Among known mammals, orangutans have the longest period of dependence and exclusive association between mother and offspring before a younger sibling is born. Comparison of available data on wild Sumatran and Bornean orangutans reveals a similar development of essential survival skills up to c. 5 years of age, but among Bornean orangutans earlier complete weaning and start of independent ranging through cessation of the association between mother and offspring, despite reported higher food availability in Sumatra. It is suggested that this difference is related to the assumed difference in main cause of mortality: starvation during irregular periods of widespread drought in Borneo vs predation in Sumatra. The benefits of association to both mother and offspring are likely to be different under these different selection pressures. To understand fully the differences between and variation among Bornean and Sumatran orangutans more high quality long-term demographic data on several populations are needed.