The domestic pig (Sus scrofa domesticus or only Sus domesticus), often called SwineHog, or simply Pig when there is no need to distinguish it from other Pigs, is a domesticated large, Even-toe ungulate. It is variously considered a sub-species of the Wild-Boar or a distinct Species. The domestic pig’s head-plus-body-length ranges from 0.9 to 1.8 m (35 to 71 in), and adult pigs typically weigh between 50 and 350 kg, with well-fed individuals often exceeding this weight range. The size and weight of a hog largely depends on its breed. Compared to other artiodactyls its head is relatively long, pointed, and free of warts with mustaches. Even-toed ungulates are generally herbivorous, but the domestic pig is an Omnivorous, like its wild relative.

When used as livestock, domestic pigs are farmed primarily for the consumption of their flesh, called Pork. The animal’s bones, hide, and bristles are also used in commercial products. Domestic pigs, especially miniature breeds, are kept as pets.

 Pigs are also less able than many other mammals to dissipate heat from wet mucous membrane in the mouth through panting. Their thermoneutral zone is 16 to 22 °C (61 to 72 °F). 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, ectoparasite control, and scent-marking.

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 a neurotoxin from binding. These represent four separate, independent mutations.

Domestic pigs have small lungs in relation to their body size and are thus more susceptible than other domesticated animals to fatal bronchitis and pneumonia.

Reproduction

Female pigs (gilt) reach sexual maturity at 3–12 months of age, and come into estrus every 18–24 days if they are not successfully bred. The gestation period averages 112–120 days. Estrus lasts two to three days, and the female’s displayed receptiveness to mate is known as standing the 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. The female cervix contains a series of five interdigitating pads, or folds, that will hold the boar’s corkscrew-shaped penis during copulation. Females have bicornuate uteruses and two conceptuses must be present in both uterine horns for pregnancy to be established. 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 (CL). To avoid luteolysis by PGF2α, rescuing of the CL must occur via embryonic signaling of estradiol 17β and PGE2. 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. During mid to late pregnancy, the CL relies primarily on luteinizing hormone (LH) for maintenance until parturition.

Behavior

In many ways, their behaviour appears to be intermediate between that of other artiodactyls and of carnivorous. Domestic pigs seek out the company of other pigs, and often huddle to maintain physical contact, although they do not naturally form large herds. They typically live in groups of about 8-10 adult sows, some young individuals, and some single males.

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. They do not submerge completely under the mud, but vary the depth and duration of wallowing depending on environmental conditions. Typically, adult pigs start wallowing once the ambient temperature is around 17-21 °C. They cover themselves from head to toe in mud. Pigs may use mud as a sunscreen, or as a method of keeping parasites away. Most bristled pigs will “blow their coat”, meaning that they shed most of the longer, more-coarse stiff hair once a year, usually in Spring or early Summer, to prepare for the warmer months ahead.

If conditions permit, domestic 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 behavior. As they are foraging animals, they primarily eat leaves, stems, roots, fruits, flowers, and meat. Domestic pigs are highly intelligent animals. They are “widely considered the smartest domesticated animal in the world. Pigs can move a cursor on a video screen with their snouts and even learn to distinguish between the scribbles they knew from those they saw for the first time. Pigs use their grunts to communicate with each other. Pigs like humans and they enjoy being with humans.

Farrowing

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, grasses and leaves, and carries these in her mouth to the depression, building them into a mound. She distributes the softer, finer material to the center of the mound using her feet. When the mound reaches the desired height, she places large branches, up to 2 meters in length, on the surface. She enters into the mound and roots around to create a depression within the gathered material. She then gives birth in a lying position, which, again, is different from other artiodactyls, which usually give birth in a standing position.

Nest-building behavior is an important part in the process of pre and post-partum maternal behavior. Nest-building will occur during the last 24 hours before the onset of farrowing, and becomes most intense during 12 to 6 hours before farrowing. Nest-building is divided into two phases: one of which is the initial phase of rooting in ground while the second phase is the collecting, carrying and arranging of the nest material. The sow will separate from the group and seek a suitable nest site with some shelter from rain and wind that has well-drained soil. This nest-building behavior is performed to provide the offspring with shelter, comfort, and thermoregulation. The nest will provide 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 that other piglets are not stealing milk from the sow. 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 behavior. 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. The longer time spent on nest-building will increase pre-partum oxytocin.

Nursing and suckling behavior

Compared to most other mammals, pigs display complex nursing and suckling behavior. Nursing occurs every 50–60 minutes, and the sow requires stimulation from piglets before milk let-down. Sensory inputs (vocalization, odors from mammary and birth fluids and hair patterns of the sow) are particularly important immediately post-birth to facilitate teat location by the piglets. Initially, the piglets compete for position at the udder, then each piglet massages around its respective teat with its snout, 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.

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. 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 greater the future milk release from that teat will be.

Teat order

In pigs, dominance hierarchies can be formed at a very early age. Domestic piglets are highly precocious and within minutes of being born, or sometimes seconds, will attempt to suckle. The piglets are born with sharp teeth and fight to develop a teat order as the anterior teats produce a greater quantity of milk. Once established, this teat order remains stable with each piglet tending to feed from a particular teat or group of teats.  Stimulation of the anterior teats appears to be important in causing milk letdown, so it might be advantageous to the entire litter to have these teats occupied by healthy piglets. Using an artificial sow to rear groups of piglets, recognition of a teat in a particular area of the udder depended initially on visual orientation by means of reference points on the udder to find the area, and then the olfactory sense for the more accurate search within that area.

Senses

Pigs have panoramic vision of approximately 310° and binocular vision of 35° to 50°. It is thought they have no eye accommodation. Other animals that have no accommodation, e.g. sheep, lift their heads to see distant objects. The extent to which pigs have color 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 color vision is present.

Pigs have a well-developed sense of smell, and use is made of this in Europe where they are trained to locate underground truffles. Olfactory rather than visual stimuli are used in the identification of other pigs. Hearing is also well developed, and localization of sounds is made by moving the head. Pigs use auditory stimuli extensively as a means of communication in all social activities. Alarm or aversive stimuli are transmitted to other pigs not only by auditory cues but also by pheromones. Similarly, recognition between the sow and her piglets is by olfactory and vocal cues.

In human medical applications

The domestic pig, both as a live animal and source of post-mortem tissues, is one of the most valuable animal models used in biomedical research today, because of its biological, physiological and anatomical similarities to human beings. For instance, human skin is very similar to pig skin, therefore pig skin has been used in many preclinical studies. Porcine are used in finding treatments, cures for diseases, xenotransplantation and for general education. They are also used in the development of medical instruments and devices, surgical techniques and instrumentation, and FDA-approved research. As part of animal conservation, these animals contribute to the reduction methods for animal research, as they supply more information from fewer animals used, for a lower cost.X

In human medical applications

The domestic pig, both as a live animal and source of post-mortem tissues, is one of the most valuable animal models used in biomedical research today, because of its biological, physiological and anatomical similarities to human beings. For instance, human skin is very similar to pig skin, therefore pig skin has been used in many preclinical studies. Porcine are used in finding treatments, cures for diseases, xenotransplantation and for general education. They are also used in the development of medical instruments and devices, surgical techniques and instrumentation, and FDA-approved research. As part of animal conservation, these animals contribute to the reduction methods for animal research, as they supply more information from fewer animals used, for a lower cost.

Xenotransplantation

Pigs are currently thought to be the best non-human candidates for organ donation to humans. The risk of cross-species disease transmission is decreased because of their increased phylogenetic distance from humans. They are readily available, their organs are anatomically comparable in size, and new infectious agents are less likely since they have been in close contact with humans through domestication for many generations.

To date, no xenotransplantation trials have been entirely successful due to obstacles arising from the response of the recipient’s immune-system generally more extreme than in allotransplantations, ultimately results in rejection of the xenograft, and in some cases result in the death of the recipient-including hyperacute rejection, acute vascular rejection, cellular rejection and chronic rejection. An early major breakthrough was the 1,3 galactosyl transferase gene knockout. Examples of viruses carried by pigs include porcine herpesvirus, rotavirus, parvovirus, and circovirus. Of particular concern are PERVs (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine 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 any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis

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