The narwhal’s spiral-shaped tusk is one of the most unusual structures in the animal kingdom. It can grow to more than two metres in length, yet remains almost perfectly straight. Researchers have now, for the first time, investigated at the nanometre level how this astonishing growth is possible. The findings were published in the journal Nature Communications.
The so-called tusk is in fact a greatly elongated canine tooth. In most male narwhals, the left tooth grows outwards through the upper lip, forming the characteristic left-handed spiral. Unlike the tusks of elephants or walruses, however, it is barely curved.
To find out why, an international research team led by Henrik Birkedal from Aarhus University examined the tusk using modern three-dimensional X-ray techniques. In doing so, the scientists were able to reconstruct its internal architecture, from the tiny mineralised collagen fibres right down to the visible spiral.
The result is surprising: Two structures with opposite twists interact within the narwhal tusk. In the outer layer, known as cementum, the collagen fibres run in a left-handed helix. In the underlying dentine, however, they form a right-handed structure. Most of the fibres run almost parallel to the tooth’s longitudinal axis.
According to the researchers, this creates a kind of mechanical equilibrium. The opposing twists may prevent the tusk from curving as it grows. At the same time, they give it stability against bending and torsional forces. The narwhal tusk is considered to be the only known tusk in nature that grows practically straight.
Narwhal tusks grow throughout the animal’s entire life. Their annual growth layers resemble the annual rings of a tree and can provide information about the animals’ age and living conditions. Studies also show that the chemical properties of the tusks can be linked to the narwhals’ habitats around Greenland.
However, the original purpose of these striking tusks has not yet been conclusively established. Much suggests they play a role in mate selection and the social hierarchy among male animals. Narwhals have also been observed using their tusks playfully.
The discovery could even have implications beyond narwhal research. The combination of hardness, flexibility and opposing spiral structures could serve as a model for new, particularly durable composite materials in the future.
Heiner Kubny, PolarJournal