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    Through his teaching and writing, Ken Norris inspired us to ask the “why” questions about nature; thus we dedicate to Ken our effort to understand why baleen whales migrate. During our literature search we were delighted to find an unpublished manuscript by Norris, written in the early 1980s, that anticipates some of the ideas reviewed here. We include two insightful quotes from that manuscipt, titled “Broad Patterns of Cetacean Zoogeography and Evolution.” Our answer to the migration question provides another link to Norris-an observation by one of us (Connor) in the spring of 1982, during Ken Norris's Natural History of California Field Class, that forcefully brought home the magnitude of the threat killer whales present to baleen whales and their young. Hiking along Donner Ridge, over 200 m above the Big Sur coast line, we spotted a group of killer whales milling close to shore. Just what the killer whales were doing was unclear until, some minutes later, a bloodied gray whale surfaced in their midst. Ken, waylaid by a nasty flu, staggered up to the cluster of undergraduates glued to their binoculars, and still had enough presence of mind to remark that the unfortunate gray was unusually far out from the kelp bed. On their return migration north after breeding in protected Mexican lagoons, the gray whales, with newborns in tow, must run a gauntlet of killer whales to return to their summer feeding grounds. As we witnessed, not all make it through safely. Those same killer whales, we argue in this review, may provide the primary selection pressure for the migration itself.


The annual migrations of baleen whales are a conspicuous but unexplained feature of their behavioral repertoire. Some hypotheses offered to explain whale 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 orca) predation on their newborn calves in low-latitude waters. Killer whale abundance in high latitudes is substantially greater than that in lower latitudes, and most killer whales do not appear to migrate with baleen whales. 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 problems with all current hypotheses, most of which stem from a lack of directed research. We explore variation in migratory habits between species, populations, and individuals that may provide a “natural laboratory” for discriminating among the competing hypotheses.