not an exact science.
For more accurate dating, Smith and colleagues turned to information preserved in teeth. Primate teeth grow with a circadian rhythm, laying down a microscopic layer every day, starting before birth. Babies grow bones and teeth using milk calcium, which their moms pull from their own skeletons. A similar element, barium, hitchhikes along and ends up in bones and teeth, too. “Mothers dissolve parts of themselves to feed their children,” as Smith puts it. Greater concentrations of barium in a tooth layer mark a time when the tooth was being built up with a greater proportion of mother’s milk.
To read the history of mother’s milk, Smith used a method to track barium concentrations that was developed with colleagues at the Icahn School of Medicine at Mount Sinai in New York City. The researchers sampled sets of molars from four immature specimens, two of each orangutan species, which were preserved in museum collections. The teeth came from decades ago when collectors “went around randomly shooting endangered species,” Smith says.
Now, both Bornean and Sumatran orangutans rank as critically endangered. A fever of logging and oil palm planting is eating away their scraps of forest, and the pet trade rewards hunters who shoot a mom to bag a cute baby to sell. Neither species had lush resources to begin with, as the animals evolved in forests with booms and long busts in food supplies. Prolonged nursing of young may be part of their slow-lane accommodation to continuing uncertainty and scarcity in their environment.
Researchers debate whether some similar uncertainty shaped human evolution. Among apes, the human species has evolved a “stretched-out” childhood, though with different pacing from that of orangutans, Smith says. “Studying our cousins puts our own history in context.”
A wild orangutan feeds her month-old baby in Gunung Palung National Park on Borneo, the beginning of what could be years of nursing. The young ape’s growing teeth pick up enough barium from the mother’s milk for researchers to track how long the animal suckles. Copyright Gunung Palung Orangutan Project
T.M. Smith et al. Cyclical nursing patterns in wild orangutans. Science Advances. Vol. 3, published online May 17, 2017, p. e1601517. doi: 10.1126/sc1adv.1601517.
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