Orangutans have one of the slowest life histories among mammals, and this study now shows just how long orangutan mothers continue to breastfeed their offspring. An international team has demonstrated that wild orangutan juveniles keep consuming their mother's milk continuously until at least six and a half years of age, confirming one of the longest breastfeeding periods known among mammals.
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 2‑year‑7‑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 6.5 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. This continuous and prolonged breastfeeding likely supports the exceptionally high survival of orangutan offspring and contributes to their slow reproductive strategy. At the same time, it underscores why populations of this critically endangered great ape recover very slowly from declines, highlighting the urgency of protecting their remaining rainforest habitats.
