Multiple social benefits drive the formation of mixed-species groups of Australian humpback and Indo-Pacific bottlenose dolphins

• Published in: Behavioral ecology and sociobiology 2023-04, Vol.77 (4), p.43, Article 43
• Main Authors: Syme, Jonathan, Kiszka, Jeremy J., Parra, Guido J.
• Format: Article
• Language: English
• Subjects: Age composition; Aggressive behavior; Animal Ecology; Anti-predator behavior; Aquatic mammals; Behavioral Sciences; Biomedical and Life Sciences; Cetacea; Composition; Dolphins; Dolphins & porpoises; Life Sciences; Original Article; Sexual behavior; Social behavior; Social factors; Social interactions; Sousa sahulensis; Species; Sympatric populations; Tursiops aduncus; Zoology
• ISSN: 0340-5443; 1432-0762
• DOI: 10.1007/s00265-023-03320-y

Mixed-species groups are common amongst diverse taxa including fishes, birds, and mammals. Antipredator, foraging, and social benefits have been proposed as functional explanations for mixed-species group formation. Amongst delphinids, mixed-species groups are widespread, but little is known about their function. To investigate the potential benefits of delphinid mixed-species groups, we compared the number of individuals, the age composition, and the behaviour of single- and mixed-species sightings of Australian humpback ( Sousa sahulensis ) and Indo-Pacific bottlenose dolphins ( Tursiops aduncus ) observed around the North West Cape, Western Australia. We found no difference in the number of individuals or the age composition of humpback dolphins present in single- and mixed-species sightings, whereas bottlenose dolphins were present in larger numbers in single-species sightings than in mixed-species sightings due to a higher number of adults. Socialising was the initial observed behavioural state of 36.1% of mixed-species sightings, compared to only 5.1% and 10.3% of humpback and bottlenose dolphin single-species sightings, respectively. Furthermore, both species travelled and foraged less frequently when in mixed-species groups. Of 93 mixed-species groups observed during a focal follow of ≥ 10 min, 32 (34.4%) involved aggressive and/or sexual behaviours typically initiated by bottlenose dolphins towards humpback dolphins while the remaining 61 (65.6%) involved only neutral and affiliative behaviours. The results of this study suggest that the observed mixed-species groups provide multiple social benefits, particularly those pertaining to socio-sexual behaviours and the development and care of young. Significance statement Numerous species of dolphin are known to form mixed-species groups. Few studies have, however, investigated the antipredator, foraging, and/or social benefits that these species may gain by grouping with other species. Investigating these benefits is key to understanding the impacts of mixed-species groups on the species and individuals involved. We evaluated the potential functions of mixed-species groups of two sympatric, inshore dolphin species — the Australian humpback dolphin and the Indo-Pacific bottlenose dolphin. We found that both species engage in socialising behaviour more frequently when in mixed-species groups and that they engage in a diverse variety of interspecific behavioural interactions. Our results not only indicat