Large organ (Image: J. Craig George)

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: Balaena mysticetus

Habitat: in and around the Arctic pack ice

It’s September, and bowhead whales are migrating west along the northern coast of Canada. Behind them the sea ice is spreading as winter approaches, and the whales must find patches of open water.


As they swim they often hold their vast mouths – the biggest in the animal kingdom – slightly open. It’s curious behaviour considering the whales have thousands of kilometres to travel, and an open mouth will increase the drag from the water. But there’s method to this apparent madness. They’re cooling off. It turns out that bowheads have an organ on the roofs of their mouths, bizarrely similar to a penis, which sheds body heat into the cold water.

Whopping whales

Bowhead whales are a superlative species. In addition to having a record-breaking mouth their long, slow lives may make them the longest-lived mammal. We know they can live more than 100 years, and some may reach 200 (although they can’t hold a candle to the immortal jellyfish). They are also the second largest animal on the planet: only blue whales are bigger.

Their massive heads make up one-third of their body length, and contain sheets of enormous bristles called baleen. These sheets look rather like Venetian blinds and trap enormous numbers of tiny animals. They feed by opening their mouths wide and lunging forwards into swarms of plankton, gulping in huge volumes of water. Studies of their brains suggest that, for whales, they have an unusually good sense of smell, which probably helps them find food.

Chill factor

To survive in freezing Arctic waters, bowheads have an arsenal of tricks to retain their bodily warmth. Their bulbous shape reduces their surface area, compared to the elongated shape of most other whale species, which keeps them warm. They are also insulated by a 0.5-metre thick layer of blubber.

But to regulate their temperature, they also need a way of cooling off. Alexander Werth of Hampden-Sydney College in Virginia and colleagues think they know what that is. In the early 1990s, they dissected the heads of seven bowhead whales, which had been killed by Inupiat hunters in Alaska. The results have only just been published.

Each whale had a rod of tissue running along the middle of its palate. When Werth dissected it, he found that it was made of soft, spongy tissue, and filled with blood vessels. The team says it probably swells and becomes rigid when extra blood is pumped through it – much like a penis. They call it the corpus cavernosum maxillaris.

Despite the whale having died several hours earlier, the organ was 6 to 8°C warmer than the other surfaces of the whale’s body. That suggests it sheds heat, especially when it is engorged with warm blood.

Bowhead whales are almost too well insulated, Werth says. This keeps them warm in the freezing Arctic, but it may also make them prone to overheating when they’re working hard – while migrating for instance. “They have very few options to radiate heat,” Werth says.

Werth speculates that the corpus cavernosum maxillaris may have additional functions. It contains a lot of nerve endings, so it might also be sensory. “It would benefit the whales to have a structure in the mouth to detect prey.” Last year it emerged that other massive whales have a sensory organ in their mouths that coordinates lunge feeding – when a whale accelerates to engulf a huge mouthful of prey and water for filtering through its great baleen sheets.

Journal reference: The Anatomical Record, DOI: 10.1002/ar.22681