Whale of the Month - Narwhal

Species fact file

Common names: narwhal

Icelandic name: náhvalur

Scientific name: Monodon monoceros

Family: Monodontidae (narwhal and beluga)

Max length: 3.95m (females), 5.5m (males, excluding tusk)

Distribution: Arctic

Narwhals are also known as the ‘unicorns of the sea’ – this is a fitting nickname for the animal that likely inspired the myth of unicorns in the first place. Because narwhals inhabit such remote areas of the Arctic, few Europeans would ever have seen them until the 19th Century, but would pay vast sums for a unicorn’s horn which they believed to have healing abilities.

In fact, the horn of the ‘unicorn of the sea’ is not a horn at all but a tusk. The front left tooth grows in a long spiral shape, piercing through the skin of the top lip. It is typically male narwhals that have a tusk – some even grow two – but an estimated 15% of females grow a tusk as well, although they are usually much smaller than those of males.

The function of the narwhal’s tusk has been a mystery for a long time. How does it benefit them? Some male narwhals have tusks over two and a half metres long, which seems like it would be quite a burden to carry around all the time. Now, we believe this could actually be the point!

Scientists generally agree that the narwhal’s tusk evolved by sexual selection. Sexually selected traits are those which increase the likelihood of an animal reproducing successfully – but doesn’t necessarily provide any survival benefit. In fact, some sexually selected traits are actually detrimental to survival: the peacock is a great example of this. While peahens are fairly inconspicuous with mostly brown plumage and a short tail, peacocks sport bright, flashy colours all over their body and, of course, that magnificent tail. The brilliant colour of their feathers makes camouflage very difficult, and with the burden of their long, heavy tail peacocks cannot so easily escape predators. These traits are actually putting peacocks on the back foot, so they cannot have evolved through natural selection; however, peahens are more likely to choose a male with brighter feathers and longer tails as a mate. This means that her male offspring inherit similarly bright feathers and long tails, and the cycle continues.

We know that, whatever the primary function of the tusk is, it doesn’t provide any benefit to their survival as females have longer lifespans than males. This, along with the fact that the tusk is an overwhelmingly male trait, is strong evidence that it evolved through sexual selection. Just like a peacock’s tail, male narwhals use their tusk to show off to females, essentially advertising their worth as a potential mate.

However, the tusk can be used for other purposes. Recent drone footage captured in the Canadian Arctic showed male narwhals tapping small fish with their tusk, which stuns the fish and makes them much easier for the narwhals to capture. Some scientists have proposed that the tusk serves also as a sensitive organ: analysis of the tusk’s structure revealed that it is hollow and carries 10 million nerve endings, which may be able to sense changes in water temperature and salinity.

Male narwhals compare the length of their tusks in a dominance contest. Photo by Gavin Williams, NIST via NOAA.

The narwhal’s name comes from nár, the Old Norse word for corpse. This is probably because their mottled grey and white skin is reminiscent of that of drowned sailors floating in the water! Narwhals are born dark grey all over and develop white patches in the first few years of life. They continue to get lighter as they age, especially on the belly, and some old males are almost entirely white.

Narwhals have strong seasonal migrations. As summer approaches, they travel with the receding ice edge into more coastal areas. Narwhals spend most of the summer quite close to land, in fjords, inlets and bays; they then migrate into deeper, offshore waters in autumn as the sea ice begins to form. They are remarkably well adapted to survival in one of the most inhospitable habitats on Earth: during winter, they thrive in areas of the Arctic with as much as 97% ice coverage. During the winter months, narwhals rely on leads and polynyas, areas of open water in the midst of the sea ice, that they can return to for air and to rest.

Narwhals travelling up a lead in the sea ice. Photo by Paul Gierszewski via Wikipedia Commons.

As well as migrating between different regions of the Arctic, narwhals change what and how much they eat depending on the season. They hunt less often during summer – polar cod and squid are their main prey during this time – and at shallower depths, typically diving to around 500m below the surface. Feeding increases throughout autumn and into winter, coinciding with their migration to deeper waters. In these areas, narwhals hunt squid and halibut at depths of over 800m, performing 5-25 dives per day. Narwhals are capable of diving to over 1,500m – this makes them one of the deepest diving cetaceans on Earth!

Analysis of their diet suggests that, regardless of season, narwhals feed on quite a limited range of prey species. This could be cause for concern if a main prey species decreases: because of their fairly specialised diet, narwhals may not be able to switch to another type of prey.

As well as a decline in their food supply, it’s possible that in the future narwhals could be exposed to a greater risk of being hunted themselves. The only animals other than humans that regularly hunt narwhals are polar bears and killer whales. Killer whales are a top ocean predator but, unlike narwhals, they are not adapted to life in the far north – they avoid sea ice and cannot survive long periods in polar temperatures. However, with a decline in sea ice coverage, the Arctic is becoming more accessible to killer whales. Where once the sea ice could provide some protection for narwhals, they are becoming increasingly vulnerable.

Climate change is one of the largest concerns for Arctic specialists like narwhals. The seasonal formation and breakup of sea ice is essential for the stability of the ecosystem; as water temperatures rise and sea ice becomes less predictable, Arctic food webs are at risk. But how a change at one level of the ecosystem affects other levels can be really hard to predict, which - unfortunately for narwhals and other Arctic cetaceans - makes conservation in the face of climate change quite a challenge.

Learn more about...

The evidence that the narwhal’s tusk is a sexually selected trait

How climate change could increase killer whale predation on narwhals