22. Mar, 2020

The function of the Narwhals tusk: the longer the better.

The narwhals (Monodon Monoceros) live year-round in the Arctic waters around Greenland, Canada, and Russia.  They have been harvested for hundreds of years by Inuit people in northern Canada and Greenland for meat and ivory, and a regulated hunt continues. It is one of two living species of whale in the family  Monodontidae, along with the beluga whale. Narwhals can live up to 50 years and grow to 6 m long. Their pigmentation is a mottled pattern, with blackish-brown markings over a white background. They are darkest when born and become whiter with age. During the winter, narwhals make some of the deepest dives recorded for a marine mammal, diving to at least 800 meters, several times per day, with some dives reaching 1,500 meters.

Normally, the canine tooth only on the left side of the upper jaw becomes a tusk. The tusk is actually an enlarged spiraled ivory tooth with sensory capability (somewhat like a feeler) and up to 10 million nerve endings inside, that can grow as long at 10 feet. Rarely, males develop two tusks. Only about 15 percent of females grow a smaller tusk.

What is the function of the strange rapier-like tusk of the male narwhal: is it a weapon or rather a signal?  The general scientific consensus is that the narwhal tusk is not directly necessary for survival but serves as a sexual trait, much like the manes of a lion, or the feathers of a peacock  Perhaps the sensors in the tusk have also a communicative function, the reason why some males have been seen ‘tusking’:  that is crossing the tusks like rapiers in a simulated fight. Based on the disproportional growth and large variation in male tusk length biologist Zackary Graham from Arizona and colleagues found morphological evidence that narwhal tusks are indeed sexually selected during male-male contests. Parts of the body that are sexually selected are often disproportionally larger. The variation is tusk size among male narwhals is indeed much larger (between 45-250 cm) than the tail (or fluke, between 45-90 cm). Large tusks thus benefit male narwhals in sexual acts, probably signaling  ’I am bigger than you'', and avoid potentially dangerous fights by impressing rivals (see insert). Source:  Royal Society Biology Letters,  March 2020