Every winter, humpback whales Megaptera novaeangliae congregate in warm, low-latitude waters to mate and give birth. While in winter waters, male humpback whales produce long complex songs. Song content is dynamic and singers incorporate changes as they occur. Average source levels suggest that most sounds in the song would become masked by ambient noise levels at distances less than a few tens of kilometres. Thus, singers must be relatively close to one another, much closer than the distance separating wintering regions, in order to match the whole complex acoustic structure of the local song type. The tendency for singers to converge on current song themes, plus limited propagation distance, allows the potential for stable geographic variation to occur. The use of geographic variation in song to map population structure was evaluated by comparing songs recorded in winter migratory termini in Madagascar, Western Australia, Eastern Australia, New Caledonia, Tonga and Colombia in winter of 1996. Differences in regional variants were most pronounced between Madagascan, Australian and Colombian song. Eastern Australian, New Caledonian and Tongan song were most similar. Some evidence of song sharing between Western and Eastern Australian waters was present. Phenetic analysis was consistent with Discovery tag models of humpback stock .structure in the southern hemisphere. The results suggest some migratory exchange among wintering regions of Area V and slight exchange between Areas IV and V; but the time and location at which song sharing occurs remains speculative.
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