A new technique has revealed orangutans are dependant on their mothers' milk longer than any other primates, and probably any mammal, sometimes not being weaned until eight or nine. The method used to demonstrate this could tell us a lot about how our ancestors raised their young.
Few animals produce new offspring before weaning siblings, so the age of weaning is an important indicator of reproduction rate. For elusive species such as orangutans, it is not so easy to find out when weaning occurs in the wild, however. A paper in Science Advances notes “Nursing behavior is notoriously difficult to study in arboreal primates, particularly when offspring suckle inconspicuously in nests.”
First author Dr Tanya Smith of Griffith University previously demonstrated milk consumption leaves a residue in teeth, distinguishing milk contribution to diet as the teeth were formed. In the new paper, she applies this technique to teeth held in museums from the days when orangutans were still shot for collections.
In addition to showing these orangutans were still drinking their mother's milk until the age of eight for a Bornean orangutan, and almost nine for one from Sumatra, Smith's work revealed seasonal cycles in food intake.
The orangutans fed on milk almost exclusively for the first year of life. After this their food consumption varied, living on fruit when it was abundant but surviving on their mother's milk when times were tough. The authors suggest this is because infant orangutans lack the fat stores that allow adults to survive periods of scarcity, so they access these indirectly, through the mother. Nevertheless, increases in milk consumption were associated with periods of weight loss.
Smith's work relies on barium levels. Barium shares transport pathways in the body with calcium (and lead, but hopefully there's not too much of that). Consequently, when nursing mothers draw on calcium in their bones to feed their young, the milk also concentrates barium, leaving a lasting legacy in the young ones' mouths.
Smith told IFLScience her technique is harder to apply for species whose teeth develop more rapidly, but for primates, and particularly apes, it should be widely applicable. Although teeth from early human fossils are hard to access for destructive testing, her work potentially provides an opportunity to trace the history of breastfeeding through human evolution. She's also keen to study ancestral orangutans from the days when they roamed through much of Asia, to see if other environments provided more reliable food supplies.
Smith said her work could also help assess when orphaned orangutans are ready to be released into the wild. Primarily, however, she “Hopes it highlights how vulnerable they are... since they can't replace themselves fast.”