THE densest known bone in the world has been found in the skull of a
whale. Four times harder than the previous record holder, the bone is so brittle
that its purpose is a mystery.
Blaineville鈥檚 beaked whale, Mesoplodon densirostris, is a rare type
of whale renowned for its unusually heavy skull. The rostrum bone, a beaklike
part of its skull, forms part of the upper jaw. A team led by Peter Zioupos and
John Currey of the University of York have now studied the bone鈥檚 properties
using a sample from the Natural History Museum in Paris.
The team says in the current issue of the Journal of Zoology (vol
241, p 725) that the hardness of the whale bone is four times that of the
tympanic bulla of a fin whale, the skull bone that previously held the record.
鈥淎t 2.7 grams per cubic centimetre, it is about half as dense again as the
average mammalian bone,鈥� says Currey.
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The bone also turned out to have unique chemical properties. It contains 35
per cent calcium by weight鈥�13 per cent more than the highest value known
previously. Using microscopes, the team showed that the bone is riddled with
tiny tunnels containing highly concentrated minerals. These channels would have
formed when bone-destroying cells called osteoclasts chewed cylindrical holes
that other specialised cells later filled with more mineral-rich bone.
But this channelled structure makes the bone like a brittle ceramic material.
鈥淥nce started, microcracks travel easily through it,鈥� says Currey. 鈥淭his makes
it a very fragile material.鈥� Because of this, the bone鈥檚 purpose is a mystery.
The densest rostra occur in males, suggesting that they are used in fighting or
jousting. 鈥淏ut if males rammed each other it is very likely the bone would
crack,鈥� says Currey. 鈥淚t really is an inappropriate material for conflicts.鈥�
The team says that the bone might instead be used to reflect and 鈥渂roadcast鈥�
the whale鈥檚 sonar signals. The rostral bones do not form the most efficient
parabolic shape for this, says Currey. 鈥淏ut they might help to conduct and focus
sound waves,鈥� he adds.