Editor’s note: This is the final entry in a series by island whale biologist Phil Clapham exploring the whales that make up our part of the Salish Sea.
Having dealt with the big and flashy local cetaceans — the humpback, gray and killer whales — I’ll conclude this series with the animal that we see more of than any of the rest, albeit usually very briefly.
You’re on the ferry and gazing out at the water, lost in thought. Suddenly you see a blip at the surface: a small dark back with a dorsal fin, there for a second and then disappearing beneath the waves. You’ve just witnessed a pretty typical view of the humble harbor porpoise.
One of the smallest of all the cetaceans, the harbor porpoise clocks in at a maximum length of around five and a half feet. Females are a bit bigger than males, and can weigh up to 170 pounds. They’re one of only eight species of porpoises worldwide.
Which begs the question frequently asked by passengers on whalewatching trips: what’s the difference between a dolphin and a porpoise?
Unlike dolphins, porpoises lack a prominent “beak”, and they have broader, spade-shaped teeth rather than the pointed teeth found in dolphins. The harbor porpoise’s dorsal fin is triangular in shape rather than curved and falcate, as it is in most dolphins.
When you catch that quick roll of the back at sea, you see the porpoise as dark gray, but the underside of the animal is actually white.
Found only in the Northern Hemisphere, harbor porpoises have a range that includes temperate to arctic waters, generally between latitudes 38 to 70 degrees North (although in the eastern North Atlantic they can be found in the cold currents off tropical northwestern Africa). They’re usually found in coastal waters rather than offshore.
Their range is discontinuous, which results in numerous separate (“discrete”) populations. The typically smaller sizes of these populations make the species particularly vulnerable to human threats, notably entanglement in fishing gear (especially bottom-set gill nets).
It’s likely that thousands of porpoises every year die from being trapped in fishing nets, and in some areas (such as Southeast Alaska) the local population has declined as a result. Because of widespread gill-netting, the populations in the Black Sea and Baltic Sea are generally considered the most endangered of all.
Harbor porpoises have a gestation period of 10-11 months, but females will often reproduce several years in a row. This means that they are simultaneously pregnant and lactating, almost continuously, which imposes a huge energetic burden on the mother.
The production of milk — which contains far more fat than the human equivalent — is an expensive business for any cetacean: a large whale such as a humpback can lose a third of her body weight (many tons) between the end of pregnancy and when she weans her calf a year later. Consequently, a porpoise’s body condition will undoubtedly deteriorate after repeatedly popping out calves for several years.
Speaking of reproduction: while harbor porpoises usually offer only the briefest glimpses of themselves when they surface to breathe, that inconspicuous nature belies a fascinating mating system.
Males are, to put it bluntly, extremely well-endowed relative to most cetaceans; and females have a vaginal anatomy that seems designed to repel mating, with internal folds that create a barrier to penetration. Probably because of an asymmetrical anatomical configuration, males exclusively approach from the female’s left side, and attempts to mate involve them sometimes jumping out of the water very energetically.
It’s hard to imagine how this aerial approach ever results in copulation, but it does.
The length of the male’s penis, and the large size of the (internal) testes make it clear that the mating system involves what’s known as sperm competition, where males don’t fight for females but instead try to out-compete each other with a large volume of semen (which also means that a female will likely mate with more than one male).
We see this mating system in some other cetaceans. The most spectacular example is the right whale, which has a penis that can reach eight feet in length, along with the largest testes in the animal kingdom: one ton the pair (yes, you read that right). Female right whales mate serially and sometimes even simultaneously with more than one male.
Harbor porpoises are largely solitary, though it’s common to see them in small groups, especially when they’re feeding. They prey on small schooling fish such as herring or capelin. Like all toothed whales, they use sonar (echolocation) to navigate and to find food.
They don’t live very long. Typical life expectancy is between eight and twelve years, though ages exceeding twenty years have been recorded in captive animals. Many porpoises end their lives as prey for killer whales.
The word porpoise comes from mediaeval Latin porcopiscus, which is a combination of the words for “pig” and “fish”, and indeed in some places harbor porpoises are known to fishermen as “puffing pigs”. The scientific name is Phocoena phocoena; that word comes to us from Aristotle, and is actually a misnomer — it translates to “big seal”.
Phil Clapham is a whale biologist who lives on Maury Island. Prior to retiring in 2019, he directed the Cetacean Assessment and Ecology Program at NOAA’s Alaska Fisheries Science Center in Seattle.
