Prevalence, Composition, and Behaviour of Humpback Whale (Megaptera novaeangliae) Groups Containing Multiple Mother–Calf Pairs in East Australia

Humpback whale (Megaptera novaeangliae) mothers and their calves predominantly avoid social interactions with conspecifics. However, observations from East Australia indicate that mother–calf pairs occasionally associate with other mother–calf pairs. We investigated the prevalence, composition, and behaviour of multiple-mother-calf (multi-MC) groups in East Australia from 2009 to 2023. In 2009 and 2010, data were collected from two tropical breeding areas (Port Douglas and the Whitsunday Islands) and two coastal migratory locations (Hervey Bay and Eden). No multi-MC groups were detected in Port Douglas out of 15 calf groups observed. Only one (4%) of 23 calf groups encountered in the Whitsunday Islands was a multi-MC group. In Hervey Bay, seven (4%) multi-MC groups were recorded out of 180 calf groups; while in Eden, seven (11%) out of 63 calf groups were multi-MC groups. Between 2009 and 2023, 978 calf groups were encountered in Hervey Bay over 12 years, including 56 (6%) multi-MC groups; while 39 (13%) out of 293 calf groups encountered in Eden over six years were multi-MC groups. Multi-MC groups exhibited more social behaviours compared to single MC groups. Social behaviour was common in both Hervey Bay and Eden; however, groups in Hervey Bay displayed lower arousal behaviours, consistent with the area’s importance as a resting habitat for whales. We did not detect behavioural differences in multi-MC groups with or without escort whales. These findings suggest that humpback whale mothers may gradually socialise their calves by forming multi-MC groups, with an increasing prevalence along the southerly migration route as calves mature. While multi-MC groups have only been detected in Australia to date, it is possible that they occur elsewhere, particularly if social development is a key driver of the phenomenon. Our results enhance our understanding of humpback whale mother–calf sociality and highlight the importance of migration routes for calf social development.