Scientists have documented the first case of sibling adoption among wild Bornean orangutans in Gunung Palung National Park, Indonesia, where genetic analysis confirmed that Rossa, an adolescent female, adopted her younger sister Ronnie after their mother disappeared, according to Amy Scott and colleagues in the journal Ethology.
Ronnie, orphaned between ages 4.2 and 5, reached developmental milestones earlier than average orangutans, but spent less time playing, suggesting that maternal loss accelerated her independence while Rossa’s alloparental care played a key role in her survival during this vulnerable period.
Although the sisters maintained a greater distance compared to typical mother-offspring pairs, reflecting the orangutans' semi-solitary nature, Rossa still showed exceptional caregiving by sharing food and providing the same level of feeding tolerance that biological mothers typically offer.
Rossa’s adoption of Ronnie did not delay her own reproductive timeline, but researchers observed some behavioral changes in Rossa’s social interactions, indicating adjustments due to her caretaking responsibilities.
Amy Scott pointed out that this discovery raises important questions about the frequency of sibling adoption in wild orangutans, highlighting its significance for conservation and reintroduction programs, as orphaned orangutans face challenges in acquiring the species’ complex diet—consisting of around 250 food types—that is typically learned through years of maternal care.
