The supermoms of the mammal world are big, shy redheads. These orangutans are great apes. They also are great at feeding and caring for their young. To learn this, scientists didn’t just watch how long young animals spent with mom. Some also studied growth layers in their teeth. These showed youngsters can rely on mothers’ milk for eight years or more. That’s a record for wild mammals.
The teeth came from a museum specimen of a young orangutan (Pongo pygmaeus) captured in Borneo. These teeth showed no sign that the animal had been weaned — switched from mother's milk to solid food — until it was 8.1 years old. Teeth from another orangutan (P. abelii) from Sumatra may have nursed even longer. Its teeth showed signs the animal was still nursing during the few months before it was killed at 8.8 years old.
Researchers reported these lengthy mothering behaviors May 17 in Science Advances.
Their tests also showed that youngsters don’t just stop depending on mom all at once. At times they taper off drinking their mother’s milk as they begin eating solid food. Then, if that other food gets scarce, they may return to what looks like an all-mom diet.
Such on-again, off-again nursing cycles aren’t known in other wild mammals, says Tanya Smith. She is an evolutionary anthropologist at Griffith University in Nathan, Australia. She worked on the new study.
Before her team collected these data, nobody had good numbers for how long a baby orangutan nursed. The best estimate was 7.5 years, Smith says. The only other estimate was 5.75 years. It came from a Bornean youngster whose age was known. (Smith knows of no similar estimates for Sumatra’s orangutans.)
Serge Wich is an ecologist in England who did not take part in the new study. He works at Liverpool John Moores University. Orangutans in their native forests in Indonesia don’t make weaning easy to see, Wich notes. For his own research, he started watching orangs in 1993. Nursing “happens very high up in trees,” he says. Having to look way up through branches makes it hard to tell whether a baby is nursing or just cuddling. Adding to the confusion is mom’s fur. It can partly hide her baby’s face.
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For more accurate dates, Smith and colleagues turned to teeth. Apes and other primates grow their teeth in a circadian cycle. Their bodies lay down a new microscopic layer of tooth daily, which starts before birth. Babies grow bones and teeth using the calcium in milk. To get that mineral, a mom’s body pulls it from her bones. As Smith explains it, “Mothers dissolve parts of themselves to feed their children.”
In the body, the element barium acts a bit like the calcium. Barium occurs naturally in a host of food. As such, It will hitchhike into a baby along with the calcium in its mom’s milk. This barium then ends up in bones and teeth, just as calcium does. When babies switch to non-mom feeding, they don’t get as much barium. So tooth layers rich in barium mark when the youngster drank a lot of milk.
To read that history of milk consumption, the researchers tested molars from museum specimens of these apes. The teeth were decades old. Explains Smith, they came from when collectors “went around randomly shooting endangered species.”
Now, Bornean and Sumatran orangutans both rank as species critically endangered with extinction. Why? Their forests are shrinking as people cut down trees to sell for wood or just to clear land on which to grow oil palms. Wild orangutans may also end up in the pet trade. Hunters shoot the moms and sell their cute babies.
Neither orang species had a lush life. The animals evolved in forests with booms and long busts in food supplies. Prolonged nursing may be one way orangutans evolved to deal with this uncertain access to food.
Researchers debate whether similar uncertainties shaped human evolution. It’s hard to tell how people would grow up if we were still wild animals. Smith estimates that humans might not have drunk their mothers’ milk for as long as orangutans do. Yet humans don’t get their last adult teeth — the so-called wisdom teeth — until long after they start dining on food other than milk.
In this and some other ways, human species have evolved a “stretched-out” childhood, too. Says Smith, “Studying our cousins puts our own history in context.”
(for more about Power Words, click here)
ape A group of rather large “Old World” primates that lack a tail. They include the gorilla, chimpanzees, bonobos, orangutans and gibbons.
barium An abundant, silvery-white metal that can be founds in air, water and soils across the planet. From there it often enters foods, such as fish, nuts and certain plants.
Borneo The largest island in Asia and third largest in the world. It is mountainous, covered by vast expanses of rainforest and sparsely populated by people. Part of its land belongs to the nation of Indonesia, a small part to the sultanate of Brunei and the rest to the nation of Malaysia.
calcium A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
circadian rhythm Biological functions such as body temperature and sleeping/waking times that operate on a roughly 24-hour cycle.
colleague Someone who works with another; a co-worker or team member.
concentration (in chemistry) A measurement of how much of one substance has been dissolved into another.
diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health.
dissolve To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
element (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
endangered An adjective used to describe species at risk of going extinct.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
evolution (adj. evolutionary) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
nurse (in biology) A term for suckling a young mammal so that it can feed on its mother’s milk.
palm A type of evergreen tree that sprouts a crown of large fan-shaped leaves. Most of the roughly 2,600 different species of palms are tropical or semitropical.
primate The order of mammals that includes humans, apes, monkeys and related animals (such as tarsiers, the Daubentonia and other lemurs).
species A group of similar organisms capable of producing offspring that can survive and reproduce.
Sumatra A part of the island nation of Indonesia, this is one of its bigger islands (and indeed, the sixth largest island in the world).
wean (adj. and v. weaning) The process in young mammals of transitioning from a diet of mother’s milk to other foods.
Journal: T.M. Smith. Cyclical nursing patterns in wild orangutans. Science Advances. Vol. 3, Published online May 17, 2017, p. e1601517. doi: 10.1126/sc1adv.1601517.
Journal: C. Austin et al. Barium distributions in teeth reveal early-life dietary transitions in primates. Nature. Vol. 498, June 13, 2013, p. 216. doi:10.1038/nature12169.