Thepig (Sus domesticus), also calledswine (pl.: swine) orhog, is anomnivorous,domesticated,even-toed, hoofed mammal. It is named thedomestic pig when distinguishing it from other members of the genusSus. It is considered asubspecies ofSus scrofa (thewild boar or Eurasian boar) by some authorities, but as a distinctspecies by others. Pigs were domesticated in theNeolithic, both in East Asia and in the Near East. When domesticated pigs arrived in Europe, they extensively interbred with wild boar but retained their domesticated features.
Pigs are farmed primarily for meat, calledpork. The animal's skin orhide is used forleather. China is the world's largest pork producer, followed by the European Union and then the United States. Around 1.5 billion pigs are raised each year, producing some 120 million tonnes of meat, often cured asbacon. Some are kept aspets.
Pigs have featuredin human culture since Neolithic times, appearing in art and literature for children and adults, and celebrated in cities such asBologna for their meat products.
Description
The pig has a large head, with a long snout strengthened by a special prenasal bone and a disk ofcartilage at the tip.[2] The snout is used to dig into the soil to find food and is an acute sense organ. Thedental formula of adult pigs is3.1.4.33.1.4.3, giving a total of 44teeth. The rear teeth are adapted for crushing. In males, the canine teeth can formtusks, which grow continuously and are sharpened by grinding against each other.[2] There are fourhoofed 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 sparselybristled hair on their skin, though there are somewoolly-coated breeds such as theMangalitsa.[4] Adult pigs generally weigh between 140 and 300 kg (310 and 660 lb), though some breeds can exceed this range. Exceptionally, a pig called Big Bill weighed 1,157 kg (2,551 lb) and had a shoulder height of 1.5 m (4 ft 11 in).[5]
Pigs possess bothapocrine andeccrine sweat glands, although the latter are limited to the snout.[6] Pigs, like other "hairless" mammals such as elephants, do not use thermal sweat glands in cooling.[7] Pigs are less able than many other mammals to dissipate heat from wetmucous membranes in the mouth by panting. Theirthermoneutral zone is 16–22 °C (61–72 °F).[8] At higher temperatures, pigs lose heat bywallowing 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 among four mammalian species with mutations in thenicotinic acetylcholine receptor that protect againstsnake venom.Mongooses,honey badgers,hedgehogs, and pigs all have different modifications to the receptor pocket which preventsα-neurotoxin from binding.[10] Pigs have small lungs for their body size, and are thus more susceptible than other domesticated animals to fatalbronchitis andpneumonia.[11] Thegenome of the pig has been sequenced; it contains about 22,342protein-codinggenes.[12][13][14]
Skeleton
Skull
Bones of the foot
Evolution
Phylogeny
Domestic pigs are related to other pig species as shown in thecladogram, based on phylogenetic analysis usingmitochondrial DNA.[15]
The initial emergence of wild pigs, followed by the genetic divergence between boars and pigs and the domestication of pigs[20]
Archaeological evidence shows that pigs weredomesticated from wild boar in theNear East in or around theTigris Basin,[21] being managed in a semi-wild state much as they are managed by some modern New Guineans.[22] There were pigs inCyprus 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 theBronze Age, as rural populations instead focused on commodity-producing livestock, but it was sustained in cities.[27]
Domestication did not involve reproductive isolation with population bottlenecks. Western Asian pigs were introduced into Europe, where they crossed with wild boar. There appears to have been interbreeding with a now extinctghost population of wild pigs during thePleistocene. 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 createddomestication islands in the genome.[28][29] Pigs arrived in Europe from the Near East at least 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.[30]
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.[31][32] Historical records indicate that Asian pigs were again introduced into Europe during the 18th and early 19th centuries.[25]
History
Columbian Exchange
Among the animals that the Spanish introduced to theChiloé Archipelago in the 16th centuryColumbian Exchange, pigs were the most successful in adapting to local conditions. The pigs benefited from abundantshellfish andalgae exposed by the largetides of the archipelago.[33] Pigs were brought to southeastern North America from Europe byde Soto and other early Spanishexplorers. Escaped pigs becameferal.[34]
Pigs have escaped from farms and goneferal in many parts of the world. Feral pigs in the southeastern United States have migrated north to theMidwest, where many state agencies have programs to remove them.[35][36][37] Feral pigs in New Zealand and northernQueensland have caused substantial environmental damage.[38][39] Feral hybrids of the European wild boar with the domestic pig are disruptive to both environment and agriculture, as they destroy crops, spread animal diseases includingfoot-and-mouth disease, and consume wildlife such as juvenile seabirds and youngtortoises.[40] Feral pig damage is especially an issue in southeastern South America.[41][42]
Female pigs reach sexual maturity at 3–12 months of age and come intoestrus 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. The gestation period averages 112–120 days.[43]
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.[44] The female cervix contains a series of five interdigitating pads, or folds, that hold the boar's corkscrew-shaped penis during copulation.[45] Females havebicornuate uteruses and twoconceptuses must be present in both uterine horns to enable pregnancy to proceed.[46] The mother's body recognises that it is pregnant on days 11 to 12 of pregnancy, and is marked by thecorpus luteum's producing the sex hormoneprogesterone.[47] To sustain the pregnancy, the embryo signals to the corpus luteum with the hormonesestradiol andprostaglandin E2.[48] This signaling acts on both the endometrium and luteal tissue to prevent the regression of the corpus luteum by activation of genes that are responsible for corpus luteum maintenance.[49] During mid to late pregnancy, the corpus luteum relies primarily onluteinizing hormone for maintenance until birth.[48]
Archeological evidence indicates that medieval European pigs farrowed, or bore a litter of piglets, once per year.[50] By the nineteenth century, European piglets routinely double-farrowed, or bore two litters of piglets per year. It is unclear when this shift occurred.[51] Pigs have amaximum life span of about 27 years.[52]
Nest-building
A characteristic of pigs which they share with carnivores isnest-building. Sows root in the ground to create depressions the size of their body, and then build nest mounds, using twigs and leaves, softer in the middle, in which to give birth. 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.[53]
Nest-building occurs during the last 24 hours before the onset of farrowing, and becomes most intense 12 to 6 hours before farrowing.[54] The sow separates from the group and seeks a suitable nest site with well-drained soil and shelter from rain and wind. This provides 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.[55] The onset of nest-building is triggered by a rise in prolactin level, caused by a decrease in progesterone and an increase in prostaglandin; the gathering of nest material seems to be regulated more by external stimuli such as temperature.[54]
Nursing and suckling
Pigs have complex nursing and suckling behaviour.[56] 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.[57] 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.[58]
The phase of competition for teats and of nosing the udder lasts for about a minute, ending when milk begins to flow. The piglets then 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.[59] 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 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.[60]
Sows typically have 12–14 nipples.
A sow with suckling piglets
Teat order
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 theanterior 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.[53] Stimulation of the anterior teats appears to be important in causing milk letdown,[61] 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.[62]
Pig behaviour is intermediate between that of otherartiodactyls and ofcarnivores.[53] Pigs seek out the company of other pigs and often huddle to maintain physical contact, but they do not naturally form large herds. They live in groups of about 8–10 adult sows, some young individuals, and some single males.[54] Pigs confined in a simplified, crowded, or uncomfortable environment may resort totail-biting; farmers sometimesdock the tails of pigs to prevent the problem, or may enrich the environment with toys or other objects to reduce the risk.[63][64]
Temperature control
Because of their relative lack of sweat glands, pigs often control their body temperature using behavioural thermoregulation.Wallowing, coating the body with mud, is a common behaviour.[9] They do not submerge completely under the mud, but vary the depth and duration of wallowing depending on environmental conditions.[9] 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.[9] They may use mud as a sunscreen, or to keep parasites away.[9] 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.[65]
Eating, feeding, sleeping
Pigs around a rotary feeder
Where pigs are allowed to roam freely, they walk roughly 4 km daily,scavenging within a home range of around a hectare. Farmers in Africa often choose such a low-input, free-range production system.[66]
If conditions permit, pigs feed continuously for many hours and then sleep for many hours, in contrast toruminants, which tend to feed for a short time and then sleep for a short time. Pigs areomnivorous and versatile in their feeding behaviour. They primarily eat leaves, stems, roots, fruits, and flowers.[67]
Rooting is an instinctual comforting behaviour in pigs characterized by nudging the snout into something. It first happens when piglets are born to obtain their mother's milk, and can become a habitual, obsessive behaviour, most prominent in animalsweaned too early. Pigs root and dig into the ground to forage for food. Rooting is also a means of communication.[68]
Pigs are relatively intelligent animals, roughly on par withdogs. They distinguish each other as individuals, spend time in play, and form structured communities. They have good long-term memory and they experience emotions, changing their behaviour in response to the emotional states of other pigs. In terms of experimental tasks, pigs can perform tasks that require them to identify the locations of objects; they can solve mazes; and they can work with a simple language of symbols. They displayself-recognition in a mirror. Pigs have been trained to associate different sorts of music (Bach and a military march) with food and social isolation respectively, and could communicate the resulting positive or negative emotion to untrained pigs.[70][71] Pigs can be trained to use ajoystick with their snout to select a target on screen.[69]
Senses
Atrained pig using its sensitive nose to assist the search for wildtruffles in France
Pigs havepanoramic vision of approximately 310° andbinocular vision of 35° to 50°. It is thought they have noeye accommodation.[72] Other animals that have no accommodation, e.g. sheep, lift their heads to see distant objects.[73] The extent to which pigs have colour vision is still a source of some debate; however, the presence ofcone cells in the retina with two distinct wavelength sensitivities (blue and green) suggests that at least some colour vision is present.[74]
Pigs have a well-developed sense of smell; this is exploited in Europe wheretrained pigs find undergroundtruffles.[75] Pigs have 1,113 genes for smell receptors, compared to 1,094 in dogs; this may indicate an acute sense of smell, but against this, insects have only around 50 to 100 such genes but make extensive use of olfaction.[76] Olfactory rather than visual stimuli are used in the identification of other pigs.[77] Hearing is well developed; sounds are localised by moving the head. Pigs use auditory stimuli extensively for communication in all social activities.[78] Alarm or aversive stimuli are transmitted to other pigs not only by auditory cues but also bypheromones.[79] Similarly, recognition between the sow and her piglets is by olfactory and vocal cues.[80]
Some parasites of pigs are apublic health risk as they can be transmitted to humans in undercooked pork. These are the pork tapewormTaenia solium; a protozoan,Toxoplasma gondii; and a nematode,Trichinella spiralis. Transmission can be prevented by thorough sanitation on the farm; by meat inspection and careful commercial processing; and by thorough cooking, or alternatively by sufficient freezing and curing.[82]
Pigs have been raised outdoors, and sometimes allowed toforage in woods or pastures. In industrialized nations, pig production has largely switched to large-scaleintensive pig farming. This has lowered production costs but has caused concern about possiblecruelty. As consumers have become concerned with the humane treatment of livestock, demand for pasture-raised pork in these nations has increased.[83] Most pigs in the US receiveractopamine, abeta-agonist drug, which promotes muscle instead of fat and quicker weight gain, requiring less feed to reach finishing weight, and producing lessmanure. China has requested that pork exports be ractopamine-free.[84] With a population of around 1 billion individuals, the domesticated pig is one of the most numerous large mammals on the planet.[85][86]
Like all animals, pigs are susceptible to adverse impacts fromclimate change, such asheat stress from increased annual temperatures and more intenseheatwaves. Heat stress has increased rapidly between 1981 and 2017 on pig farms in Europe. Installing aground-coupled heat exchanger is an effective intervention.[87]
Indoor pig farm, Sweden, 1911
Sow in stall with separate piglet balcony to prevent crushing, Germany, 1959
Free range pigs with field shelters, England, 2006
Around 600breeds of pig have been created by farmers around the world, mainly in Europe and Asia, differing in coloration, shape, and size.[88] According toThe Livestock Conservancy, as of 2016, three breeds of pig are critically rare (having a global population of fewer than 2000). They are theChoctaw hog, theMulefoot, and theOssabaw Island hog.[89] The smallest known pig breed in the world is theGöttingen minipig, typically weighing about 26 kilograms (57 lb) as a healthy, full-grown adult.[90]
Pigs are intelligent, social creatures. They are consideredhypoallergenic and are known to do quite well with people who have the usual animal allergies. Since these animals are known to have a life expectancy of 15 to 20 years, they require a long-term commitment.
Given pigs are bred primarily as livestock and have not been bred as companion animals for very long, selective breeding for a placid or biddable temperament is not well established. Pigs have radically differentpsychology and behaviours compared to dogs, and exhibitfight-or-flight instincts, an independent nature, and naturalassertiveness.[91] Male and female swine that have not been de-sexed may express unwanted aggressive behavior, and are prone to developing serious health issues.[92] Asrooting is found to be comforting, pigs kept in the house may root household objects, furniture or surfaces. Pet pigs should be let outside to allow them to fulfill their natural desire of rooting around.
In 2023, China produced more pork than any other country, 55 million tonnes, followed by the European Union with 22.8 million tonnes and the United States with 12.5 million tonnes. Global production in 2023 was 120 million tonnes.[96] India, despite its large population, consumed under 0.3 million tonnes of pork in 2023.[97] International trade in pork (meat not consumed in the producing country) reached 13 million tonnes in 2020.[98]
Uses
Products
Pigs are farmed primarily for meat, calledpork. Pork is eaten in the form of pork chops, loin or rib roasts, shoulder joints, steaks, and loin (also called fillet). The many meat products made from pork includeham,bacon (mainly from the back and belly), andsausages.[99] Pork is further made intocharcuterie products such asterrines,galantines,pâtés andconfits.[100] Some sausages such assalami are fermented and air-dried, to be eaten raw. There are many types, the original Italian varieties including Genovese, Milanese, and Cacciatorino, with spicier kinds from the South of Italy including Calabrese, Napoletano, and Peperone.[101]
The hide is made into pigskinleather, which is soft and durable; it can be brushed to formsuede leather. These are used for products such asgloves,wallets, suede shoes, andleather jackets.[102] In the16th century, pig skin was the most popular book-binding material inGermany, thoughcalf skin was more common elsewhere.[103]
Salami, a fermented and air-dried sausage, originally made in Italy
A 16th century book bound in pig skin
A woman's suede gloves, England, c. 1820
In medicine
The growth in publication of medical research papers using pigs and miniature pigs, and the research done on miniature pigs by organ system[104]
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. For instance, human skin is very similar to the pigskin, therefore pigskin has been used in many preclinical studies.[105][106]
Pigs are good non-human candidates fororgan donation to humans, and in 2021 became the first animal to successfullydonate an organ to a human body.[107][108] 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.[109] Pigs are good for human donation as the risk of cross-species disease transmission is reduced by the considerablephylogenetic distance from humans.[110] They are readily available, and the danger of creating new human diseases is low as domesticated pigs have been in close contact with humans for thousands of years.[111]
As with the other forms of meat, producing pork is more energy-intensive than plant-based foods, and it is associated with moregreenhouse gas emissions percalorie. However, emissions from pork are many times smaller than those ofbeef,veal andmutton, though larger than ofchicken meat.[115]
Intensive pig production is also associated withwater pollution concerns, as the swine waste is often stored above ground in so-called lagoons. These lagoons typically have high levels ofnitrogen andphosphorus, and can containtoxic heavy metals likezinc andcopper, microbialpathogens, or hold elevated concentrations of pharmaceuticals fromsubtherapeutic antibiotic use in swine.[116] This wastewater from lagoons is liable to reachgroundwater on farms, though there is little evidence for it reaching deeper into localdrinking water supplies.[117] However, lagoon spills, such as from heavy rains in the wake of ahurricane, can lead to fish kills and algal blooms in local rivers.[116] In theUnited States, 35,000 mi (56,000 km) of river across over 20 states were estimated to have been contaminated by manure leakage as of 2015.[118] There is also evidence that evaporation from lagoons can cause nitrogen and phosphorus to spread through the air as dry particles then reach other water basins when they fall out throughdry deposition. This process then also contributes to watereutrophication.[116]
Intensive pig production involves practices such ascastration,earmarking, tattooing for litter identification, taildocking, which are often done without the use ofanesthetic.[119][120] Painfulteeth clipping of piglets is also done to curtailcannibalism, behavioural instability and aggression, andtail biting, which are induced by the cramped environment.[121][122] In English indoor farming, young pigs (less than 110kg in weight) are allowed to be kept with less than one square meter of space per pig.[123]
Pigs often begin life in afarrowing or gestation crate, which is a small pen with a central cage, designed to allow the piglets to feed from their mother while preventing her from attacking or crushing them.[124] The crates are so small that the mother sows cannot turn around.[125][126] While wild piglets remain with their mothers for around 12 to 14 weeks, farmed piglets areweaned and removed from their mothers at between two and five weeks old.[127][128] Of the piglets born alive, 10% to 18% will not reach weaning age, instead succumbing to disease, starvation, dehydration, or accidental crushing by their mothers.[121][129] Unusually smallrunt piglets are typically killed immediately by staff through blunt trauma to the head.[130][131] Further, intensive farming involves sows giving birth to large litter sizes at an unnatural frequency, which increases the rate ofstillborn piglets, and causes as many as 25%-50% of sows to die ofprolapse.[132][133]
Pigs, widespread in societies around the world sinceNeolithic times, have been used for many purposes in art, literature, and other expressions of humanculture. In classical times, theRomans consideredpork the finest of meats, enjoyingsausages, and depicting them in their art.[134] Across Europe, pigs have been celebrated incarnivals since theMiddle Ages,[135] becoming specially important inMedieval Germany in cities such asNuremberg,[136] and inEarly Modern Italy in cities such asBologna.[137][138] Pigs, especially miniature breeds, are occasionally kept as pets.[139][140]
In literature, both for children[141] and adults, pig characters appear in allegories, comic stories, and serious novels.[135][142][143] In art, pigs have been represented in a wide range of media and styles from the earliest times in many cultures.[144] Pig names are used in idioms andanimal epithets, often derogatory, since pigs have long been linked with dirtiness and greed,[145][146] while places such asSwindon are named for their association with swine.[147] The eating of pork isforbidden in Islam andJudaism,[148] but pigs are sacred in some other religions.[149][150]
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