The findings are published in the journal Communications Biology.

Previous studies using stable isotopes or trace elements such as nitrogen or barium had suggested inconsistent breastfeeding patterns in orangutans and could not clearly separate signals of breast milk from other dietary sources.

In this study, researchers used a technique called fecal proteomics, which identifies specific proteins in feces. Because orangutan milk contains several proteins that are unique to milk, finding these milk-specific proteins in a juvenile's feces is direct evidence that the individual is still consuming breast milk.

The team applied fecal proteomics to fecal samples from wild Bornean orangutans in the Danum Valley Conservation Area in Sabah, Malaysian Borneo. Over a two-year, seven-month period, they collected fecal samples from identified individuals.

Milk-specific proteins were detected in all 20 samples from individuals aged under six and a half years, indicating that the juveniles still consumed breast milk at least six and a half years after birth.

By comparing milk-specific proteins with biological defense proteins and probiotic intestinal bacteria proteins, the study also suggests that the more milk a juvenile consumes, the stronger its biological defenses and the higher the presence of probiotic intestinal bacteria.