Young bowhead whales may cease growing lengthwise and undergo severe bone loss to help grow their enormous head and baleen plates, according to a study published June 22, 2016 in the open-access journal PLOS ONE by John George from North Slope Borough Department of Wildlife Management, Alaska, and colleagues.
As filter feeders, bowhead whales depend on baleen, about 640 plates suspended from their upper jaws, to catch their tiny prey. But after weaning, bowhead calves have so little baleen that they can't eat enough to keep up with rapid growth. To resolve this issue, the head and baleen grow faster than the rest of the body, dwarfing it for a few years. To see if the whales redistribute resources from their bodies to the baleen to spur their growth, the authors of this study examined bowheads living along the Bering, Chukchi, and Beaufort seas. The researchers took measurements of the whales to estimate their age, body size, and baleen area, as well as CT scans to determine the density of their rib bones.
The authors' results confirmed that after weaning, bowhead bodies almost stop growing until the whales are about five years old, in contrast to baleen that continues to grow. The researchers found that the rib bones were extremely dense in one-year old bowheads, but lost up to 40% of their mass over the next few years, presumably to the new baleen plates that grow at such a clip in the young whales.
While rare, strategic severe bone loss is not unprecedented in mammals, like some deer, elk and moose that deplete other bones to rapidly grow antlers. The authors suggest that this bone mass variation may complicate the study of life history strategy in bowheads and modern species, as well as the recognition of new species in the fossil record.
Co-author Dr. Hans Thewissen notes: "This work shows that the ribs of whales can look very differently at different ages. I work a lot with fossils, we often only have one or two specimens for some species, and this reminds us that we need to take intraspecific variation very much into account for whales, they can change a lot over their lifetime."
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Citation: George JC, Stimmelmayr R, Suydam R, Usip S, Givens G, Sformo T, et al. (2016) Severe Bone Loss as Part of the Life History Strategy of Bowhead Whales. PLoS ONE 11(6): e0156753. doi:10.1371/journal.pone.0156753
Funding: North Slope Borough funded collection of samples, maintenance of database, and analysis. NEOMED funded analysis of data. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Givens Statistical Solutions LLC provided support in the form of salaries for author GG but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific role of this author is articulated in the 'author contributions' section.
Competing Interests: The authors have the following interests: NEOMED funded the analysis of data for this study. Geof Givens is affiliated to Givens Statistical Solutions LLC. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.