Opposite handedness in nested layers is a geometric fact, and the authors turn it into a mechanical story: two twists that counteract, so the tooth lengthens instead of curving the way an elephant's tusk does [7]. Birkedal's picture of the growth is a screw. "Imagine you turn a screw into a piece of wood, and with each turn, the screw penetrates deeper" [8], he said, except that this screw drives outward from the head.
What the scans do not settle is whether the two twists actually cancel. Handedness is structure; cancellation is torque. AFP's account of the paper does not say how many tusks were scanned, and it reports no curved tusk put through the same instruments for comparison [19], which leaves the elephant contrast sitting in the explanation rather than in data a reader can weigh [7]. Two mineralized layers wound against each other is the kind of design a materials group would want to imitate, and that account reports neither mechanical loading nor any synthetic copy [20].
The layering may travel further than the helix. A narwhal can live 80 years [10], and the longest tusks reach three metres [2]; 300 centimetres spread across 80 years is roughly 3.8 centimetres a year [18]. Both inputs are maxima and probably not from the same animal, so treat that as an order of magnitude for how much tooth a whale adds annually, not a growth rate.
The function argument runs on thinner evidence than the structure. The team's case against the hunting and salinity-sensing hypotheses [12] is an argument from who carries a tusk: almost only males do, and females would have needed those capabilities too [13]. That narrows the plausible functions without establishing what males do with the tooth. On the positive side, AFP reports a single observation of two males in what looked like a tusk-measuring contest in front of a female, which Mads-Peter Heide Jorgensen of the Greenland Institute of Natural Resources called "definitely the most likely purpose" [14]. Martin Nweeia, the Harvard dental anthropologist who was not involved, told AFP the primary function is a sensory organ system and pointed to his own earlier work [15], while calling the new study "one more valuable contribution to our knowledge of the tusk microanatomy and morphology" [16].
So one paper carries two claims resting on different footings: mineral structure, which keeps in a museum drawer, and behaviour, which has to be caught under Arctic ice, where these whales spend most of their lives [17].
