Letter: A twist in the narwhal's tail

Sir: Ken Mantel (letter, 17 June) nicely illustrates, but omits comment on, the most exciting peculiarity of the narwhal's tusk. The old Kearsly print which you reproduce shows that when, exceptionally, a right tusk is developed as well as a left, both surprisingly spiral in the same direction. Contrast the symmetrical twisting and coiling of antelope, sheep and goat horns on opposite sides of the head.

The explanation of the phenomenon is that the propulsive motion of the narwhal's tail in swimming has a rotating component about a centre behind the head and so this leverage imparts a rotary motion to the head itself. This consistent twisting strain or torque is applied equally and in the same direction to both right and left tusks and their identical spiral form is, like the twisting of a wind-blown tree-trunk, a consequence of mechanical strains imposed during growth.

Yours faithfully,

DENYS W. TUCKER

London, SW19

17 June