Among the heroic responses to the terrorist attack on London Bridge on Friday, one stands out for its strangeness. A man at Fishmongers’ Hall, where the incident began, grabbed a narwhal tusk that was displayed by the side of the hall’s doorway and, wielding it like a lance, deployed it against the assailant outside. He acted according to instinct, yet his brave gesture, with its knightly resonance, has an emblematic power.

In medieval times, the narwhal’s tusk was invested with magic powers: ground up as a powder it was thought to protect monarchs against poisoning. Elizabeth I was given a tusk by her Arctic explorer, Sir Martin Frobisher; it was so valuable she could have bought a castle with it. Another was turned into a royal sceptre. This heavy ivory spiral – it is difficult to hold in one hand – was long believed to have belonged to the mythical unicorn, as emblazoned on the royal coat of arms along with the lion. The Holy Roman Emperor Maximilian I had the animal’s image etched on his jousting armour. Nowadays, tusks – which cannot be traded internationally – can change hands for £25,000 in north America, where the animals are still hunted with high-velocity rifles.

This enigmatic Arctic mammal’s name itself has a fated aspect, being old Norse for “corpse whale”, since the cetacean’s spotted hide was said to resemble a drowned man. For centuries it was believed that the narwhal itself only used its tusk in jousts between males. In fact, recent research has established that the tusk – actually an extended tooth that erupts through the whale’s upper lip, and spirals out for up to five metres – is sensitive with nerve endings at the surface, and that the animals rub tusks together in sensory communication.

If one animal’s tusk gets broken off, another narwhal may even break off the tip of its own tusk in the gap, perhaps the ultimate altruistic gesture. Perhaps more apposite is another new discovery: that in a fearful situation the narwhal’s heart beat slows down to just four beats a minute.

It is strange to note that only hours after the London Bridge attack, a dead minke whale stranded on the banks of the same river, at Battersea Bridge, its sleek black carcass lying within the pooled bright lights of the city’s back side at night. That whale joined two others that have perished in this waterway in the past two months, one of them killed by a ship-strike. It is hard not to see this river as a mortal place, out of Joseph Conrad’s Heart of Darkness, or TS Eliot’s The Waste Land.

Yet whales have forever been seen as omens – of good as well as ill. And perhaps there is another, more human poetry to be found in the terror of what happened in the heart of a metropolis that is often seen as an unfeeling, lonely place. That for people, as for whales, it is our sense of community that offers a real kind of hope.

• Philip Hoare is an author whose books include Leviathan, or The Whale