Using Fossil Whale Barnacles to Reconstruct Prehistoric Whale Migrations
Baleen whales, as we know them today, lead lives that are largely defined by their annual migrations. Every year, whales spend their winters breeding and reproducing in tropical waters, then travel to poleward feeding areas each summer. For North Pacific humpback whales, winter breeding areas cluster around Central America and Hawaii, and then they travel to the Gulf of Alaska to feed in the summer (small numbers also feed on the California coast). Likewise, gray whales travel from Baja California to the Bering Sea. These are the longest migrations made by any mammal, and they come at extreme energetic costs, as a whale will lose 25% of its body weight between summer feeding sessions.
Because of this cost, whales rely on taking in enormous amounts of food each summer, and thus they are tracking down the most productive areas of the ocean. In a sense, the migration routes of whales tell us something about ocean productivity and how it’s distributed, both in time and in space. This is where things get interesting: perhaps whale movements in the prehistoric past can yield clues about ocean productivity patterns millions of years ago. What’s more, being massive allows whales to afford this type of lifestyle: they have enough energy stores to last through the lean seasons, and their great size allows them to travel the thousands of miles necessary to reach their summer feeding areas. This link between body size and lifestyle has led some scientists to believe that it’s not just coincidence, and that perhaps whales are so big specifically because they evolved to better handle the demands of migration.
Can whale migrations of the past tell us something about productivity and nutrient distribution in the ancient oceans? Did whales really get so big because they evolved to migrate? Both of these questions rely on figuring out if whale were migrating in the past, and if so, how those migrations changed through time. My research seeks to answer these questions by taking advantage of fossil whale barnacles. Whale barnacles incorporate stable isotope signatures of the surrounding seawater into their shells, and as they grow, they continually deposit new shell beside older shell. That means that their shells end up with an isotopic signature of the water they’ve been growing in over their life. As it turns out, this signature can be decoded to give clues about where in the ocean each shell layer was formed. Thus, a whale barnacle acts like as a tracking mechanism, recording signatures of everywhere its host whale has been traveling.
My approach is relatively straightforward: I collect samples of shell powder from a barnacle by drilling into it, then these samples are isotopically analyzed by UC Berkeley’s Center for Stable Isotope Biogeochemistry. I then use an equation derived by Killingley and Newman (1982) to help reconstruct where the barnacle (and the whale it was riding on) has been. I first tried this approach with modern barnacles to verify the technique, and found that barnacle isotope profiles accurately reconstructed the migration of humpback and gray whales, including humpbacks that take multiple different migratory routes. Now my advisor, Seth Finnegan, and I are working purely in the fossil record, using specimens that have been loaned by museums as well as fossils we recently discovered in Panama. Results thus far are promising, as some of the fossil isotope profiles retain basic patterns also seen in the modern barnacles. There is a lot of work left to do, but these preliminary results give me confidence that we can extract useful information from the fossil barnacles. With luck and proper analysis, this work will shed light on just how prehistoric whales were moving, and what that means for the evolution of the ocean and of the whales themselves.
This work is generously supported by grants from the UC Museum of Paleontology, National Sigma Xi, the Berkeley Chapter of Sigma Xi, the Geological Society of America, the Paleontological Society, and by collaborators from the NOAA, the San Diego Natural History Museum, the California Academy of Sciences, the Smithsonian Tropical Research Institute, and the National Museum of Natural History.