Why do baleen whales migrate?

The annual migrations of baleen whales are a conspicuous but unexplained fe ature of their behavioral repertoire. Some hypotheses offered to explain wh ale migration focus on direct benefits to the calf (thermoregulation, calm water) and some do not (resource tracking, and the "evolutionary holdover" hypothesis). Here, we suggest that a major selective advantage to migrating pregnant female baleen whales is a reduced risk of killer whale (Orcinus a rea) predation on their newborn calves in low-latitude waters. Killer whale abundance in high latitudes is substantially greater than that: in lower l atitudes, and most killer whales do not appear to migrate with baleen whale s. We suggest that the distribution of killer whales is determined more by their primary marine mammal prey, pinnipeds, and that following the baleen whale migrations would remove them from their pinniped prey. There are prob lems with all current hypotheses, most of which stem from a lack of directe d research. We explore variation in migratory habits between species, popul ations, and individuals that may provide a "natural laboratory" for discrim inating among the competing hypotheses.