The pig (Sus domesticus), often called swine, hog, or domestic pig when distinguishing from other members of the genus Sus, is an omnivorous, domesticated even-toed ungulate. It is variously considered a subspecies of the Eurasian boar or a distinct species, but the American Society of Mammalogists considers it the latter.[2] The pig's head-plus-body length ranges from 0.9 to 1.8 m (3 to 6 ft), and adult pigs typically weigh between 50 and 350 kg (110 and 770 lb), with well-fed individuals even exceeding this range. The size and weight of hogs largely depends on their breed. Compared to other artiodactyls, a pig's head is relatively long and pointed. Most even-toed ungulates are herbivorous, but pigs are omnivores, like their wild relative. Pigs grunt and make snorting sounds.
When used as livestock, pigs are farmed primarily for the production of meat, called pork. A group of pigs is called a passel, a team, or a sounder. The animal's bones, hide, and bristles are also used in products. Pigs, especially miniature breeds, are kept as pets.
Biology



The pig typically has a large head, with a long snout which is strengthened by a special prenasal bone and a disk of cartilage at the tip.[3] The snout is used to dig into the soil to find food and is a very acute sense organ. The dental formula of adult pigs is
3.1.4.3
3.1.4.3
, giving a total of 44 teeth. The rear teeth are adapted for crushing. In the male, the canine teeth can form tusks, which grow continuously and are sharpened by constantly being ground against each other.[3]
Four hoofed toes are on each foot, with the two larger central toes bearing most of the weight, but the outer two also being used in soft ground.[4]

Reproduction
Female pigs reach sexual maturity at 3–12 months of age and come into estrus every 18–24 days if they are not successful
Pigs possess both apocrine and eccrine sweat glands, although the latter appear limited to the snout and dorsonasal areas.[6] Pigs, however, like other "hairless" mammals (e.g. elephants, rhinos, and mole-rats), do not use thermal sweat glands in cooling.[7] Pigs are also less able than many other mammals to dissipate heat from wet mucous membranes in the mouth through panting. Their thermoneutral zone is 16 to 22 °C (61 to 72 °F).[8] At higher temperatures, pigs lose heat by wallowing in mud or water via evaporative cooling, although it has been suggested that wallowing may serve other functions, such as protection from sunburn, ecto-parasite control, and scent-marking.[9]

Pigs are one of four known mammalian species which possess mutations in the nicotinic acetylcholine receptor that protect against snake venom. Mongooses, honey badgers, hedgehogs, and pigs all have modifications to the receptor pocket which prevents the snake venom α-neurotoxin from binding. These represent four separate, independent mutations.[10]

Pigs have small lungs in relation to their body size, and are thus more susceptible than other domesticated animals to fatal bronchitis and pneumonia.[11]

Reproduction
Female pigs reach sexual maturity at 3–12 months of age and come into estrus every 18–24 days if they are not successfully bred. The variation in ovulation rate can be attributed to intrinsic factors such as age and genotype, as well as extrinsic factors like nutrition, environment, and the supplementation of exogenous hormones.[43] The gestation period averages 112–120 days.[44]


Piglets keeping warm together
Estrus lasts two to three days, and the female's displayed receptiveness to mate is known as standing heat. Standing heat is a reflexive response that is stimulated when the female is in contact with the saliva of a sexually mature boar. Androstenol is one of the pheromones produced in the submaxillary salivary glands of boars that will trigger the female's response.[45] The female cervix contains a series of five interdigitating pads, or

History

Bronze pig sculpture, Zhou dynasty
Archaeological evidence suggests that pigs were domesticated from wild boar in the Near East in the Tigris Basin,[16][page needed] Çayönü, Cafer Höyük, Nevalı Çori[17] being managed in the wild in a way similar to the way they are managed by some modern New Guineans.[18] Remains of pigs have been dated to earlier than 11,400 years ago in Cyprus. Those animals must have been introduced from the mainland, which suggests domestication in the adjacent mainland by then.[19] There was also a separate domestication in China which took place about 8,000 years ago.[20][21]

In the Near East, pig husbandry spread for the next few millennia. It reduced gradually during the Bronze Age, as rural populations focused instead on commodity-producing livestock. It was sustained urbanized regions, however.[22]

DNA evidence from subfossil remains of teeth and jawbones of Neolithic pigs shows that the first domestic pigs in Europe had been brought from the Near East. This stimulated the domestication of local European wild boar, resulting in a third domestication event with the Near Eastern genes dying out in European pig stock. Modern domesticated pigs have involved complex exchanges, with European domesticated lines being exported, in turn, to the ancient Near East.[23][24] Historical records indicate that Asian pigs were introduced into Europe during the 18th and early 19th centuries.[20]

In August 2015, a study looked at over 100 pig genome sequences to ascertain their process of domestication, which was assumed to have been initiated by humans, involved few individuals, and relied on reproductive isolation between wild and domestic forms. The study found that the assumption of reproductive isolation with population bottlenecks was not supported. The study indicated that pigs were domesticated separately in Western Asia and China, with Western Asian pigs introduced into Europe, where they crossed with wild boar. A model that fit the data included a mixture with a now extinct ghost population of wild pigs during the Pleistocene. The study also found that despite back-crossing with wild pigs, the genomes of domestic pigs have strong signatures of selection at DNA loci that affect behavior and morphology. The study concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars and created domestication islands in the genome. The same process may also apply to other domesticated animals.[25][26] In 2019, a study showed that the pig had arrived in Europe from the Near East 8,500 years ago. Over the next 3,000 years they then admixed with the European wild boar until their genome showed less than 5% Near Eastern ancestry, yet retained their domesticated features.[27]

Among the animals that the Spanish introduced to the Chiloé Archipelago in the 16th century, pigs were the most successful to adapt. The pigs benefited from abundant shellfish and algae exposed by the large tides of the archipelago.[28] Pigs were brought to southeastern North America from Europe by de Soto and other early Spanish explorers. Escaped pigs became feral and caused a great deal of disruption to Native Americans.[29] Feral pig populations in the southeastern United States have since migrated north and are a growing concern in the Midwest. Considered an invasive species, many state agencies have programs to trap or hunt feral pigs as means of removal.[30][31][32] Domestic pigs have become feral in many other parts of the world (e.g. New Zealand and northern Queensland) and have caused substantial environmental damage.[33][34] Feral hybrids of the European wild boar with the domestic pig are also very disruptive to both environment and agriculture (among the 100 most damaging animal species),[35] especially in southeastern South America from Uruguay to Brazil's Mato Grosso do Sul and São Paulo.[36][37][38][39][40]

With around 1 billion individuals alive at any time, the domesticated pig is one of the most numerous large mammals on the planet.[41][42]