Porpoises are small aquatic mammals from the cetacean family Phocoenidae that fall within a large group of toothed whales, Odontoceti. Often confused with dolphins, porpoises have several distinct traits. Unlike dolphins, they have rounded faces without a prominent beak, spade-shaped teeth instead of pointed ones, and are generally smaller in size. Despite their resemblance to true dolphins (family Delphinidae), porpoises are more closely related to belugas and narwhals of the family Monodontidae.
These cetaceans occupy diverse habitats, from rivers and coastal waters to the open ocean, thriving in waters from tropical to polar regions. Their diet mainly consists of squid and fish, which they pursue by tracking the seasonal movements of these prey.
Along with whales and dolphins, porpoises are descendants of land-living ungulates that entered the ocean almost 50 million years ago. Currently, there are 8 extant species under three genera: Neophocaena (Finless porpoises), Phocoena, and Phocoenoides.
These mammals vary in size from the smallest, the vaquita, measuring 4 ft 7 in (1.4 m) in length and weighing around 119 lb (54 kg), to the largest, the Dall’s porpoise, measuring 7 ft 7 in (2.3 m) and weighing 490 lb (220 kg).
They exhibit sexual dimorphism, with the female porpoises being larger than males.
Porpoises have a torpedo-shaped, streamlined body with a rounded head, a rigid neck, two flippers (modified forelimbs) with four digits each, and a tail fin. Their hindlimbs are vestigial and may contain remnants of feet and digits. A thick layer of fat or blubber beneath their skin insulates them from cold temperatures underwater.
Although porpoises resemble dolphins, they can be distinguished by several key features. Porpoises are generally smaller and lack the elongated, beak-like snout, or rostrum, that is characteristic of dolphins. Additionally, porpoises have a distinct, triangular dorsal fin, while dolphins usually have a curved dorsal fin.
The body coloration varies across the different species of porpoises. For instance, members of the genus Phocoena are dorsally black and ventrally white. However, the Dall’s porpoise (member of Phocoenoides), though black throughout, is distinctly patched with white on each side.
They have a three-chambered stomach consisting of a fore-stomach, a fundic, and a pyloric chamber.
The skull of a porpoise has two small orbital sockets for its eyes and a large depression that houses an organ called the melon. This fat-filled organ, also found in dolphins, aids in generating high-frequency ultrasonic clicks used for communication within a species. Positioned at the top of the head is a blowhole, which serves as a spiracle (functionally similar to the nostrils in other mammals), through which the porpoise exhales stale air in forceful, misty bursts and inhales fresh air.
Unlike dolphins, which have conical teeth, porpoises possess spade-shaped teeth covered with a calcified layer of cementum over the dentine.
The ears are separated from the skull by air-filled sinus pockets, allowing them to receive sound from the throat. These sound waves travel through a fat-filled, low-impedance cavity that connects directly to the inner ear.
Porpoise eyes are relatively small compared to their body size and are positioned on either side of the head, providing them with clear underwater vision. To maximize light intake, both the eyeballs and corneas are slightly flattened, and the pupils are enlarged in most species. They also have a layer behind the retina, known as the tapetum lucidum, which reflects light through the retina for enhanced vision.
Their sense of taste and smell is limited due to atrophied taste buds and the absence of olfactory lobes.
The 8 extant species fall under three genera: Neophocaena (Finless porpoises), Phocoena, and Phocoenoides. Some extinct genera include Semirostrum, Septemtriocetu, Piscolithax, and Numataphocoena.
It is believed that porpoises, along with whales and dolphins, evolved from terrestrial ungulates that switched to an aquatic lifestyle around 50 million years ago. With time, the cetaceans diversified, and around 15 million years ago, during the Miocene Epoch, both porpoises and dolphins diverged from their last common ancestor (LCA).
The oldest fossils suggest that these animals were initially restricted to the North Pacific Ocean and eventually spread to the European coasts and Southern Hemisphere during the Pliocene Epoch.
Porpoises are distributed across a wide range of habitats, from the tropical waters of the Sea of Cortez, where the vaquita is found, to the polar waters around Greenland, home to the harbor porpoise. Both of these species are generally found in coastal and continental shelf waters. Finless porpoises (Neophocaena genus) are commonly found in rivers, whereas Dall’s porpoise and the spectacled porpoise primarily inhabit the open ocean.
Like most toothed whales, porpoises primarily feed on fish (both benthic and pelagic species) and marine invertebrates such as cephalopods following a seasonal migration of their prey. They occasionally consume small amounts of algae, such as sea lettuce (Ulva lactuca).
They are generally shy and live in small social groups.
These cetaceans are fast swimmers, propelling at speeds of 9 to 28 km/h. They move their flippers constantly to steer forward and utilize their tail fin to move their body vertically up and down. Since their neck vertebrae are fused, porpoises are unable to turn their heads and thus are extremely stable when swimming at high speeds. They may also rest or move slowly at the water surface (logging) or leap out of the water in long, energetic jumps (porpoising) to avoid drag and travel at ease.
Some porpoises, such as the finless porpoises in the Indus River mouth, migrate to the sea between April and October to feed on prawns spawning in that region.
The lifespan of porpoises varies widely across species and the type of habitat (living in the wild or captivity):
Porpoise females typically give birth to a single calf following a gestation period of about a year. The calf is delivered underwater, and the fetus receives a tail-first delivery to prevent drowning.
Following its birth, the mother squirts milk into its mouth since the young cannot suck milk of their own (due to the shape of their mouth). This milk is rich in fat and helps the young develop its insulating blubber.
It is the females that rear the young and wean them at around 11 months. However, they may also remain dependent for up to 2 years, after which they attain sexual maturity between 7 to 10 years.
They are preyed upon by sharks, killer whales (orcas), and, rarely, dolphins.
These cetaceans have been traditionally hunted for their meat and blubber, especially in Asia, Europe, and North America. In Japan, around 17,000 Dall’s porpoises are hunted every year, making it the largest cetacean hunt worldwide.
They are also victims of accidental bycatch during fishing. The vaquita, the world’s most endangered cetacean, continues to fall prey to gillnetting, a fishing technique that involves vertical panels of nets hanging from a line. In El Golfo de Santa Clara, Mexico, an estimated 39 vaquitas are accidentally killed each year due to gillnetting.
These cetaceans are also extremely sensitive to anthropogenic disturbances, like offshore construction, ship traffic, and military practices. They often consume heavy metals and plastic, which make them more susceptible to diseases and also affect their fecundity.
Currently, seven of the eight extant porpoise species are listed in the IUCN Red List under the following categories.
| IUCN Category | Species |
|---|---|
| Least Concern (LC) | Harbor porpoise, Dall’s porpoise, and spectacled porpoise |
| Critically Endangered (CR) | Vaquita |
| Near Threatened (NT) | Burmeister’s porpoise |
| Vulnerable (VU) | Indo-Pacific finless porpoise |
| Endangered (EN) | Narrow-ridged Finless porpoise |
Four species, the harbour porpoise, spectacled porpoise, Burmeister’s porpoise, and Dall’s porpoise, are listed under Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). The harbour porpoise also falls under the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea, and Contiguous Atlantic Area (ACCOBAMS).
Since these animals may drown if they remain unconscious for too long, porpoises sleep with one side of their brain active and breathe consciously during their period of rest.
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