Continuous and prolonged breastfeeding in wild Bornean orangutans verified with fecal proteomics

Orangutans have a slow life history with one of the longest interbirth intervals and the lowest reported infant mortality rates among primates or even mammals. Breastfeeding is a key factor in their life history because it possibly promotes offspring health and increases maternal interbirth intervals. However, quantifying milk intake is difficult, and existing estimates for their weaning age are contradictory. Here, we use fecal proteomics to predict the breastfeeding and weaning patterns in wild Bornean orangutans in Danum Valley, Sabah, Malaysia. Age changes in milk-specific proteins identified from 20 feces of five immature individuals revealed that the orangutans in Danum Valley consistently consumed milk for ≥6.5 years after birth, consistent with the behavioral evidence as having one of the longest breastfeeding periods in mammals. Milk intake was significantly correlated with higher levels of biological defense and probiotic bacterial proteins. Mothers were not pregnant with their next offspring during the breastfeeding period. These results indicate that a continuous and long breastfeeding period is a key component of the slow life history of orangutans and shows that fecal proteomics can be applied to a wide range of wild animal populations, with the potentials to uncover novel aspects of behavior and physiology.