The annual migration of gray whales from northern Alaska to Baja California, a round-trip of some 10,000 miles, is one of the most extreme examples of long-distance travel in the animal world. Now a new study suggests that this feat may be a relatively recent phenomenon, and that only a few thousand years ago, these marine mammals stayed much closer to home.
Gray whales eat by sucking up invertebrates living in and above the sea floor. Today there are plenty of shallow, near-shore areas where gray whales can feed, especially in the Bering Sea off the coast of Alaska. During the last Ice Age, however, shallow areas would have been much less common. Large glaciers covering much of the Northern Hemisphere during the Ice Age trapped large amounts of water; lowering sea levels by more than 200 feet and dramatically decreasing the shallower area of the continental shelf.
“Over the past million or so years, the available feeding area for gray whales on the sea floor was completely eliminated many times,” says Nicholas Pyenson, curator at the Smithsonian’s National Museum of Natural History, and lead author of the study. In the new study, “we argue that these changes would have restricted the extent of gray whale migration and forced them to use alternative feeding modes – similar to what we see in the so-called ‘resident,’ non-migrating gray whales off Vancouver Island today.”
“Gray whales are one of the great conservation success stories, but we don’t know much about their deeper history, prior to the arrival of humans in North America,” says study co-author David Lindberg, professor of Integrative Biology at University of California, Berkeley. “This study tried to address their ecological history using a known constraint.”
Pyenson and Lindberg used detailed maps of the ocean floor to model how the amount of gray whale feeding habitat would have changed during the last 120,000 years, as giant ice sheets formed and disappeared, changing overall sea-level. The scientists then calculated how this would have affected the size of the gray whale population over this time, given their restriction to shallow areas that support their primary food resource.
They determined that the gray whale population would have plummeted during glacial times if they had stuck to feeding in shallow waters. But because there is no genetic evidence that populations ever were so small – intervals of time that might have caused genetic bottlenecks – gray whales must have changed their feeding strategy.
This study has important implications for other marine mammals that forage on the seafloor of the Bering Sea, like walruses, and other animals that are connected to this food web, like eider ducks. It also suggests that gray whales may still retain an ancestral ability to deal with relatively rapid changes to their feeding habitat, especially in the Arctic, where human induced changes in the environment are already occurring