Narwhals are cetaceans that inhabit the Arctic Ocean with a very curious particularity: a long tusk that has been the inspiration for many fantastic stories. For example, the existence of a horse-like creature with a long horn – the unicorn. But what is the function of this tusk?
The narwhal tusk is basically an overgrown spiralized tooth, similar to those of walruses and elephants, protruding from the head. It can reach 3 m in length! Almost all males have a tusk, with few females developing this structure. While male narwhals are typically able to grow a long tusk, some lose it over time and females tend to lose theirs if they ever grow one. Many have been the theories that try to explain the purpose of the tusk, such as serving as a sensor for chemical changes in the surrounding environment, used as a weapon in fights between males, used to communicate during interactions with other narwhals, or hunting prey by tapping it with the tusk.
To better understand the function of the tusk, scientists from the USA, Greenland and Brazil studied the morphological relationship between the size of the tusk and body size. This methodology is commonly used to have insight on the type of natural selection acting on a specific trait. For example, when comparing individuals of the same age, body structures that have been sexually selected are often disproportionally larger, so these individuals will have a better chance to reproduce. The large variability observed among individuals for sexual traits is normally not observed for non-sexual traits. Under the hypothesis that the tusk is a sexual trait, the scientists compared body length, tusk length, and fluke width (a non-sexual trait) of adult male narwhals sampled in Greenland from 1983 to 2018.
The larger variation observed in tusk length among male narwhals compared to fluke width supports the idea that the tusk has a sexual purpose. It is likely to act as a signal in male-male competition: males with larger tusks are signaling to others that they are bigger and more resourceful, which reduces the occurrence of dangerous fights. In this context, tusking behavior – which has been observed when two narwhals cross and rub their tusks together – may be a way of signaling that happens through this quick assessment of the opponent’s fighting ability.
In addition to this function, narwhal tusks could also be used to attract females, yet little is known about mating behavior of this intriguing Arctic species. And it is also possible that tusks serve both sexual and non-sexual purposes. But after all, a question remains: why do some females develop a tusk?
Source: Graham ZA, Garde E, Peter Heide-Jørgensen M, Palaoro A V. 2020. The longer the better: evidence that narwhal tusks are sexually selected. Biol Lett 16: 20190950. doi: 10.1098/rsbl.2019.0950
Author: Sara Pedro