Pig

For other uses, see Pig (disambiguation).

"Swine" redirects here. For other uses, see Swine (disambiguation).

The pig (Sus domesticus), often called swine (pl.: swine), hog, or domestic pig when distinguishing from other members of the genus Sus, is an omnivorous, domesticated, even-toed, hoofed mammal. It is variously considered a subspecies of Sus scrofa (the wild boar or Eurasian boar) or a distinct species. The pig's length ranges from 0.9 to 1.8 m (3 to 6 ft); adult pigs weigh between 50 and 350 kg (110 and 770 lb), and well-fed individuals can exceed this range. The size and weight of pigs largely depends on their breed. Compared to other artiodactyls, a pig's head is relatively long and pointed.

Pig
Pig farm in Finland
Conservation status
Domesticated
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	Suidae
Genus:	Sus
Species:	S. domesticus
Binomial name
Sus domesticus Erxleben, 1777
Synonyms^[1]
Sus domestica Sus scrofa domesticus Linnaeus, 1758

When used as livestock, pigs are farmed primarily for meat, called pork. The animal's bones, hide, and bristles are also used in products. Pigs, especially miniature breeds, are kept as pets.

Biology

 

Skull

 

Skeleton

 

Bones of the foot

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.^[2] The snout is used to dig into the soil to find food and is an acute sense organ. The dental formula of adult pigs is 3.1.4.33.1.4.3, 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.^[2] There are four hoofed toes on each foot; the two larger central toes bear most of the weight, while the outer two are also used in soft ground.^[3] Most pigs have rather sparsely bristled hair on their skin, though there are some woolly-coated breeds such as the Mangalitsa.^[4]

Pigs possess both apocrine and eccrine sweat glands, although the latter are limited to the snout and dorsonasal areas.^[5] Pigs, like other "hairless" mammals such as elephants, do not use thermal sweat glands in cooling.^[6] Pigs are less able than many other mammals to dissipate heat from wet mucous membranes in the mouth by panting. Their thermoneutral zone is 16–22 °C (61–72 °F).^[7] 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.^[8]

Pigs are among four mammalian species with mutations in the nicotinic acetylcholine receptor that protect against snake venom. Mongooses, honey badgers, hedgehogs, and pigs all have different modifications to the receptor pocket which prevents α-neurotoxin from binding.^[9]

Pigs have small lungs for their body size, and are thus more susceptible than other domesticated animals to fatal bronchitis and pneumonia.^[10] Pigs have a maximum life span of about 27 years.^[11]

The genome of the pig has been sequenced; it contains about 22,342 protein-coding genes.^[12][13][14]

Evolution

Phylogeny

Domestic pigs and wild boar, Sus scrofa, are related to other pig species as shown in the cladogram, based on phylogenetic analysis using mitochondrial DNA.^[15]

Suidae	Babirusa Indonesia Red river hog Warthogs Africa Visayan warty pig Philippine warty pig Bornean bearded pig Javan warty pig Sus scrofa East Asian pigs European pigs Eurasia

Taxonomy

The pig is most often considered to be a subspecies of the wild boar, which was given the name Sus scrofa by Carl Linnaeus in 1758; following from this, the formal name of the pig is Sus scrofa domesticus.^[16][17] However, in 1777, Johann Christian Polycarp Erxleben classified the pig as a separate species from the wild boar. He gave it the name Sus domesticus, still used by some taxonomists.^[18][19] The American Society of Mammalogists considers it a separate species.^[20]

Domestication in the Neolithic

Archaeological evidence suggests that pigs were domesticated from wild boar in the Near East in or around the Tigris Basin,^[21] being managed in the wild in a way similar to the way they are managed by some modern New Guineans.^[22] There were pigs in Cyprus more than 11,400 years ago, introduced from the mainland, implying domestication in the adjacent mainland by then.^[23] Pigs were separately domesticated in China, starting some 8,000 years ago.^[24][25][26] 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, but it was sustained in cities.^[27]

DNA evidence from subfossil remains of teeth and jawbones of Neolithic pigs shows that the first domestic pigs in Europe were 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. More recently there have been complex exchanges, with European domesticated lines being exported, in turn, to the ancient Near East.^[28][29] Historical records indicate that Asian pigs were introduced into Europe during the 18th and early 19th centuries.^[25]

Domestication did not involve reproductive isolation with population bottlenecks. Pigs were domesticated separately in Western Asia and China. Western Asian pigs were introduced into Europe, where they crossed with wild boar. There appears to have been interbreeding with a now extinct ghost population of wild pigs during the Pleistocene. The genomes of domestic pigs show strong selection for genes affecting behavior and morphology. Human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars and created domestication islands in the genome.^[30][31] Pigs arrived in Europe from the Near East by 8,500 years ago. Over the next 3,000 years they interbred with European wild boar until their genome showed less than 5% Near Eastern ancestry, yet retained their domesticated features.^[32]

History

Columbian Exchange

Among the animals that the Spanish introduced to the Chiloé Archipelago in the 16th century Columbian Exchange, pigs were the most successful in adapting to local conditions. The pigs benefited from abundant shellfish and algae exposed by the large tides of the archipelago.^[33] Pigs were brought to southeastern North America from Europe by de Soto and other early Spanish explorers. Escaped pigs became feral, disrupting the lives of Native Americans.^[34]

With a population of around 1 billion individuals, the domesticated pig is one of the most numerous large mammals on the planet.^[35][36]

Feral pigs

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 remove feral pigs.^[37][38][39] 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.^[40][41] Feral hybrids of the European wild boar with the domestic pig are disruptive to both environment and agriculture (among the 100 most damaging animal species),^[42] especially in southeastern South America from Uruguay to Brazil's Mato Grosso do Sul and São Paulo.^{[43][44][45][46][47]}

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.^[48] The gestation period averages 112–120 days.^[49]

 

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 trigger the female's response.^[50] The female cervix contains a series of five interdigitating pads, or folds, that hold the boar's corkscrew-shaped penis during copulation.^[51] Females have bicornuate uteruses and two conceptuses must be present in both uterine horns for pregnancy to be established.^[52] Maternal recognition of pregnancy in pigs occurs on days 11 to 12 of pregnancy and is marked by progesterone production from a functioning corpus luteum.^[53] To avoid luteolysis by PGF2α, rescuing of the CL must occur via embryonic signaling of estradiol 17β and PGE2.^[54] This signaling acts on both the endometrium and luteal tissue to prevent the regression of the CL by activation of genes that are responsible for CL maintenance.^[55] During mid to late pregnancy, the corpus luteum relies primarily on Luteinizing hormone for maintenance until birth.^[54][56]

Archeological evidence indicates that medieval European pigs farrowed, or bore a litter of piglets, once per year.^[57] By the nineteenth century, European piglets routinely double-farrowed, or bore two litters of piglets per year. It is unclear when this shift occurred.^[58]

Behavior

 

Pigs in a wallow

In many ways pig behaviour appears to be intermediate between that of other artiodactyls and of carnivores.^[59] Pigs seek out the company of other pigs and often huddle to maintain physical contact, although they do not naturally form large herds. They live in groups of about 8–10 adult sows, some young individuals, and some single males.^[60]

Because of their relative lack of sweat glands, pigs often control their body temperature using behavioural thermoregulation. Wallowing, which often consists of coating the body with mud, is a behaviour frequently exhibited by pigs.^[8] They do not submerge completely under the mud, but vary the depth and duration of wallowing depending on environmental conditions.^[8] Adult pigs start wallowing once the ambient temperature is around 17–21 °C (63–70 °F). They cover themselves in mud from head to tail.^[8] Pigs may use mud as a sunscreen, or as a method of keeping parasites away.^[8] Most bristled pigs "blow their coat", meaning that they shed most of the longer, coarser stiff hair once a year, usually in spring or early summer, to prepare for the warmer months ahead.^[61]

If conditions permit, pigs feed continuously for many hours and then sleep for many hours, in contrast to ruminants which tend to feed for a short time and then sleep for a short time. Pigs are omnivorous, and are highly versatile in their feeding behaviour. As they are foraging animals, they primarily eat leaves, stems, roots, fruits, and flowers.^[62] Pigs are highly intelligent animals,^[63] on par with dogs,^[64] and according to David DiSalvo's writing in Forbes, they are "widely considered the smartest domesticated animal in the world. Pigs have demonstrated the ability to move a cursor on a video screen with their snouts and understand what is happening onscreen, and have learned to distinguish between the scribbles they had seen before and those they were seeing for the first time."^[65][a][69]

Rooting

Rooting is an instinctual behavior in pigs characterized by nudging the snout into something; it is found comforting. It first happens when piglets are born to obtain their mother's milk, and can become a habitual, obsessive behavior, most prominent in animals weaned too early. Pigs root and dig into the ground to forage for food. Rooting is also a means of communication.^[70]

Nest-building

A behavioural characteristic of pigs which they share with carnivores is nest-building. Sows root in the ground to create depressions and then build nests in which to give birth. First, the sow digs a depression about the size of her body. She then collects twigs and leaves, and carries these in her mouth to the depression, building them into a mound. She distributes the softer, finer material to the centre of the mound using her feet. When the mound reaches the desired height, she places large branches, up to 2 metres in length, on the surface. She enters the mound and roots around to create a depression within the gathered material. She then gives birth in a lying position, unlike other artiodactyls which usually stand while birthing.^[59]

Nest-building is an important part of pre and post-partum maternal behaviour. It occurs during the last 24 hours before the onset of farrowing, and becomes most intense during 12 to 6 hours before farrowing.^[71] Nest-building is divided into two phases: one of which is the initial phase of rooting in the ground while the second phase is the collecting, carrying and arranging of the nest material.^[71] The sow separates from the group and seeks a suitable nest site with well-drained soil and shelter from rain and wind. This nest-building behaviour is performed to provide the offspring with shelter, comfort, and thermoregulation. The nest provides protection against weather and predators, while keeping the piglets close to the sow and away from the rest of the herd. This ensures they do not get trampled on and prevents other piglets from stealing milk from the sow.^[72] Nest-building can be influenced by internal and external stimuli. Internal hormonal changes and the completion of one nesting phase are indicators of this maternal behaviour.^[72] The onset is triggered by the rise in prolactin levels, which is caused by a decrease in progesterone and an increase in prostaglandin, while the gathering of the nest material seems to be regulated more by external stimuli such as temperature.^[71]

Nursing and suckling

 

Sow with prominent nipples. Pigs typically have 12–14 nipples.

 

Two piglets suckling

Pigs display complex nursing and suckling behaviour.^[73] Nursing occurs every 50–60 minutes, and the sow requires stimulation from piglets before milk let-down. Sensory inputs (vocalisation, odours from mammary and birth fluids, and hair patterns of the sow) are particularly important immediately post-birth to facilitate teat location by the piglets.^[74] Initially, the piglets compete for position at the udder; then the piglets massage around their respective teats with their snouts, during which time the sow grunts at slow, regular intervals. Each series of grunts varies in frequency, tone and magnitude, indicating the stages of nursing to the piglets.^[75]

The phase of competition for teats and of nosing the udder lasts for about one minute and ends when milk flow begins. In the third phase, the piglets hold the teats in their mouths and suck with slow mouth movements (one per second), and the rate of the sow's grunting increases for approximately 20 seconds. The grunt peak in the third phase of suckling does not coincide with milk ejection, but rather the release of oxytocin from the pituitary into the bloodstream.^[76] Phase four coincides with the period of main milk flow (10–20 seconds) when the piglets suddenly withdraw slightly from the udder and start sucking with rapid mouth movements of about three per second. The sow grunts rapidly, lower in tone and often in quick runs of three or four, during this phase. Finally, the flow stops and so does the grunting of the sow. The piglets may then dart from teat to teat and recommence suckling with slow movements, or nosing the udder. Piglets massage and suckle the sow's teats after milk flow ceases as a way of letting the sow know their nutritional status. This helps her to regulate the amount of milk released from that teat in future sucklings. The more intense the post-feed massaging of a teat, the more milk that teat later releases.^[77]

Teat order

 

A sow with suckling piglets

In pigs, dominance hierarchies are formed at an early age. Piglets are precocious, and attempt to suckle soon after being born. The piglets are born with sharp teeth and fight for the anterior teats, as these produce more milk. Once established, this teat order remains stable; each piglet tends to feed on a particular teat or group of teats.^[59] Stimulation of the anterior teats appears to be important in causing milk letdown,^[78] so it might be advantageous to the entire litter to have these teats occupied by healthy piglets. Piglets locate teats by sight and then by olfaction.^[79]

Senses

 

A trained pig assisting the search for wild truffles in France

Pigs have panoramic vision of approximately 310° and binocular vision of 35° to 50°. It is thought they have no eye accommodation.^[80] Other animals that have no accommodation, e.g. sheep, lift their heads to see distant objects.^[81] The extent to which pigs have colour vision is still a source of some debate; however, the presence of cone cells in the retina with two distinct wavelength sensitivities (blue and green) suggests that at least some colour vision is present.^[82]

Pigs have a well-developed sense of smell; use is made of this in Europe where trained pigs find underground truffles.^[83] Olfactory rather than visual stimuli are used in the identification of other pigs.^[84] Hearing is well developed; sounds are localised by moving the head. Pigs use auditory stimuli extensively for communication in all social activities.^[85] Alarm or aversive stimuli are transmitted to other pigs not only by auditory cues but also by pheromones.^[86] Similarly, recognition between the sow and her piglets is by olfactory and vocal cues.^[87]

Breeds

Main article: List of pig breeds

Many breeds of pig exist, with different colors, shapes, and sizes. According to The Livestock Conservancy, as of 2016, three breeds of pig are critically rare (having a global population of fewer than 2000). They are the Choctaw hog, the Mulefoot, and the Ossabaw Island hog.^[88] The smallest known pig breed in the world is the Göttingen minipig, typically weighing about 26 kilograms (57 lb) as a healthy, full-grown adult.^[89]

In agriculture

Main article: Pig farming

Global pig stock
in 2019
Number in millions
1. China (Mainland)	310.4 (36.5%)
2. European Union	143.1 (16.83%)
3. United States	78.7 (9.26%)
4. Brazil	40.6 (4.77%)
5. Russia	23.7 (2.79%)
6. Myanmar	21.6 (2.54%)
7. Vietnam	19.6 (2.31%)
8. Mexico	18.4 (2.16%)
9. Canada	14.1 (1.66%)
10. Philippines	12.7 (1.49%)
World total	850.3
Source: UN Food and Agriculture Organization

The pig is mostly farmed for its meat, pork. Other food products made from pigs include sausage (which may have casings made from the intestines), bacon, gammon, ham and pork rinds. The head of a pig can be used to make a preserved jelly called head cheese, which is sometimes known as brawn. Liver, chitterlings, blood (for black pudding), and other offal from pigs are widely used for food.

Approximately 1.5 billion pigs are slaughtered each year for meat.^[90]

The skin of pigs is eaten and used to produce seat covers, apparel, and other items.

Pigs have been raised outdoors, and sometimes allowed to forage in woods. In industrialized nations such as the United States, pig production has switched from the pigsty to large-scale intensive pig farming. This has lowerrf production costs but has caused concern about possible cruelty. As consumers have become concerned with the humane treatment of livestock, demand for pasture-raised pork in these nations has increased.^[91]

•  
  Interior of pig farm at Bjärka-Säby Castle, Sweden, 1911
•  
  Exterior of pig farm in Vampula, Finland, 2021
•  
  A Large White, a breed commonly used in meat production

 

Pigs have the fourth highest emissions intensity of any agricultural commodity.

Like all animals, pigs can be susceptible to adverse impacts from climate change, such as heat stress from increased annual temperatures and more intense heatwaves. Heat stress has increased rapidly between 1981 and 2017 on pig farms in Europe. Installing a ground-coupled heat exchanger is an effective intervention.^[92]

As pets

Pigs are occasionally kept as pets. In the mid-1980s pigs of the small Vietnamese Lon I breed were brought into the United States from Canada; by the end of the decade the "Vietnamese Pot-bellied" was being marketed as a pet. Not all of these were purebred, and some grew to considerable size;^[93] the fad was short-lived.^[94]

In medicine

Pigs, both as live animals and as a source of post-mortem tissues, are valuable animal models because of their biological, physiological, and anatomical similarities to human beings.^[95][96] For instance, human skin is very similar to the pigskin, therefore pigskin has been used in many preclinical studies.^[95][96]

Xenotransplantation

Main article: Xenotransplantation

Pigs are good non-human candidates for organ donation to humans, and in 2021 became the first animal to successfully donate an organ to a human body.^[97][98] The procedure used a donor pig genetically engineered not to have a specific carbohydrate that the human body considers a threat–Galactose-alpha-1,3-galactose.^[99]

Besides similarity between pig and human organs, pigs are among the best animals for human donation due to the lower risk of cross-species disease transmission. This is caused by pigs' increased phylogenetic distance from humans.^[100] Furthermore, they are readily available, and new infectious agents are less likely since they have been in close contact with humans through domestication for thousands of years.^[101]

Viruses carried by pigs include porcine herpesvirus, rotavirus, parvovirus, and circovirus. Of particular concern are porcine endogenous retroviruses, vertically transmitted viruses that embed in pig genomes. The risks with xenosis are twofold, as not only could the individual become infected, but a novel infection could initiate an epidemic in the human population. Because of this risk, the FDA has suggested that any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis.^[102]

Pig cells have been engineered to inactivate all 62 porcine endogenous retroviruses in the genome using CRISPR Cas9 genome editing technology, and eliminated infection from the pig to human cells in culture.^[103]

In culture

Main article: Pigs in culture

Pigs, widespread in societies around the world since neolithic times, have been used for many purposes in art, literature, and other expressions of human culture. In classical times, the Romans considered pork the finest of meats, enjoying sausages, and depicting them in their art.^[104] Across Europe, pigs have been celebrated in carnivals since the Middle Ages,^[105] becoming specially important in Medieval Germany in cities such as Nuremberg,^[106] and in Early Modern Italy in cities such as Bologna.^[107][108]

In literature, both for children^[109] and adults, pig characters appear in allegories, comic stories, and serious novels.^{[105][110][111]} In art, pigs have been represented in a wide range of media and styles from the earliest times in many cultures.^[112] Pig names are used in idioms and animal epithets, often derogatory, since pigs have long been linked with dirtiness and greed,^[113][114] while places such as Swindon are named for their association with swine.^[115] The eating of pork is forbidden in Islam and Judaism,^[116][117] but pigs are sacred in some other religions.^[118][119]

•  
  Two men sacrificing a pig to Demeter. red-figure pot, Ancient Greece
•  
  Bronze pig sculpture, Zhou dynasty
•  
  Painting of Saint Anthony with a pig in background by Piero di Cosimo c. 1480
•  
  Canzone Sopra La Porcellina ("Song Upon the Piglet") by Giulio Cesare Croce, Bologna, 1622
•  
  Pigling Bland setting out on his adventures
•  
  Hams, pigs' trotters, sausages, and mortadella in Bologna, 2019

Glossary of terms

Main article: Pig farming § Terminology

See also

• Farming
  • Mycoplasma hyorhinis
  • Peccary (domestication)
• List of individual pigs
• Pet
  • Pigs in culture
  • Truffle hog
  • Xenotransfusion
• List of pig breeds

Notes

1. David DiSalvo's article in Forbes refers to via an article in Penn State Agricultural Magazine^[66] referenced from 'Pork' by Catherine Becker at The Ohio State University^[67] referencing work by Candace Croney, now head of Purdue center for animal welfare science.^[68]

References

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15. {[cite journal |last1=Wu |first1=Gui-Sheng |last2=Pang |first2=Jun-feng |last3=Zhang |first3=Ya-Ping |title=Molecular phylogeny and phylogeography of Suidae |journal=Zoological Research |volume=27 |issue=2 |year=2006 |pages=197-201 |url=https://www.researchgate.net/profile/Gui-Sheng-Wu/publication/43563862_Molecular_Phylogeny_and_Phylogeography_of_Suidae/links/09e4150cf2af663202000000/Molecular-Phylogeny-and-Phylogeography-of-Suidae.pdf}}
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Sources

• Bateman, Heather; Curtis, Steve; McAdam, Katy, eds. (2006). Dictionary of Agriculture (3rd ed.). A & C Black. ISBN 978-0-7136-7778-2.

Further reading

• Keuling, O.; Leus, K. (2019). "Sus scrofa". IUCN Red List of Threatened Species. 2019: e.T41775A44141833. doi:10.2305/IUCN.UK.2019-3.RLTS.T41775A44141833.en. Retrieved 11 November 2021.

External links

 

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