Narwhal Tusks Stay Straight by Twisting in Two Opposite Directions

Narwhals are known for a feature that looks like a horn but is actually an elongated tooth—and researchers have now identified an unusual structural arrangement that helps it grow straight. According to Smithsonian Magazine, a study published August 18 in Nature Communications examined several narwhal tusks. Some came from Greenlandic Inuit subsistence hunters, while others were provided by the Greenland Institute of Natural Resources. Researchers studied sections of the tusks with powerful microscopes and intense X-rays, and used a CT scanner to examine an entire narwhal skull with its tusk attached. The analysis found that the tusk is made of two principal layers with different internal structures. Cementum, the outer layer, contains collagen fibers arranged in a left-handed spiral. Dentine, the inner layer, forms a spiral in the opposite direction. Henrik Birkedal, a chemist at Aarhus University and an author of the study, described the consistent leftward direction of narwhal tusk spirals as unique to the best of the researchers’ knowledge. The opposing helical layers appear to help the tusk maintain a straight shape, while mechanical testing suggested they also make it better able to withstand bending and twisting than a simple spiraled or nonspiraled rod. The tusk’s purpose in the animal’s life remains an area of research. Because tusks are found mostly in males, scientists suspect they may play a role in competition for mates. They may also function as sensory organs. Aerial footage included in a study published last year showed narwhals prodding and flipping a fish, an activity that appeared to be play, though the new structural study does not establish that this is a primary tusk function. Researchers also observed that the opposing spiral pattern appears in annual growth layers, similar to tree rings. Birkedal and his colleagues hope those layers may eventually help trace aspects of a narwhal’s life and Arctic living conditions. They also plan to investigate the part of the skull where the tusk develops.