Longest weaning period for a mammal

No other primate – or mammal – weans their offspring for longer than the orangutans (genus Pongo) of Borneo and Sumatra, with suckling occurring from birth until six to eight years old. A study published in the journal Science Advances on 17 May 2017, which examined the teeth of four immature orangutans from museum collections, revealed that a Bornean female nursed until 8.1 years old, while an individual from Sumatra weaned until approximately 8.8 years of age.

Observing nursing behaviour in orangutans is notoriously difficult, given that mothers normally suckle their young in the treetops and at night. To estimate the age that these specimens nursed to, the researchers analysed the level of barium in the teeth – a trace element that young orangutans absorb from their mother's milk.

The results showed that barium levels dropped significantly after the first year, but then would recur intermittently – up to post eight years of age in two of the studied skeletons. Barium levels corresponded to abundance of fruit, indicating that breast milk is used as a supplement in periods of shortage of other food.

The study was a collaboration between Griffith University (Australia), Harvard University, Arizona State University, The State University of New Jersey and the Icahn School of Medicine at Mount Sinai (all USA), led by human evolutionary biologist Dr Tanya Smith at Griffith University.