Social Foraging of the Southeast Alaskan Humpback Whale, Megaptera Novaengliae
Author | : Fred Sharpe |
Publisher | : |
Total Pages | : 129 |
Release | : 2001 |
Genre | : Humpback whale |
ISBN | : |
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Relative to other baleen whale populations, the humpback whales Megaptera novaeangliae inhabiting Southeast Alaska are noteworthy in that they form large, enduring groups when foraging on schools of Pacific herring Clupea harengus pallasi. These groups use a variety of unusual feeding tactics when attacking prey, including the production of loud feeding calls, the release of bubbles, and the waving of their large pectoral flippers. Past observations of these groups have been largely anecdotal so little is known about their social behavior or the function of their feeding tactics. In particular, it is unclear if these pods are based on cooperative interactions or if they represent groups of individuals who are competing for prey that occurs in dense aggregations. In addition, little is known about the underlying social structure of these pods and whether they are composed of kin. The objective of this study was to employ field and laboratory techniques to gain insight into the function of the unusual feeding tactics, and to describe the basic social structure of these pods. In order to test the prediction that bubbles, feeding calls, and flipper movements represent prey manipulation tactics, herring schools were brought into the laboratory and subjected to various simulated humpback whale feeding behaviors. I found that these simulated behaviors produced strong avoidance responses from the herring schools, suggesting that humpback whales use these tactics to manipulate the behavior of their prey. Seven years of field observations revealed that humpbacks with enduring social bonds (i.e., high coefficients of association) specialized on herring and represented a small proportion of the entire whale population. Calves born to these "core members" were never observed to return and feed with their mothers in subsequent years, suggesting that these groups are not composed of close kin. This was verified by analysis of haplotype variation in the mitochondrial genome and microsatellite variation in the nuclear genome. Individuals within these pods appear to invest in by-product benefits, with the enduring bonds between whales in Chatham Strait (cf. krill feeders in Frederick Sound) possibly representing combinations of individuals performing compatible tasks (Le., bubble blower, herder, vocalizer).