Humpback whale adult females and calves balance acoustic contact with vocal crypsis during periods of increased separation

Acoustic communication is important for animals with dependent young, particularly when they are spatially separated. Maternal humpback whales (Megaptera novaeangliae) use acoustic calling to help minimise the risk of separation from their young calves during migration. These pairs also use acoustic crypsis to minimise detection by males. How they balance a restricted active space with the need to maintain acoustic contact during periods of separation is not yet understood. Here, we analysed movement metrics of tagged adult female-calf pairs during migration to identify two behavioural states, 'resting/milling' and 'travelling'. When travelling, these pairs dived synchronously and exhibited little to no spatial separation. Alternatively, adult females had significantly longer dive durations (p < 0.01) when resting, and while they spent prolonged times at depth, calves would surface several times independently. This demonstrated that these pairs are frequently separated during periods of rest. We then determined if the call rates and acoustic levels of these pairs increased with more frequent separation, finding that both adult females and calves significantly increased their call rates, but not levels, when resting. We also found that adult female-calf pairs have a restricted active space, with less than 15% of calls estimated to be detectable beyond 2 km. However, as with call level, detection distance did not differ significantly between the two behavioural states. In summary, adult female-calf pairs maintain successful communication during periods of separation by calling more frequently rather than by producing louder calls. This strategy aids in maintaining acoustic contact while simultaneously limiting detectability by conspecifics.Please see the README document for information regarding the data spreadsheets.This dataset was collected using animal-borne recording tags and accompanying boat- and land-based visual observations, as well as recordings from a moored hydrophone array. The non-hierarchical partitioning clustering method 'k-means' was used to determine adult female-calf behavioural states; a glmer was used to examine whether fine-scale dive parameters of the adult females were significantly different between behavioural states; GLMM's were used to determine if call rate (calls per hour) varied per individual between behavioural states; an lmer was used to determine if adult females modified their received call levels (dB re 1 µPa) in response to the behavioural state of the pair; and a site-specific frequency- and distance-dependent transmission loss equation was used to determine the communication active space of adult female-calf vocal signals, while a glmer was used to determine whether these pairs modified this active space in response to their behavioural state