A new study published at the end of May in the scientific journal Communications Biology showed that orangutans in the tropical forests of Borneo, Indonesia, breastfeed their offspring for an average of 6.5 years. This is one of the longest breastfeeding periods ever recorded among mammals in the wild.

Scientists already knew that orangutan infants nursed for a considerably longer period compared to other animals, but no previous study had been able to accurately quantify milk intake, leading to contradictory estimates regarding weaning age.

In this study, the team (Malaysian and Japanese scientists) spent nearly three years collecting and analyzing fecal samples from wild Bornean orangutans in the Danum Valley Conservation Area, in Sabah, Malaysia.

They used a technique known as fecal proteomics, which identifies specific proteins in feces. Orangutan milk contains several proteins that are not present in other food sources. Therefore, when these proteins are found in the feces of a young orangutan, it indicates that the animal is still consuming its mother’s milk.

The researchers explain that orangutans have a slow life history—they can live for around 40 years in the wild, with long intervals between births and one of the lowest infant mortality rates recorded among primates. Breastfeeding is a key factor in this, as it promotes offspring health while also increasing the interval between births (females only reproduce again after weaning).

Read the full study at: https://www.nature.com/articles/s42003-026-09968-2