Whale of the Month - Harbour Porpoise

Species fact file

Common names: Harbour porpoise, common porpoise

Icelandic names: hnísa, selhnísa

Scientific name: Phocoena phocoena

Family: Phocoenidae (porpoises)

Max length: 1.8m (females), 1.7m (males)

Distribution: Coastal North Atlantic, North Pacific, and the Black Sea

The harbour porpoise is the smallest cetacean found in the waters around Iceland. This species is thought to be fairly common here, but it has proven difficult to get an accurate estimate of how many harbour porpoises inhabit this part of the North Atlantic. Traditionally, we can estimate the size of cetacean populations from data collected on surveys – this relies on observers being able to spot and identify different species. But harbour porpoises are notoriously tricky to spot! As well as their small body size, they rarely breach out of the water and tend to avoid boat traffic.

Porpoises are close relatives of dolphins, and if you catch just a glimpse of them it can be easy to mistake a harbour porpoise for a small dolphin. There are a few differences to keep an eye out for that can help you to tell them apart. Porpoises have stout, triangular dorsal fins - very different to the crescent-shaped dorsal fins of dolphins - and they have a short beak, making their faces appear quite small and round-looking. Another key difference, although you’d have to get pretty close to notice it, is inside their mouth: porpoises have spade-shaped teeth, whereas dolphins have conical, pointed teeth.

Porpoises have distinctively flat faces, whereas most dolphins have an elongated beak. Photo via Wikimedia Commons.

Porpoises are also less social than dolphins; they may form small groups, but are very often seen alone. They tend to hunt on their own, too, but recently we have discovered that they are also capable of hunting in groups. In fact, harbour porpoises demonstrate a particularly sophisticated form of cooperative hunting, in which each porpoise in the group has its own individual role to play. This is seen rarely in nature – even amongst cetaceans!

A pair of harbour porpoises. This species is rarely seen in groups of larger than two or three. Photo by Drew Collins.

Due to their small size, porpoises that live in cold regions like here in Iceland have been described as living on a ‘knife-edge’. They are mammals like us and have to maintain their internal body temperature, but with such small bodies they lose heat very quickly in cold water. This means that porpoises have to eat almost constantly to stay alive – one study estimated that they can hunt over 500 fish in an hour and have a success rate of greater than 90%!

Harbour porpoises are one of the few toothed whale species that exhibit what scientists call ‘reverse sexual size dimorphism’ - this is essentially a fancy way of saying that females grow to larger sizes than males. This has been attributed to their incredibly rapid metabolism and the energetic costs of pregnancy. Male and female harbour porpoises have very different reproductive strategies: males grow faster and reach maturity at a younger age and smaller body size, while females mature later in life but are longer and heavier when they do so. This is because at larger body sizes they are better able to survive the ordeal of pregnancy and nursing a calf, so for female harbour porpoises it is better to put off reproduction until they are older and bigger. Males, on the other hand, prefer to begin reproducing as early as possible in order to maximise the number of calves they can father in their lifetime.

One side effect of this is that females can dive deeper and hunt for larger prey than males are capable of. This isn’t all that unusual amongst marine mammals and can actually be beneficial, as it means that males and females of the same species don’t compete with one another for food. Harbour porpoises mainly hunt fish species, including cod, hake, and sprat.

Harbour porpoises are one of a handful of toothed whales that have lost the ability to whistle. This trait has evolved separately in porpoises, a handful of small dolphins including one river dolphin, and the pygmy sperm whale. What do all these species have in common? Killer whales as their main predator! In order to avoid being overheard by eavesdropping orcas, these species have stopped using whistles to communicate and instead have developed very specialised clicks that are too high-pitched for killer whales to hear – so porpoises can chatter away to one another, safe from the prying ears of predators!

These clicks are so high-pitched that they are not only inaudible to killer whales, but to humans as well. Harbour porpoises produce sounds at frequencies as high as 150 kHz; our hearing range only extends to about 20 kHz. Although porpoise clicks can be detected on recordings from hydrophones, we have to modify the sound in order to hear it. Because they communicate with one another at ultrasonic frequencies, porpoises have specially adapted hearing, which is tuned to be particularly sensitive to the echolocation clicks produced by killer whales as well as to their own vocalisations.

Harbour porpoises have been given a few odd nicknames over the years – mostly comparing them to pigs! The word porpoise itself is thought to originate from a Medieval Latin word meaning ‘pig-fish’, and they are still referred to as pigs in many Germanic languages today. For example, the Danish marsvin means ‘sea-pig’ and the German Schweinswal translates literally to ‘pig-whale’, while in parts of Canada, harbour porpoises are sometimes referred to as a ‘puffing pig’ due to the noise they make when they exhale at the surface. This rather sweet characteristic has also given them their Icelandic name, which is derived from the Old Norse word for sneeze!

Learn more about...

How harbour porpoises hunt in groups

Differences in growth and reproduction between male and female harbour porpoises

The evolution of high-frequency clicks in certain toothed whales, including porpoises