Pigs

(Suidae)

Class: Mammalia

Order: Artiodactyla

Suborder: Suiformes

Family: Suidae

Thumbnail description
Medium-sized omnivores, characterized by elongated head and discoid snout, bristly pelage, tusks, and short tail

Size
34–83 in (86–211 cm); 77–770 lb (35–350 kg)

Number of genera, species
5 genera; 16 species

Habitat
Grassland, forest, steppe, desert, swamp, and agricultural fields

Conservation status
Critically Endangered: 2 species; Endangered: 1 species; Vulnerable: 2 species; Data Deficient: 1 species

Distribution
Global, in association with humans; all continents, except Antarctica

Evolution and systematics

The family Suidae is one of three extant families belonging to the suborder Suiformes that has its origins about 48 million years ago (mya) in the middle to late Eocene. The Suiformes (pigs, peccaries, and hippopotamuses) are anatomically the most primitive among the Artiodactyls, being characterized by having simple stomachs, many low-crowned, bunodont teeth, and a less-advanced unguligrade limb structure.

During the Eocene epoch 60 mya, there were giant pigs (entelodonts) that may have been distant relatives of today's pig; however, there is no evidence of a direct lineage. Paleontological evidence suggests that the origin of the pig is in the Miocene about 40 mya. At this time, their distribution was restricted to the Old World, and there is not any evidence that North America was the site of origin for any modern genera. The African suids underwent a major radiation during the Pliocene and early Pleistocene epochs, with approximately 17 species known from the fossil record; most of this diversity had disappeared by the mid-Pleistocene. The African suid radiation has served as a useful tool in reconstructing hominid paleoecology because hominids and suids had some ecological characteristics in common. In addition, there are many suid taxa that appeared, evolved, and disappeared in a relatively short period of time in Africa, making them useful markers in biostratigraphy for aging archaeological sites. It has also been suggested that Pleistocene suids competed with Homo erectus for subterranean food sources.

The genus Sus is thought to have originated at the latest in the Miocene or near the Miocene/Pliocene boundary, 5 mya. Genetic studies suggest that the origin of the babirusa (Babyrousa babyrussa) is between two and 10 mya, and that of the warthog (Phacochoerus africanus), between five and 15 mya; these estimates are approximately 50% more recent than those suggested by paleontological and anatomical data. The only babirusa fossils to date are from the Pleistocene, and known fossils of the warthog are no older than two million years.

The 1993 action plan of the IUCN/SSC Pigs, Peccaries, and Hippo specialist group recognizes three subfamilies of Suidae: the Suinae, or true pigs, the Phacochoerinae (warthogs), and the Babirousinae (babirusa). The Suinae comprises three genera (Sus, Potamochoerus, and Hylochoerus) and 11 species. The Phacochoerinae includes two extant species (P. aethiopicus and P. africanus), and the Babirousinae includes one species (Babyrousa babyrussa). However, recent reports by Groves in 2001 and 2002 suggest that some previously identified subspecies may in fact warrant full species status. Anatomical studies suggest that there may be as many as 40 subspecies within the Suidae. Approximately 16 more or less distinct subspecies have been found in the Sus scrofa lineage.

Physical characteristics

The Suidae are medium-sized mammals with stocky, sometimes barrel-shaped, bodies. Body weight ranges from 77 to 770 lb (35–350 kg) and may be as high as 990 lb (450 kg) in some domesticated breeds. The length of the body is 34–83 in (86–211 cm), while body height is 21–43 in (53–109 cm). The exception is the pygmy hog (Sus salvanius), which is only 20–28 in (51–71 cm) in length, 10–12 in (25–30 cm) in height, and 14.5–21 lb (6.5–9.5 kg) in weight. The neck is short, whereas the head is long and pointed with a mobile snout. The tip of the snout is cartilaginous and discoid. The eyes are fairly small, and the ears are generally long, with a tassel of hair at the end in some species. The first digit is absent. Each foot has four digits, the middle two of which are flattened and bear hooves. The outer digits are higher up the leg and bear smaller hooves.

The complete dental formula is 44 teeth (3143/3143 per quadrant) in Sus, Potamochoerus, and Hylochoerus, while Babyrousa and Phacochoerus have 34 teeth (2123/3123 and 1133/3123, respectively). The upper canines are large and curve upward, protruding from the mouth to varying degrees. The exception is the babirusa, in which the upper canines protrude upwards through the skin of the head, never entering the mouth. The lower canines are generally sharper and may or may not be visible outside of the mouth. Suids have a simple, non-ruminating stomach composed of two chambers.

Skin color varies greatly in the suids, from brown to almost black, whereas the pelage may be dark gray or black to light red and of varying length. Some babirusa appear almost naked. Some species have a mane or dorsal crest of hair. The male Visayan warty pig (S. cebifrons) bears a long mane that is shed after the breeding season. In many species, the young bear stripes. Some members of the Suidae bear warts or fleshy ridges on the face. These are fleshy structures with no bony core or support. Females have from two to four pairs of mammae.

Distribution

Today, wild suids are found on every continent, except Antarctica, as well as on many oceanic islands. Their presence in the New World and on some islands (e.g., New Guinea, New Zealand, and possibly Madagascar) is due to introductions by humans. The domesticated pig has a global distribution due to its association with people, and has returned to a feral state in some areas (e.g., North America). The historical range of the Suidae was restricted to the Old World. Wild pigs occurred throughout much of sub-Saharan Africa, Europe, Asia, Asia Minor, India, and the East Indies as far southeast as the Philippines and Sulawesi. The ranges of many species have decreased in modern times because of expanding human populations and the associated loss of habitat and hunting pressure. In some cases, and for some species (e.g., wild boar), the conversion of land for agriculture has benefited local populations of wild pigs and allowed them to expand their range.

Habitat

The wild suids are found in a diverse array of habitats ranging from semi-arid environments and temperate woodlands to tropical rainforests and swamps. They also often take advantage of agricultural lands. They occupy altitudes ranging from sea level to over 13,000 ft (4,000 m). Habitat selection is dictated by the availability of energy-rich foods, climactic extremes, and predation pressure. Some species may occupy a range of habitat types as long as they have adequate food resources (e.g., wild boar), whereas others specialize on a particular habitat (e.g., pygmy hog). Forest type, elevation, and forest age have been found to influence habitat selection in some populations of wild boar. It has been suggested that wild pigs play an important role in forest diversity, regeneration, and structure through their depredation of seeds and young saplings. Studies of the effects of wild pigs on the environment have had varying results. Wild pigs may increase or decrease forest species' richness, negatively impact regeneration of trees, remove competitive vegetation such as weeds, and decrease soil macroinvertebrate populations. There is also evidence the Eurasian wild pigs will take advantage of acorn caches created by small mammals.

The suids can be divided into two groups in terms of territoriality or home range use. The African suids are generally more sedentary than other wild pigs, occupying small territories or home ranges that may overlap. The other suids are also more or less sedentary, but changing ecological conditions may cause these species to roam in search of better places to forage, sometimes over long distances. Whether territorial or not, areas occupied by wild suids tend to have several features in common. Resting places are one such feature, and these tend to take the form of nest sites or burrows. Most wild suids construct nests for farrowing, whereas others build nests for protection in bad weather (e.g., P. larvatus), but some construct nests all year round (S. salvanius). Most wild suids make nests of varying complexity and structure, using vegetation. Nests may be used year round or seasonally. Warthogs often occupy burrows dug by aardvarks (Orycteropus afer) and do not build their own nests. Wild suid home ranges also include tree trunks, rocks, or anthills that are used as spots for rubbing and scratching the body. These home ranges also include sources of shade and water, as well as mud wallows. These features are of critical importance as some suids do not have functional sweat glands for cooling the body. In forest hogs and warthogs, traditional defecation sites have been reported. Finally, all home ranges include sites for foraging. These features are often connected by a well-worn set of trails. Home range size fluctuates in response to food availability, reproductive condition, population density, age, and sex. Home ranges of 125–965 acres (52–390 ha) have been reported for wild boar.

Behavior

Suids are generally characterized by a loose social organization in which the basic unit is the mother-offspring pair. Group sizes vary from one to 15 individuals. Females may live alone or in groups with other females called "sounders." Offspring may remain in the natal group up to two years. Natal males almost always disperse, whereas natal females may sometimes remain in the group permanently. Males associate closely with females only during the breeding season; however, the African suids differ in this regard. Male forest hogs live with family groups year round, and male bush pigs associate with females longer to help rear the young. Male warthogs join females for breeding, leave, and then return after the young are born. Group size sometimes increases in proximity to fixed resources such as saltlicks or in response to availability of resources. Large groups may sometimes split into smaller ones if hunting pressure is high. Territorial or home range marking has not been well documented, but it has been suggested that the plowing behavior of babirusa may have a scent-marking function. The tusk gland has also been suggested to play a role in territorial marking. Phermones produced in the salivary glands of boars have been shown to induce standing in estrous females.

Suids communicate using a variety of sounds and displays. Vocalizations are used to convey fear, pain, comfort or well being, warning, or for breeding and establishing contact. Olfactory investigation of the snout and genitals are common features of greetings in wild pigs. Displays are often used in aggressive encounters but also during courtship and breeding. When displays fail to diffuse aggression, body contact is made. Several combat postures have been noted, including "boxing" on the hind limbs in babirusa. Wild pigs are fast runners and good swimmers. They will generally flee when threatened, but they will fight vigorously when they are wounded or cornered. The upper and lower tusks are formidable weapons in some species. There has been some suggestion of a dominance order both within and between the sexes among babirusa. Cannibalism and infanticide have been observed in some species. Solitary, social, and object play behavior have been documented in wild piglets.

The warthog is diurnal, but other wild pigs are often more active at night or at dawn and dusk. They generally become more nocturnal when hunting pressure is high. They may be active 40–65% of the time. There is a tendency for there to be two peaks in activity; however, this is affected by climate, reproductive condition, and resource availability.

Feeding ecology and diet

Wild pigs are usually omnivorous, consuming diets of leaves, grasses, young saplings, seeds, roots, tubers, fruits, fungi, eggs, invertebrates, carrion, and small vertebrates. The suid diet fluctuates considerably during the year to take advantage of energy-rich foods such as fruits and acorns. Most species also visit mineral licks where they ingest soil or water. Warthogs will chew animal bones, possibly as a source of calcium and other minerals. Wild pigs are known for their rooting behavior, but the extent to which they do so depends on the species and the soil conditions. The babirusa lacks a well-developed rostral bone and so can only root in very soft, moist soils. Warthogs, on the other hand, have a very strong rhinarium and will go down on their wrists to dig in the hard soil to excavate roots and tubers. Pigs will forage alone or in small groups, though in some cases they will form very large groups to feed on ephemeral resources. Bearded pigs (S. barbatus) have been known to form groups of hundreds of individuals and make mass migrations following the fruiting of dipterocarps. These migrations occur over a period of months during which the animals may travel 150–400 mi (250–650 km). Aggregations of 30–60 red river hogs (Potamochoerus porcus) have been observed in Guinea and eastern Democratic Republic of the Congo (Zaire). Studies of Eurasian wild pig and bearded pig suggest a strong link between nutritional status and reproduction in these species. The populations of both of these species will increase dramatically following mass fruiting events. Alternatively, populations will decline significantly in drought years.

It has been argued that wild suids play a significant role in forest structure. A study in Queensland, Australia found that the germinability of mesquite seeds (Porsopis pallida) remained very high after passing through the gut of wild boar, suggesting that wild pigs may be important agents of seed dispersal. Exclosure studies in Malaysia have demonstrated that soil rooting and seed predation by Eurasian wild pigs decrease stem density and species richness; however, other studies have concluded that the foraging behavior of wild pigs may increase species richness in forests. The amount of rooting pigs do depends on food availability. In the temperate zone, most rooting occurs in mid-autumn to spring. After this period, rooting decreases as pigs switch to foraging on herbs and foliage.

Reproductive biology

Suid reproduction differs from that of other ungulates in several ways: the gestation is relatively short, newborns are very small compared to the mother, suids are the only truly multiparous ungulates, and all show nest-building behavior prior to parturition. In some species, the young are unable to regulate their body temperature and, also in contrast to many other ungulates, mothers nurse from a recumbent position as opposed to standing. In most suids, males associate with females only during estrus. The mating system is generally polygynous, but there has been some suggestion of monogamy in warthogs.

Estrous females urinate more frequently than usual, and the voided urine is sniffed and sometimes licked by males. Courtship behavior includes broadside displays, chasing, vocalizations, female solicitation of the male, and male nuzzling of the sides and vulva of the female. Female estrus is accompanied by swelling of the labia and mucus discharge. Males also tend to salivate excessively during this time. There may also be mutual grooming of the genitals. There are multiple mounts before intromission, and copulation sessions may last from 15 to 30 minutes. Adults will copulate several times per day. The estrous cycle is 21–42 days in length, with estrus lasting one to four days. Gestation lasts 100–175 days, depending on the species. In some species, sexual maturity is reached as early as eight months whereas in others, maturity is attained at two to five years of age.

Prior to parturition, sows will separate from their sounders, if they live in one, and construct a nest in which to give birth. Nests are usually located in thick cover. Females may excavate a shallow area and line it with vegetation or create a bed of vegetation on which to farrow. Babirusa sows often farrow at night in captivity. In domestic pigs, the mother does not help free the young from membranes and usually does not eat the placenta. Litter sizes in wild pigs vary from one to 12 piglets, with litters of babirusa and Visayan warty pig having the smallest litters (one to three piglets). In domesticated pigs, litter sizes generally increase with age and depend on breed and may reach 18 piglets. Piglets will compete among themselves to establish a teat order shortly after birth and will nurse from the same teat throughout lactation. In domestic pigs, sows may nurse up to 20 times per day, and they wean their litters at eight to 14 weeks of age, whereas bearded piglets are weaned at five or six weeks, and babirusa piglets are weaned at 26–32 weeks. Piglets may leave the nest with their mothers as early as a few days postpartum. Males generally provide little, if any, parental care, but may defend the young in some species. Females will vigorously defend their young and will sometimes work with other females to repel predators.

Reproduction in the temperate zone is generally seasonal with parturition occurring in spring and mating in autumn or winter; however, these periods are variable and seem to be dictated in large part by food resources. Drought conditions tend to decrease the percent of adult females that breed in a given year. Similarly in the tropics, reproduction can occur year round when the availability of energy-rich foods is high. Studies in the Cape Provinces have shown that life history characteristics reflected differences in nutrient availability. Bush pigs (P. larvatus) in the eastern areas of the Province had a higher quality diet and showed a higher reproductive investment (e.g., small, young females had large, frequent litters with low survival rates). Southern bush pigs bred at a later age and larger size, and their litters were smaller and had higher survival rates. In Borneo, female bearded pigs appear to require a certain thickness of subcutaneous fat in order to be responsive to mating stimuli. In this species, young born to fat mothers grow and mature quickly and can breed within one year. Among domestic pigs, it has been shown that nutritional status can affect hormone secretion and fertility.

Conservation status

Though some species of wild pigs are very widespread and abundant, others are Critically Endangered with very limited distributions. According to the 1993 Survey and Action Plan of the IUCN/SSC Pigs, Peccaries, and Hippos Specialist Group, the pygmy hog is considered Critically Endangered. Indeed, its distribution is now restricted to only one national park in Assam. The common warthog is still considered to be secure, but the remaining desert warthog subspecies (P. a. delamerei) is considered Vulnerable. The forest hog is considered Rare to Endangered depending on the subspecies. Bush pigs and the red river hog are still considered widespread and abundant, or secure. The majority of subspecies of Eurasian wild pigs are also considered widespread and secure; however, the subspecies S. s. riukiuanus from the Ryukyu Islands in southern Japan is considered Vulnerable to Endangered. The bearded pig is considered potentially at risk or rare, as is the Philippine warty pig (S. philippensis). Javan warty pigs (S. verrucosus) are considered Vulnerable, and the Visayan Warty pig is Endangered, perhaps critically. The babirusa is Vulnerable or Endangered, whereas the other species sharing its range, the Sulawesi warty pig (S. celebensis), is still considered Secure. The Vietnam warty pig (S. bucculentus) is known from only a few recent skulls and may be extinct. The babirusa and pygmy hog are listed on Appendix I of CITES and are both considered Endangered by the United States Fish and Wildlife Service. Babirusa are protected under Indonesian law, and the pygmy hog is on Schedule 1 of the Indian Wildlife Protection Act of 1972. Forest hogs are listed on Class B of the African Convention on the Conservation of Nature and Natural Resources. The babirusa is endemic to the island of Sulawesi. In terms of species diversity and endemicity of wild pigs, the Philippines is the most important country in the world. Of the three species known in this area, two are endemic (S. philippensis and S. cebifrons). Seven subspecies are recognized, and six of these are endemic. More taxa await description as well.

The main threats to wild pigs are hunting and loss of habitat. For some species such as the Visayan warty pig, hybridization with local domesticated, feral, or wild stock of S. scrofa represents a major concern. The only way to ensure the survival of the Visayan warty pig at this point is captive breeding of pure stock. The conversion of forests to agricultural land has in some places benefited local populations of wild pigs, but in general the impact is negative, especially for forest specialists like the babirusa. Hunting generally take ones of three forms: subsistence hunting, commercial hunting, and hunting in reprisal for crop damage. Subsistence hunting is often not a significant threat for any of the wild pigs, but is being overrun in many places by a thriving market in bush meat. Some wild pigs are protected under law; however, enforcement is nonexistent in many cases. A disturbing development in the eradication of pigs as crop pests has been the use of "pig bombs," small vessels of gunpowder placed on or in the ground that explode when pigs root or chew them. Though a couple of species are listed by CITES, trade is not a significant threat for wild pigs. Population estimates for wild pigs are difficult to obtain given the types of habitats they prefer, their shy, reclusive nature, and nocturnal habits.

As of 2003, much of the conservation work being done on wild suids involves population surveys and habitat assessment as well as basic studies of behavior and ecology, though integrated conservation programs have been developed by the IUCN Pigs, Peccaries, and Hippos Specialist Group for both pygmy hogs and Visayan warty pigs under the aegis of formal agreements with the relevant national governmental authorities. There has also been a number of studies of local markets to assess the extent of the bush meat trade in wild pigs. For some wild pigs, population models have been developed to determine what sustainable levels of harvest may be. Public education campaigns have been launched in some areas to inform people that the wild pigs living around them are threatened with extinction. Some educational initiatives have also been aimed at reducing human-pig conflict. Captive breeding programs for pygmy hog, Visayan warty pigs, and babirusa have been initiated in their countries of origin; the latter two and some others are also being bred in the United States, Europe, and elsewhere. In some cases, these populations are very inbred because of small numbers of founders. Health regulations on imports of swine and other ungulates, coupled with historically little interest in suids as zoo exhibits, have contributed to this problem.

Significance to humans

Wild pigs represent either a pest or an important source of dietary protein for most humans. In places where they co-occur with people, pigs frequently do serious damage to agricultural lands through their consumption of crops and/or rooting behavior. They have also been known to do serious damage to timber plantations. Thus, they are often persecuted in reprisal for losses. However, even in areas dominated by Islam or other religions that forbid the consumption of pork, pigs are very frequently hunted for their flesh. Pigs represent a valuable resource for both subsistence and commercial hunting throughout much of their range in Asia and parts of Africa. In some areas wild pigs may be vectors of diseases that pose threats to domestic livestock. Some pig cultures still exist wherein pigs are valued as commodities, used as currency, or have ritualistic significance; however, these cultures are becoming more rare.

The earliest domestication of the pig appears to have taken place in Anatolia, Mesopotamia, and northern Iraq approximately 9,000 years ago. The domesticated pig came to Europe via the Caucasus and Balkans and is found in Aeneolithic and Bronze Age burials. The turbary is thought to be a transitional form between the wild boar and domestic pig. However, there is some evidence that other species besides S. scrofa

were used as stock for domestication (e.g., S. celebensis in Southeast Asia). Local wild boar populations no doubt contributed to local breed development in Europe; however, Asia was the primary center of domestication, and many of today's European breeds are influenced by Asian stock. The skull of the domestic pig has changed from its wild form more than that of any other domesticated animal besides the dog. The skull is now broader, with a shortened anterior portion. The brain case is also higher. Other characteristics of this domestication include a larger body, smaller head, longer body, and shorter legs, the existence of flop ears, and a curly tail. In modern times, the major centers of domestic breed development have been England, China, and the United States, though several countries in Europe developed their own localized breeds, known as landraces. There are approximately 175 breeds and types recognized around the world. Some breeds are threatened with extinction or considered endangered, mainly because of declining popularity and changes in consumer demands (e.g., leaner pork). The American Livestock Breeds Conservancy lists nine suid breeds as critical or rare, and the Rare Breeds Survival Trust in Britain maintains populations of seven breeds of concern.

The pig is unique among domestic animals in that it has become widely used as an animal model in biomedical research given its similarity to humans in certain aspects of its anatomy, physiology, and even behavior. One of the most active areas of research involving pigs is xenotransplantation. Pigs represent a good source of organs owing to their size, availability, and limited risk of zoonosis. Potential donor organs for human recipients include porcine heart, kidney, liver, heart-lung, and pancreas tissue. Research is needed to overcome some of the immunological barriers before widespread use of porcine organs is possible; however, selective breeding and genetic engineering are also being explored as methods to reduce transplant rejection.

Species accounts

Resources

Estes, Richard D. The Behavior Guide to African Mammals. Berkeley: University of California Press, 1991.

Oliver, William L. R., ed. Pigs, Peccaries, and Hippos: Status Survey and Conservation Action Plan. Gland, Switzerland: International Union for Conservation of Nature and Natural Resources, 1993.

Pond, Wilson G., and Harry J. Mersmann, eds. Biology of the Domestic Pig. Ithaca, NY: Cornell University Press, 2001.

Cosgrove, J. R., S. T. Charlton, S. J. Cosgrove, L. J. Zak, and G. R. Foxcroft. "Interactions between Nutrition and Reproduction in the Pig." Reproduction in Domestic Animals 30 (1995): 193–200.

Groves, C. P. "Taxonomy of Wild Pigs (Sus) of the Philippines." Zoological Journal of the Linnean Society 120 (1997): 163–191.

The American Livestock Breeds Conservancy. P.O. Box 477, Pittsboro, NC 27312 United States. Phone: (919) 542-5704. Fax: (919) 545-0022. E-mail: albc@albc-usa.org Web site:

Rare Breeds Survival Trust. National Agricultural Centre, Stoneleigh Park, Warwickshire, CV82LG United Kingdom. Phone: 024 7669 6551. Fax: 024 7669 6706. E-mail: alderson@rbst.demon.co.uk Web site:

American Zoo and Aquarium Association, Pig and Peccary Taxon Advisory Group. .

IUCN Pigs, Peccaries, and Hippos Specialist Group. .

Asian Wild Pig Research and Conservation Group. .

[Article by: David M. Powell, PhD]

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