We all know humpback whales produce amazing songs that can be heard for miles, but new research has shown that during their migration from the breeding grounds to feeding areas, calves make low level vocalisations – a new paper theorises the reasons for these quiet sounds.
Acoustic tags (Dtags) were attached to mother and calf humpback whales in Exmouth Gulf, Western Australia, which is an area where humpbacks breed and rest during their migration south to feeding grounds. The acoustic data showed that calves make very weak tonal and grunting sounds (listen to the video below). These vocalisations are produced during “active” dives and are likely produced to allow the mother and calf to remain in acoustic contact during a dive. The authors suggest that these vocalisations are particularly quiet so that they do not attract unwanted attention from nearby males (whose attention would disrupt the suckling and resting time) or from predators such as killer whales. The problem with such quiet calls is that even low levels of ambient noise can reduce the distance they can be heard, and any increases in underwater noise could increase the risk of the calf being separated from the mother.
The paper also presents some really interesting information on suckling behaviour. The data shows that calves spent 20% of their time in the suckling position and that most of the suckling occurred when the mother was below the surface – where the mother rests at depth while the calf suckles. Why suckle at depth and not at the surface? The authors suggest this may be due to the reduced buoyancy at depth making it easier for the calf to maintain their suckling position. It may also be linked to thermoregulation for the mother to avoid overheating at the surface while the calf suckles.