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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