Whale of the Month - Bowhead Whale

Species fact file

Common names: Bowhead whale, greenland right whale

Icelandic name: norðhvalur, grænlandssléttbakur

Scientific name: Balaena mysticetus

Family: Balaenidae (bowhead and right whales)

Max length: 19m (females), 16m (males)

Distribution: Arctic

Bowhead whales are the only baleen whale species to remain in Arctic and subarctic waters their whole lives. As warm-blooded mammals living in one of the coldest environments on Earth, their bodies are well adapted to survive such harsh conditions. Bowhead whales have a thick layer of blubber to insulate them against the freezing temperatures, which, at 50cm, is the thickest layer of fat in the animal kingdom!

The most impressive of the bowhead whales’ adaptations is their slow life cycle. These characteristics of slow growth, long lifespans, and late maturation are shared by many animals in polar regions; for example, polar invertebrates grow between 2 and 5 times slower than their counterparts in temperate waters. Antarctic starfish can live for more than 100 years – outstripping the average starfish lifespan of 10-35 years by some way – and Earth’s longest-lived vertebrate, the Greenland shark, is a resident of deep waters in the North Atlantic and Arctic oceans.

With a lifespan of more than 200 years, bowhead whales are the longest-lived mammal on the planet. They grow very slowly throughout their adult life, and don’t become sexually mature until they are around 25 years old – much older than all other baleen whales, which are usually between 5 and 10 when they have their first calf. A recent study found evidence that bowhead whale pregnancies may last 23 months! Much more research is needed before we can say anything for definite; but, if true, this is not only much longer than the previous estimate of 14 months, but would snatch the title of longest gestation period from its current holder, the African elephant.

A mother bowhead whale with her calf. Image by NOAA via GPA Photo Archive.

For the first year of their life, bowhead whale calves grow very rapidly, increasing their bodyweight from 1 tonne to 12 tonnes. After weaning, growth slows down, and bowheads may even stop gaining weight altogether for several years as all their resources are directed into growing baleen plates and bulking up their huge skull. During this time, many bones in other parts of the body can actually lose mass – particularly the ribs, which have been recorded losing as much as 40% of their tissue!

Bowhead whales have truly spectacular skulls: the head is absolutely massive in comparison to their body, making up as much as one third of their total length! They use the top of their head to smash through sea ice up to a metre thick, allowing them to create their own breathing holes in the frozen Arctic Ocean. The name ‘bowhead’ is in fact owed to this impressive skull, as the huge curving upper jaw is reminiscent of an archer’s bow. Hanging down from this top jaw are the baleen plates, the longest of any mysticete, which can grow to be 4 metres long. The whales use these plates to filter out their food – small crustaceans called copepods – from the seawater. Bowhead whales hunt copepods using a technique called skim feeding: they swim through dense gatherings of plankton with their mouths open, straining their food from the water as they go! Skim feeding can take place close to the water’s surface, but bowhead whales are thought to also occasionally feed along the silty seabed.

In this illustration of a bowhead whale skeleton, you can see just how huge their heads are! Image by Richard Lydekker via Wikipedia Commons.

Bowhead whales are likely to feel the impacts of climate change more keenly than many other cetaceans. The seasonal formation and breakup of sea ice is an important part of the life cycle of their main prey, small crustaceans called copepods. If this is disrupted, it could dramatically reduce the amount of food available to bowheads. Declining sea ice also makes the Arctic more accessible to killer whales, a top predator now seen more and more frequently in the far north. Where once the ice could provide some protection for bowhead whales, they are becoming increasingly vulnerable.

However, for the moment, decreasing sea ice cover doesn’t appear to be having an effect on bowhead whale populations, and in many regions their numbers are actually increasing!

A bowhead whale swimming through ice floes. Image by Kit Kovacs via UW News.

Although they are not well known for it, bowhead whales are impressive singers. Their songs are, alongside humpback whales, among the most complex of any baleen whale. Bowhead whales can produce vocalisations at a huge range of frequencies, from as low as 20 Hz to more than 5000 Hz.

A study in the north-eastern Atlantic found remarkable variation in the structure of bowhead whale songs, even within one, relatively small area and over a short time frame. Within a single population, new songs emerge very frequently, but most don’t last for very long – sometimes only a few days or even hours before they are replaced, and rarely more than a month. This extraordinary rate of change in song structure is far more rapid than that of humpback whales, who typically sing the same song type for one year; in fact, in the whole animal kingdom, only a handful of songbird species have a faster rate of song evolution.

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How juvenile bowhead whales lose a significant portion of bone mass after weaning

Variation and evolution in bowhead whale song