The coexistence of sympatric species with similar ecological requirements involves niche partitioning via interspecific differences in resource use and behaviour. Behavioural patterns of coexisting species can also be indicative of the functions of interspecific interactions, including those between species that form mixed-species groups. Here, we used Markov chain analysis applied to behavioural focal follow data of Australian humpback, Sousa sahulensis (hereafter ‘humpback dolphins’) and Indo-Pacific bottlenose dolphins, Tursiops aduncus (hereafter ‘bottlenose dolphins’) observed around the North West Cape, Western Australia, to investigate potential behavioural coexistence mechanisms and the functions of their mixed-species groups. During six austral winter (April to October) field seasons (2013–2015, 2018–2019 and 2021), we recorded 560 dolphin sightings from which we obtained 562 sequences of behavioural transitions: 222 humpback dolphin single-species sequences, 262 bottlenose dolphin single-species sequences and 78 mixed-species sequences. Humpback dolphins spent more time travelling in addition to having higher transition probabilities towards travelling and longer bout lengths of travelling, while bottlenose dolphins spent more time foraging and had longer foraging bouts. These behavioural patterns indicate that humpback and bottlenose dolphins differ in their use of the study area. In particular, they suggest interspecific temporal differences in foraging behaviour, which may facilitate coexistence. When in mixed-species groups, humpback and bottlenose dolphins altered their behavioural patterns by exhibiting higher transition probabilities towards socializing and increasing time spent socializing, suggesting that their mixed-species groups may have a social function. Overall, our results indicate that behavioural differences may play an important role in facilitating the coexistence of sympatric delphinids and that social benefits may be a key driver of the formation of mixed-species groups.
