George, J.C., Stimmelmayr, R., Suydam, R., Usip, S., Givens, G., Sformo, T., and Thewissen, J.G.M. (2016). Severe bone loss as part of the life history strategy of bowhead whales. PLoS ONE. 11(6): e0156753.
Free (open access) article available at doi:10.1371/journal.pone.0156753
Even giants start small
Members of parvorder Mysticetes, the baleen whales, include 9 of the 10 largest animals on Earth, one of which is the largest animal to have ever lived . Ironically, these ocean goliaths subsist almost entirely on tiny invertebrates like krill and copepods, which they filter out of seawater using bristly, toothbrush-like baleen plates suspended from the roof of their mouth. A whale’s baleen grows throughout its life, and is made mostly of keratin, the same material in fingernails and hair, and some bone.
Bowhead whales display an unusual growth pattern during their first decade of life, and basically stop growing (except for their head) for years after they are weaned. Inupait whale hunters have long known that the bones of yearling whales are the best to use for weights because they are heavy. Given these historical observations, a team of American researchers investigated whether these young whale redistribute resources from their bodies, particularly their ribs, to prioritize baleen growth.
Bowhead whales are regularly harvested by coastal Alaskan Native populations to meet nutritional and cultural needs. With the co-operation of the whaling ship captains of Barrows, Alaska and the Alaska Eskimo Whaling Commission, the authors were allowed to measure body dimensions and take rib samples from hunted whales. They analyzed these data to see if they supported their hypothesis that young bowhead whales used bone resources from their ribs to build baleen racks.
Selective head growth & bone loss
Weaning marks the beginning of a tough few years for young bowhead whales. Since they have smaller mouths and less filtering area, juveniles are much less effective feeders than adults. This becomes an issue when the calves are about 2 years old, and can no longer rely on their mother’s milk for nutrients. Suddenly resource limited, young whales rely on their fat stores to make up for their tiny baleen, and slim down so much that Alaskan natives once thought that fat yearlings and thin juveniles were two entirely separate species.
The only way to solve this problem is to grow a bigger, better filter that can catch more prey with each gulp of seawater. Bowhead whales can increase their filtering area by either growing the baleen longer, or making the skull bigger (to fit more baleen plates). Until they are about 5 years old, bowhead whales do this, and almost cease to grow their body so that they can put all their resources towards becoming better filter feeders.
The ribs of juvenile whales go from being extremely dense in yearlings to losing up to 40% of their mass, presumably to help reinforce the whale’s growing skull and baleen plates. This increases the effective filtering area of the individual, allowing them to become better filter feeders, though it comes at the expense of lengthwise growth. At about 5 years of age, the baleen has grown large enough to allow juveniles whales to restart body growth and take on a more adult body type.
Bone loss beyond bowheads
Such large changes in bone mass during development, which are rarely seen in mammals, may complicate studies into the life history of bowhead whales and their relatives, both modern and fossilized. Interestingly, fossil records of toothed whales also show changes in rib bone density in a pattern reminiscent of young bowhead whales. As these species do not have to reinforce baleen, it’s been suggested that changes in bone density (which is heavier than water) and blubber help whales regulate their buoyancy. Growing whales may also take advantage of this, though further work needs to be done to model the dynamics of bone and fat loss in whales from a range of life stages.