Genetic evidence for sex-specific migratory behaviour in western South Pacific humpback whales

Although predictable in its areas of occurrence, many aspects of humpback whale migration are still poorly understood. Nuclear DNA analysis has revealed a low level of relatedness among whales using the same migratory corridor, and has shown that closely related individuals tend not to travel in spatial association. Yet it still remains uncertain whether whales from different matrilineally discrete feeding stocks travel along the same migratory corridor for later mixing in common breeding waters. The western and central South Pacific Ocean is the only ocean basin where thousands of islands and reefs occur as suitable wintering habitat for humpback whales, so here their migratory behaviour may not be constrained by habitat. We analysed sex-specific and temporal distributions of 42 mitochondrial haplotypes detected in 135 humpback whales sampled off eastern Australia throughout 1 annual migration. A noticeable difference was found in haplotypic composition between northbound males and females, suggesting that the 2 sexes of any single matrilineal stock might select different and only partially overlapping migratory routes. We hypothesise that males most closely related to the N-S migrating females off eastern Australia migrate north elsewhere, perhaps past New Zealand. These findings suggest that longitudinal movements by male humpback whales are probably more extensive than currently thought, and indicate the need for reexamination of genetic data from humpback whales in the South Pacific, disaggregated by gender.