Sensory Functions of a Narwhal’s Tusk
As a hunter and outdoorsman, I’m fascinated by wildlife and ecology. Not surprisingly, I have a particular interest in understanding everything I can get my hands on about North American wildlife, especially those species that are also important table-fare in various communities. This first post may be somewhat removed from the regions and species many of us hunt, but I chose this story because it’s a species that is relevant to the areas I work in the Canadian Arctic. Some people may not even be aware of the narwhal’s existence, but it sure is a fascinating and mysterious animal.
The narwhal (Monodon monoceros) is a toothed whale species (odontoceti) that are roughly 11 million years old. They are a resident Arctic species, meaning that they spend the entire year in Arctic waters, including during winter ice-covered periods when they rely on openings in the ice to come up for air. Their Canadian range is throughout the central and eastern Arctic waters where they move between summer and winter grounds. Narwhals spend more of their time in deep offshore waters, but also move into more shallow fjords during summers. They eat fish, squid, and shrimp, and do much of their feeding under the ice throughout the winter. Narwhals are hunted by Inuit communities as an important food source. Both of the communities I do research in, Kugaaruk and Pangnirtung, Nunavut, have active summer narwhal hunts. I’ve heard from many people that the narwhal hunt is one of their favourite times of year.
The most recognizable and magnificent feature of narwhals is the long tusk that protrudes directly from the front of the mouth. The tusk is actually an erupted, protruding left canine tooth that spirals forward, growing as long as 2.6 m / 9 ft, with typically only males growing tusks (as an interesting side note, walrus tusks are also protruded upper canines, whereas elephant tusks are actually elongated incisors). It’s not surprising that this species has been the subject of lore and cultural stories throughout human history.
Since narwhals have been studied by hunters, explorers, and scientists, there has been a great deal of uncertainty over the specific function of the tusk. As may be expected, there is evidence that males use their tusks to attract mates through displays of dominance and to fight other males. In 2014, some of the mystery surrounding narwhal tusks was answered. A study examining the sensory functions of the narwhal tusk discovered that the tusk provides a type of direct line of communication between a narwhal’s brain and temperature and chemical changes in the ocean. Like many mammalian teeth, working inwards from the surface, the outer layer of a narwhal’s tusk is covered with a porous cementum, followed by a dentin layer containing tubes that channel in towards the centre of the tusk. In the core of the tooth, running the full length of the tusk, is a pulp layer full of nerve endings that connect to the brain. In other mammalian teeth, the portion of the tooth that is exposed above the gum line is covered with a hard enamel layer. What sets the narwhal’s tusk apart from other mammalian teeth is that the porous cementum layer is exposed, allowing it to detect changes in the surrounding ocean.
These findings indicate that tusks may fulfill a sensory function to help narwhal locate food sources and mates. Diet analyses show that males and females have different food sources for much of the year, overlapping particularly during the spring-summer mating period. The study also suggests that there may be a number of hypotheses for how tusks could be used to find and attract mates. For example, the tusk may be used to detect females in estrous; to detect food sources that are likely to attract feeding females (which is also supported by the fact that diets overlap during the mating period); or as an indication to females that a male with a well developed tusk could be more successful at finding food for calves.
It’s interesting to learn more about the evolutionary purposes of what is already a stunning physical trait in an elusive species like the narwhal. It’s sometimes easy to forget that we are still discovering the natural world every day. We have a great deal of knowledge about wildlife, but there are also many species we have yet to discover, and it’s a guarantee that we still have much to learn about the species we do know. Each of these little pieces we uncover should remind us to remain both humble and in awe at the diversity of species on this planet. More so, it should remind us of the importance in conserving each of these species and maintaining this diversity.