To get around that deficit, say the scientists, the animal simply stops getting longer so that the head and baleen structures can grow fast enough to keep up with its food needs. For a time, its head dwarfs the rest of its body.
The researchers found, through CT scans, that this growth-shift comes with a price tag: The whale's rib bones lose about 40 percent of their mass -- traded off, the scientists think, to keep the baleen plates growing at a healthy clip.
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The strategy isn't unheard of. Some other mammals will trade bone-mass loss in one place for growth in another. Deer, elk and moose, for example, will take from other bones to get their antlers up and running as fast as possible.
It's all to the good in the long run, though. After all, the bowhead whale needs its big head. It lives in the icy cold waters of the Arctic Ocean, where its giant noggin – taking up a good one-third of its body – comes in handy for smashing through thick ice to reach the surface.
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The researchers said their findings about bone mass might make it a bit trickier to study the animals going forward.
"This work shows that the ribs of whales can look very differently at different ages," said study co-author Hans Thewissen, of the department of anatomy and neurobiology at Northeast Ohio Medical University, in a statement.
"I work a lot with fossils," Thewissen said. "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."