Synchronous mother and calf foraging behaviour in humpback whales Megaptera novaeangliae: insights from multi-sensor suction cup tags

Abstract

Previously, all inferences regarding fine-scale baleen whale mothercalf relation- ships have come from surface observations, aerial surveys, or underwater video recordings. On May 19, 2010, we attached high-resolution digital acoustic recording tags (Dtags) to an adult female humpback whale Megaptera novaeangliae and her calf in Wilhelmina Bay (Western Antarctic Peninsula) to examine their concurrent diving and foraging behaviour. The Dtags logged ~20 h of concurrent recordings. We used cross-correlation analyses to quantify synchrony between the pair. Dive depth was positively correlated for the duration of the concurrent record and was highest when the calf's track lagged behind the mother's by 4.5 s, suggesting that the calf was 'following' its mother. Pitch and heading were positively correlated but to a lesser degree. Both animals executed feeding lunges; however, the mother foraged more intensively than the calf (792 and 118 lunges over 246 and 30 feeding dives, respectively). Also, the mother fed consis- tently once she initiated feeding at 16:22:00 h until the tag came off, whereas the calf executed 95.76% of its lunges between 17:00:08 and 19:28:21 h, local time. Correlation coefficients calcu- lated per dive were highest when both animals were feeding and lowest when only the mother was feeding. In addition, 84.26 and 79.63% of the calf's lunges were performed within ±20 s and ±20 m of its mother's lunges, respectively. Our work describes the first record of a long-term con- tinuous underwater relationship and foraging behaviour of a humpback mothercalf pair.