Successful adoption of an orphan infant in a wild group of brown howler monkeys

The rarity of infant adoption in wild primates compromises our understanding of its consequences for the participating individuals. We report the first case of successful infant adoption in a wild group of brown howler monkeys (Alouatta guariba clamitans). We evaluated the potential costs of the behavior for the adoptive mother by comparing her activity budget and diet before and after the adoption. On 18 June 2013, a domestic dog killed the mother of a 2-month-old male infant (Victorio) as she attempted to cross a canopy gap. Victorio was immediately rescued from her belly by a researcher and released in a climber near another infant-carrying female (Sofia, his likely grandmother). Sofia recovered him 2 min later. She carried and breastfed both infants during the next 4 weeks, when her own infant disappeared. We monitored Victorio until he reached adulthood in March 2018. Sofia fed more (mainly on immature leaves) when she nursed only Victorio than when nursing only her own or both infants. Assuming that the disappearance of Sofia's own infant was unrelated to the adoption of Victorio, we conclude that his successful adoption may contribute to Sofia's inclusive fitness if he sires his own infants.