Whale of the Month - Sowerby's Beaked Whale

Species fact file

Common names: Sowerby’s beaked whale, North Atlantic beaked whale, North Sea beaked whale

Icelandic name: norðsnjáldri

Scientific name: Mesoplodon bidens

Family: Ziphiidae (beaked whales)

Max length: 5.5m (males), 5m (females)

Distribution: Temperate North Atlantic, generally offshore

Sowerby’s beaked whale was identified in 1804, based on the skull of a whale which stranded in Scotland, by naturalist James Sowerby after whom the whale is named. This was the first of the so-called mesoplodont beaked whales described scientifically.

The term mesoplodont is used to describe all the species that belong to the genus Mesoplodon. This is the most poorly understood group of large mammals on Earth – there are currently 16 recognised mesoplodonts, but several of these species have never been sighted alive and are only known through a handful of stranded animals. New mesoplodonts have been discovered as recently as 2021, so there may be more additions to the genus in the future!

Sowerby’s beaked whale is the most northerly of all the mesoplodonts, ranging throughout the temperate and subarctic North Atlantic. Although it was discovered over 200 years ago, this species is rarely seen at sea, and is still mostly known to us from strandings. The majority of sightings and strandings occur around the British Isles. The last record of a Sowerby’s beaked whale in Iceland was a stranding in 2018 – before that, this species hadn’t been seen here since 1992.

This species is thought to live mostly in small groups, usually of about 5 individuals, but occasionally up to 10. Whether these groups are family units – consisting of close relatives – or are formed in some other way is still not known.

A group of Sowerby's beaked whales. The 45° angle of the beak as they break through the water's surface is a characteristic behaviour of this species. Photo by Sylvère Corre.

Beaked whales generally feed on squid and very little else, but Sowerby’s beaked whales are a little bit different; although they do hunt some squid, they are believed to favour mesopelagic and deep-water fishes as prey. They hunt more than 500m below the surface, and an average foraging trip will reach depths of over 1000m, but their dives are remarkably short for a beaked whale and normally last only about 30 minutes. In fact, the swimming and diving behaviour of Sowerby’s beaked whales sets them apart from other beaked whales: they swim faster, hunt faster, and stay underwater for much shorter lengths of time. They also cover long distances between dives by swimming just below the surface of the water, which contrasts with the typical beaked whale strategy of minimising time near the surface by travelling on shallow dives, 100-200m down.

Males have two tusks growing from their bottom jaw – the sharp, pointed tips of these large teeth grow right through the upper jaw and sometimes protrude through the top lip, leaving small holes visible in the skin. Sowerby’s beaked whales have very long, slender beaks that make them almost dolphin-like in appearance. Their small pectoral fins fit into shallow depressions, or “pockets”, in the side of the body which help them to keep a streamlined shape while swimming. This is one of the few beaked whale species that has been seen breaching.

A Sowerby's beaked whale mother and calf. Photo by Cottele Benny.

Their global population size is unknown, but they are currently not considered endangered. One of the major concerns for this species is the use of mid-frequency sonar, which is known to have serious effects on deep-diving beaked whales.

In 2020, two Sowerby’s beaked whales stranded within 8 miles of each other on the east coast of Scotland less than 24 hours apart. The cause of death was not immediately clear; however, one of the animals had a high density of gas bubbles in the lung tissue, and they each showed signs of gas embolism – this is when blood vessels are blocked by bubbles of air. These symptoms are both indicators of decompression sickness (DCS), and common findings in the bodies of other beaked whales that have stranded after being exposed to military sonar.

It is possible that this is what happened to these Sowerby’s beaked whales, as there were airborne anti-submarine sonar tests carried out in the area just a day before the first whale was beached. In combination with their otherwise good physical health, lack of other identifiable cause of death, and the fact that they were beached unusually close together in terms of both time and location, this suggests a connection between the sonar exercises and the subsequent stranding of these two whales. More details on this stranding and the necropsy results are available in the Scottish Marine Animal Stranding Scheme’s 2020 Annual Report (warning - this report contains some graphic images!).

Many other species in the Mesoplodon genus have previously been recorded in mass strandings associated with naval sonar, but this appears to be the first case involving Sowerby’s beaked whales. Sonar-related mass mortalities can consist of tens to hundreds of beaked whales – once we consider that not all cetaceans that die at sea ever reach land, the scale of these events starts to become clear. We currently understand so little about so many beaked whale species, including Sowerby’s beaked whale, in terms of their population size, distribution throughout the oceans, and social structure; this makes it impossible to know how these mass mortalities might affect a population – or even the species as a whole.

Learn more about...

The fish-based diet of Sowerby’s beaked whales

How the diving behaviour of Sowerby’s beaked whales differs from other mesoplodonts