Why bowhead whales undergo severe bone loss?

Young bowhead whales may cease growing lengthwise and undergo severe bone loss to help grow their enormous head and baleen plates, finds a new study.

Bowhead whales -- a whale with a large, bow-shaped head that is up to 40 per cent of its body length -- live in the Arctic where the annual cycle of increasing and decreasing ice cover affects their habitat, prey and migration.

"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," said John George from North Slope Borough Department of Wildlife Management, Alaska.

The bowhead bodies then almost stop growing until the whales are about five years old, in contrast to baleen that continues to grow.

The findings showed that the rib bones were extremely dense in one-year old bowheads, but lost up to 40 per cent 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.

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, suggested the authors.

"This work shows that the ribs of whales can look very differently at different ages. Whales can change a lot over their lifetime," said Hans Thewissen

For the study, published in the journal PLOS ONE, the team 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.

--IANS

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