News release

From: Springer Nature

Biology: Secrets of the narwhal tusk *IMAGES*

Hidden structures within the narwhal tusk reveal how it is able to remain so straight, according to research in Nature Communications. The findings suggest that the tusk’s distinctive left-handed spiral arises from opposing helical structures at microscopic scale.

The narwhal tusk is a protruding straight tooth that can exceed two metres in length while maintaining a characteristic left-handed spiral. The narwhal tusk is the only example of straight tusk in nature and has fascinated cultures across history. Previous studies have suggested that this shape might be linked to the organisation of the tusk’s underlying tissues, but it has remained unclear whether the macroscopic spiral structure is reflected in the tusk’s microscopic building blocks.

Henrik Birkedal and colleagues combined multiple imaging techniques to map the structure of the narwhal tusk across several scales. These observations showed that the large-scale helical structure is produced by arrangements of mineralised collagen (the building blocks of the tusk) at a microscopic level. These collagen fibrils were predominantly aligned along the length of the tusk. However, subtle but consistent changes in fibril orientation form two opposing helical arrangements: a left-handed helix in the outer cementum (the thin layer covering the tusk) and a right-handed helix in the inner dentine (an internal layer). The authors suggest that these opposing helical motifs enable the narwhal tusk to grow straight while maintaining its distinctive left-handed helix.

Further research will be needed to understand how the tusk is formed during development. However, these findings shed new light on how microscopic structural patterns can drive specific functions on a larger scale in biology.