A Case of Sibling Adoption in Wild Orangutans: Accelerated Development of Independence Following Maternal Loss

ABSTRACT Maternal loss is a detrimental early life adversity with negative consequences for the survival and development of juveniles. These negative consequences can be mitigated by adoption of the orphan. Here we provide the first detailed report on a case of adoption in wild Bornean orangutans ( Pongo pygmaeus wurmbii ) in Gunung Palung National Park, West Kalimantan, Indonesia. Among other great ape species, orphans have a greater chance of survival if adopted. In many of these cases, the orphans were adopted by siblings or other related individuals who provided critical alloparental care. We compared the behavior of this adoptive sister pairing with mother‐offspring pairs in our study population and considered costs and benefits to each sister. The orphaned younger sister survived and reached developmental milestones at a younger age than the population average. Her activity budget also showed a reduction in the percent of time spent playing. The adoptive older sister did not experience a delay in her reproductive timeline due to the adoption, but the age‐sex class profile of her social partners differed from other adolescent females in our study population. The daily average distance between the sisters was greater than the distance between mothers and their offspring, but there was no difference in time spent engaged in feeding tolerance compared to mother‐offspring pairs. Maternal loss appears to have accelerated the development of independence, while adoption and alloparental care provided by her older sister likely allowed for her survival. This study also highlights the value of genetic identification analysis along with long‐term data collection.