Simone K. A. Videsen, Lars Bejder, Mark Johnson and Peter T. Madsen
Humpback whales belong to the group of large baleen whales that embark on long distance migrations each year between foraging and breeding grounds. These long migrations result in a short time window on breeding grounds, where pregnant females must give birth and nurse their calves adequately for them to sustain the return migration to their foraging grounds. Our knowledge of nursing behavior of cetaceans is sparse, especially for baleen whales too large to observe in captivity. The aim with this study was therefore to investigate the behavior of this critical energy transfer from mother to calf to improve our understanding of their life history.
To study their nursing behavior, we deployed non-invasive multi-sensor tags on eight young humpback whale calves and two mothers. These tags (Dtags) record both depth, movement and sound data from the animals, providing unique and detailed insights into the lives of whales in the wild.
By using movement data, we could distinguish between suckling and non-suckling dives for each calf. The classification of suckling dives enabled us to estimate the proportion of time newborn calves spend on nursing. Our data show that humpback whale calves spend around 20% of their time nursing on breeding grounds. To initiate nursing, we hypothesized that calves would signal their mother vocally, however, our data show that the vocalizations are rather used during active dives, suggesting that communication between mother and calf serves to maintain cohesion. The calls between mother and calf have a lower amplitude than those produced by adults in the same area. This quiet communication may reduce the risk of predation from killer whales and harassment by male humpback whales. The cost of such quiet signalling, however, is that an increase in ship noise on breeding grounds may disrupt critical contact calling between mothers and calves and lead to separation of the two.
The findings of this study demonstrate just how critical this short time window at breeding grounds is for the fitness and survival of young calves and the importance of preserving these areas.
Photo credit: Fredrik Christiansen