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Although behavioural interactions within mixed-species groups are common among cetaceans (Herzing & Johnson, 1997; Acevedo-Gutiérrez et al., 2005; Kamaruzzan & Jaaman, 2013), interspecific alloparental care-especially in the form of adoptions-is rarely observed. Alloparental care is defined as any behaviour by an individual towards non-descendant young that benefits the young (Woodroffe & Vincent, 1994). This can occur through indirect behaviours such as herding and sentinel behaviour or directly through behaviours such as babysitting, provisioning, and adoption (Kleiman & Malcolm, 1981; Lewis & Pusey, 1997). Unlike intraspecific alloparental care, which has been observed in several cetacean species, including Atlantic white-sided dolphins (Lagenorhynchus acutus; Simard & Gowans, 2004), sperm whales (Physeter macrocephalus; Gero et al., 2009), pilot whales (Globicephala melas; Augusto et al., 2016), bottlenose dolphins (Tursiops aduncus; Mann & Smuts, 1998; Sakai et al., 2016), killer whales (Orcinus orca; Ford et al., 2000), and humpback dolphins (Sousa plumbea; Karczmarski et al., 1997), interspecific alloparental care in cetaceans has only been observed and documented in a few cases.
The best-documented case of interspecific alloparental care in cetaceans involves the complete adoption of a melon-headed whale (Peponocephala electra) calf by a female common bottlenose dolphin (Tursiops truncatus) at Rangiroa atoll, French Polynesia (Carzon et al., 2019). The association between the primiparous female, which had a dependant biological calf when the adoption occurred, and the adoptee continued for 4 years during which time nursing was evident. Interspecific alloparental care has also been observed in Indo-Pacific humpback dolphins (Sousa chinensis) on two occasions. In Malaysia, three adults associated with an Irrawaddy dolphin (Orcaella brevirostris) calf for 4 days (Kamaruzzan & Jaaman, 2013), while in a separate event in China, a finless porpoise (Neophocaena phocaenoides) calf was herded and assisted for 3 hours by eight humpback dolphins (Wang et al., 2013). Such behaviour has been recorded only once off South Africa and involves a single observation of an Indo-Pacific bottlenose dolphin calf swimming with six Indian Ocean humpback dolphins in Algoa Bay (Karczmarski et al., 1997).
Many if not all of these cases of interspecific alloparental care have been relatively brief. Herein, we describe two events of interspecific alloparental care linked to one identified individual female Indian Ocean humpback dolphin. Further, we believe one of these events entails a full-fledged interspecific adoption...