Larger threats to porpoises include extensive bycatch in gill nets, competition for food from fisheries, and marine pollution, in particular heavy metals and organochlorides.

The vaquita is nearly extinct due to bycatch in gill nets, with a predicted population of fewer than a dozen individuals.

Porpoises, along with whales and dolphins, are descendants of land-living ungulates (hoofed animals) that first entered the oceans around 50 million years ago (Mya).

The cetaceans diversified, and fossil evidence suggests porpoises and dolphins diverged from their last common ancestor around 15 Mya.

The oldest fossils are known from the shallow seas around the North Pacific, with animals spreading to the European coasts and Southern Hemisphere only much later, during the Pliocene.

[8] Porpoises have a bulbous head, no external ear flaps, a non-flexible neck, a torpedo shaped body, limbs modified into flippers, and a tail fin.

Almost all species have female-biased sexual dimorphism, with the females being larger than the males,[11][12] although those physical differences are generally small; one exception is Dall's porpoise.

The fusing of the neck vertebrae, while increasing stability when swimming at high speeds, decreases flexibility, making it impossible for them to turn their head.

[17] When swimming, they move their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering.

[19] The porpoise ear is acoustically isolated from the skull by air-filled sinus pockets, which allow for greater directional hearing underwater.

Porpoises do, however, lack short wavelength sensitive visual pigments in their cone cells indicating a more limited capacity for colour vision than most mammals.

[24] Most porpoises have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a tapetum lucidum; these adaptations allow for large amounts of light to pass through the eye and, therefore, they are able to form a very clear image of the surrounding area.

The stomach contents of Dall's porpoises reveal that they mainly feed on cephalopods and bait fish, like capelin and sardines.

It is known that populations in the mouth of the Indus River migrate to the sea from April through October to feed on the annual spawning of prawns.

[18] The tube in the head, through which this kind fish takes its breath and spitting water, located in front of the brain and ends outwardly in a simple hole, but inside it is divided by a downward bony septum, as if it were two nostrils; but underneath it opens up again in the mouth in a void.

Aristotle, however, could already see many physiological and anatomical similarities with the terrestrial vertebrates, such as blood (circulation), lungs, uterus and fin anatomy.

[citation needed] His detailed descriptions were assimilated by the Romans, but mixed with a more accurate knowledge of the dolphins, as mentioned by Pliny the Elder in his "Natural history".

The harbour porpoise was one of the most accessible species for early cetologists, because it could be seen very close to land, inhabiting shallow coastal areas of Europe.

Harbour porpoises have historically been kept in captivity, under the assumption that they would fare better than their dolphin counterparts due to their smaller size and shallow-water habitats.

In recent decades, the only place keeping the species in Denmark is the Fjord & Bælt Centre, where three rescues have been kept, along with their offspring.

[45] As part of last-ditch effort of saving the extremely rare vaquita (the tiny remaining population is rapidly declining because of bycatch in gillnets), there have been attempts of transferring some to captivity.

[18][49] Porpoises and other smaller cetaceans have traditionally been hunted in many areas, at least in Asia, Europe and North America, for their meat and blubber.

Although it is the world's most endangered marine cetacean, the vaquita continues to be caught in small-mesh gillnet fisheries throughout much of its range.

Incidental mortality caused by the fleet of El Golfo de Santa Clara was estimated to be at around 39 vaquitas per year, which is over 17% of the population size.

[58] Populations of harbor porpoises in the North and Baltic Seas are under increasing pressure from anthropogenic causes such as offshore construction, ship traffic, fishing, and military exercises.

Heavy metals and plastic waste are not biodegradable, and sometimes cetaceans consume these hazardous materials, mistaking them for food items.

Marine mammals that make use of biosonar for orientation and communication are not only hindered by the extra noise, but may race to the surface in panic.

[68] A majority of factors of this population decline are being driven by the massive growth in Chinese industry since 1990 which caused increased shipping and pollution and ultimately environmental degradation.

The Chinese Academy of Science's Wuhan Institute of Hydrobiology has been working with the World Wildlife Fund to ensure the future for this subspecies, and have placed five porpoises in another well-protected area, the He-wang-miao oxbow.

To try to prevent extinction, the Mexican government has created a nature reserve covering the upper part of the Gulf of California and the Colorado River delta.