Mother–offspring association in the humpback whale, Megaptera novaeangliae: following behaviour in an aquatic mammal

We observed 42 humpback whale mother–offspring pairs on a foraging ground in southeast Alaska during 2001 and 2002 to examine their spatial association. We found that whenever these pairs were at the surface they maintained close proximity similar to that observed in terrestrial followers. By examining their dive behaviour, however, we determined that calves dive significantly less frequently and for shorter durations than their mothers, leading to periods of separation during foraging dives. Because this is likely to increase the risk of calf predation, females responded by significantly shortening the duration of their dives when unaccompanied by their offspring, thereby reducing this risk. As the season progressed, we observed that this behaviour waned and females became significantly less responsive to their offspring. At the same time, calves appeared to increase their role in maintaining proximity by synchronizing their dives with the mothers increasingly often. We discuss this shift in proximity maintenance in terms of the energetics associated with lactation, increased foraging effort by the offspring and parent–offspring conflict.