Calf-Snatching among Cetaceans? Interspecific Associations of Adult Bottlenose Dolphins with Common and Striped Dolphin Calves in Southern Portugal

Abstract Interspecific interactions among cetaceans are diverse and have been reported on numerous occasions. However, interspecific alloparental care (an adult exhibiting nurturant behavior towards a non-descendant young of a different species) is rarely documented. Here, we describe multiple calf-directed interspecific associations, including one apparent instance of prolonged interspecific alloparental care, along the southern coast of Portugal. This behavior was directed by adult common bottlenose dolphins ( Tursiops truncatus ) towards six common dolphin ( Delphinus delphis ) calves and one striped dolphin ( Stenella coeruleoalba ) calf. Out of these calves five were classified as neonates. Observations occurred during both dedicated and opportunistic boat-based surveys. In total, 14 sightings were documented between 2016 and 2025. In 13 of these sightings, the heterospecific calf was observed swimming in echelon position alongside an adult bottlenose dolphin, and antagonistic behaviors by adult bottlenose dolphins towards the calves were observed during three encounters. Photo-identification confirmed the involvement of 94 adult bottlenose dolphins throughout all encounters, with the recurring presence of 39 individuals (41.5%). Three observations revealed a consistent pairing between one adult bottlenose dolphin and a common dolphin neonate, which persisted for nearly one month in 2024, with no other adult swimming with a heterospecific calf more than once. Although the exact reasons behind these interactions remain unclear, this study provides a unique insight into the social dynamics between different cetacean species, spanning from antagonistic to seemingly care-giving behavior. Moreover, it documents the first instance of prolonged interspecific alloparental care among cetaceans along the coast of mainland Portugal.