X-rays add new twist to narwhal’s spiral tusk

X 射线揭示独角鲸螺旋长牙的新奥秘

Narwhals are toothed whales, renowned for their distinctive straight, spiraling tusks, which many people in medieval times believed were the horns of unicorns and thus held magical healing properties. That belief was debunked long ago, but the tusks remain popular souvenirs in Canada and Greenland. And they still hold a fascination for scientists keen to learn more about their unusual structure. 独角鲸是一种齿鲸，以其独特且笔直的螺旋长牙而闻名。在中世纪，许多人认为这些长牙是独角兽的角，因此具有神奇的治愈功效。这种迷信早已被推翻，但这些长牙在加拿大和格陵兰岛依然是受欢迎的纪念品。对于那些渴望深入了解其奇特结构的科学家来说，它们依然充满魅力。

A new paper published in the journal Nature Communications reports that rather than one left-oriented spiral, narwhal tusks also have a second internal spiral oriented in the opposite direction. 发表在《自然-通讯》（Nature Communications）杂志上的一篇新论文指出，独角鲸的长牙并非只有一种左旋螺旋，其内部还存在第二个方向相反的螺旋。

Inuit legend holds that a woman was dragged into the ocean by a harpoon rope after the weapon had hit a narwhal and was transformed into the animal herself. Her hair, which she’d worn in a twisted knot, became the spiral tusk. The tusk is actually a canine tooth in the left upper jaw of male narwhals, pushing through the lip when the males reach two or three years of age and growing to lengths as long as 1.5 to 2 meters (just under 5 feet to 9 feet 10 inches). Some males may develop two long spiral tusks or, in rare cases, none at all. 因纽特人的传说中提到，一名女子在鱼叉击中独角鲸后被绳索拖入大海，随后她自己也变成了独角鲸。她那盘成发髻的头发，就变成了螺旋状的长牙。实际上，这根长牙是雄性独角鲸左上颌的一颗犬齿，在雄鲸两三岁时穿透嘴唇长出，长度可达 1.5 到 2 米（约 5 英尺到 9 英尺 10 英寸）。有些雄鲸会长出两根长螺旋牙，极少数情况下则一根也不长。

There’s a general consensus that these tusks most likely evolved via sexual selection as a form of social status. They clearly aren’t crucial for survival, since the females usually don’t develop the tusk—and if they do, the tusks are much smaller with fewer spirals—and yet typically live longer than the males. But the precise function of the tusk is still a matter of considerable debate. Narwhals have been observed using them to stun small Arctic cod while hunting. It’s possible they are used as weapons in fights, but this behavior hasn’t been directly observed, although there have been narwhals found with tusks embedded in their bodies. The presence of several million nerve endings suggests that narwhals may be able to sense temperature or salinity changes in the water with their tusks. 科学界普遍认为，这些长牙很可能是通过性选择进化而来的，作为一种社会地位的象征。它们显然对生存并非至关重要，因为雌性通常不会长出长牙——即便长出，也比雄性的短小且螺旋更少——但雌性的寿命通常比雄性更长。然而，长牙的具体功能仍存在很大争议。人们曾观察到独角鲸在捕猎时用长牙击晕北极鳕鱼。它们也可能在争斗中被用作武器，尽管这种行为尚未被直接观察到，但确实曾发现过体内嵌入了长牙的独角鲸。长牙中数百万个神经末梢的存在表明，独角鲸或许能利用长牙感知水中温度或盐度的变化。

A double helix

双螺旋结构

Scientists know that the tusk consistently twists in the left-handed direction and that it is composed of dentine covered by a thin layer of cementum encasing a central pulp chamber. The dentine and cementum, in turn, are made up of microscopic collagen fibrils mineralized with nanoparticles of hydroxyapatite. The macroscale spiral shape emerges somehow from how those fibrils organize themselves. That structure also determines the tusk’s material properties. One question is whether the helical structure is also present at the micro- and nanoscale. 科学家们已知长牙始终呈左旋扭曲，由牙本质构成，外层覆盖着一层薄薄的牙骨质，内部包裹着中央牙髓腔。牙本质和牙骨质又由被羟基磷灰石纳米颗粒矿化的微观胶原纤维组成。宏观上的螺旋形状正是源于这些纤维的排列方式。这种结构也决定了长牙的材料特性。一个悬而未决的问题是：这种螺旋结构是否也存在于微观和纳米尺度上？

But nobody had mapped the interior structure in three dimensions at the atomic, nano, and macroscales. So the authors of this latest paper studied two male narwhal tusk and skull specimens by combining multiple imaging techniques: X-ray computed tomography, scanning X-ray diffraction, scanning small-angle X-ray scattering and tensor tomography, and birefringence microscopy. That required reserving time on three large synchrotrons in Sweden, Switzerland, and France. They also performed standard morphological measurements and conducted mechanical three-point bending tests. 此前，没有人能在原子、纳米和宏观尺度上对内部结构进行三维测绘。因此，这篇最新论文的作者通过结合多种成像技术，研究了两具雄性独角鲸的长牙和头骨标本，这些技术包括：X 射线计算机断层扫描、扫描 X 射线衍射、扫描小角 X 射线散射、张量断层扫描以及双折射显微镜。为此，他们申请并使用了瑞典、瑞士和法国的三台大型同步加速器。此外，他们还进行了标准的形态学测量和机械三点弯曲测试。

The results showed that the collagen fibrils and hydroxyapatite nanoparticles orient themselves along the tusk’s longitudinal axis, so there is a consistently high degree of anisotropy at all scales. But there are tiny systemic deviations at small angles in that orientation, which in turn create the twisted structure. While the cementum layer forms the known left-handed helix, the dentine forms a right-handed helix. That double-helix structure is the secret to the tusk’s remarkable stiffness and strength, with the flexible fibers and stiff mineral matrix enabling the tusk to withstand strong forces, like bending and twisting without cracking. It also allows the narwhal tusk to grow straight, unlike, say, an elephant’s curved tusk. 结果显示，胶原纤维和羟基磷灰石纳米颗粒沿长牙的纵轴排列，因此在所有尺度上都表现出高度的一致性各向异性。但在该取向的小角度上存在微小的系统性偏差，这进而形成了扭曲的结构。虽然牙骨质层形成了已知的左旋螺旋，但牙本质却形成了右旋螺旋。这种双螺旋结构正是长牙具有卓越刚度和强度的秘诀，柔韧的纤维与坚硬的矿物基质使长牙能够承受强大的外力（如弯曲和扭转）而不开裂。这也使得独角鲸的长牙能够笔直生长，而不像大象的长牙那样弯曲。

The team also noted a finer underlying microstructure in the cementum of collagen fiber bundles radially extending outward, which they plan to study further using micro- and nano-beam experiments. “Since whales can live for up to 80 years, their teeth form a kind of historical record of changing environmental conditions throughout the animal’s lifetime,” said co-author Henrik Birkedal of Aarhus University in Denmark. “And because the North Atlantic is currently undergoing very rapid changes, it is obvious to investigate whether we can trace these changes in the hard tissue of the narwhal tusk. That is what we are now working on.” 研究团队还注意到，牙骨质中存在一种更精细的底层微观结构，即呈放射状向外延伸的胶原纤维束，他们计划利用微束和纳米束实验对此进行进一步研究。丹麦奥胡斯大学的合著者亨里克·比尔克达尔（Henrik Birkedal）表示：“由于鲸鱼的寿命可达 80 年，它们的牙齿形成了一种记录动物一生中环境条件变化的‘历史档案’。鉴于北大西洋目前正在经历剧烈变化，研究我们是否能从独角鲸长牙的硬组织中追踪到这些变化，显然非常有意义。这正是我们目前正在努力的方向。”