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.