TAXONOMY OF PIGS AND HOGS

(Linnaeus, 1758)

There are two subgenera and five species (Ansell, in Meester and Setzer 1977; Chasen 1940; Ellerman and Morrison-Scott 1966; Groves 1981a; Grzimek 1975; Laurie and Hill 1954; Medway 1977, 1978; Oliver 1984; Sanborn 1952; Taylor 1934):

subgenus Sus Linnaeus, 1758

1. S. scrofa (wild boar, or pig), originally found from southern Scandinavia and Portugal to southeastern Siberia and the Malay Peninsula, from Western Sahara to Egypt, and on Britain, Ireland, Corsica, Sardinia, Sri Lanka, Japan, the Ryukyu Islands, Taiwan, Hainan, Sumatra, Java, and many small associated islands of the East Indies as far east as Komodo;
2. S. barbatus (bearded pig), Malay Peninsula, Rhio Archipelago, Sumatra, Bangka, Borneo, Philippines;
3. S. celebensis (Celebes wild boar), indigenous to Sulawesi and some nearby small islands;
4. S. verrucosus (Javan pig, or warty pig), Java and the nearby islands of Madura and Bawean;

subgenus Porcula Hodgson, 1847

1. S. salvanius (pygmy hog), Nepal, Sikkim, Bhutan, and adjacent parts of northeastern India.

A general view, as expressed by Honacki, Kinman, and Koeppl (1982), is that S. verrucosus includes S. celebensis and that the resulting species, and not S. barbatus, is found in the Philippines. Detailed studies by Groves (1981a), however, indicate a systematic and distributional situation as given above.

In S. salvanius, head and body length is 500-650 mm, tail length is about 30 mm, and shoulder height is 250-300 mm. In the other three species, head and body length is 900-1,800 mm, tail length is about 300 mm, shoulder height is 550-1,100 mm, and weight is 50-350 kg. Some domestic breeds of S. scrofa may attain a weight of 450 kg (Grzimek 1975). Males usually are larger than females. Coloration of wild forms is dark gray to black or brown. The body is covered with stiff bristles and usually some finer fur, but the pelage often is quite scant, and the tail is only lightly covered with short hairs. Many individuals have side whiskers and a mane on the nape. The young are striped. Hogs have four continually growing tusks, two in each jaw. There are three pairs of mammae in female S. salvanius and six pairs in females of the other species.

Pigs live in many kinds of habitat but generally where there is some vegetation for cover. They are most plentiful in oak forests and in the reedbeds of Asia; a major limiting factor is thought to be snow depths greater than about 40-50 cm, which make travel and searching for food difficult (Groves 1981a). Individuals construct crude shelters by cutting grass and spreading it over a given area. They crawl under the grass and then raise themselves to lift the grass mat, which then attaches to uncut grass and forms a canopy. They wallow in mud and will do so for hours if the opportunity affords. They are swift runners and good, strong swimmers. Activity is mainly nocturnal and crepuscular. A great distance may be traveled during the night in search of food. The omnivorous diet includes fungi, tubers, bulbs, green vegetation, grains, nuts, cultivated crops, invertebrates, small vertebrates, and carrion.

In a study of feral S. scrofa in coastal South Carolina, Wood and Brenneman (1980) found a population density of 10-20 individuals per sq km and an average annual home range of 226 ha. for males and 181 ha. for females. In a nearby area, Kurz and Marchinton (1972) determined home range to average about 400 ha. and seldom observed groups of more than 3 individuals. Off the West Coast, on Santa Catalina, Baber and Coblentz (1986) found densities of 21-34 feral pigs per sq km and mean home ranges of around 200 ha. for males and 100 ha. for females. Singer (1981) listed densities of about 8-9 per sq km in Great Smoky Mountains National Park and around 1-30 per sq km in various parts of Europe.

In the Old World, S. scrofa has been seen in herds, or "sounders," of over 100, though average size seems to be about 20 (Lekagul and McNeely 1977). According to Fradrich (1974), in both wild and feral S. scrofa the basic social unit is the female and her litter. After the young are weaned, two or more families may come together. This association remains stable until the beginning of the next mating season, when the previously solitary adult males join in fighting over the females. A male usually wins control of 1-3 females but sometimes obtains as many as 8. After mating, the males depart. Observations of wild boars by Dardaillon (1988) indicate that a sounder is basically a nursery unit; a near-term female separates from her group, but within a few weeks of birth there is a reassociation, first with other lactating mothers and then with young of the previous year. Barrette (1986) reported that conflicts between pigs begin with two animals walking parallel and edging closer and closer until their shoulders touch and shoving begins. They then sometimes rear up, lean against one another, and attempt to knock each other off balance. Finally, if neither yields, they turn and begin to thrust with mouths open and tusks bared.

Breeding occurs throughout the year in the tropics, but births peak shortly before or just after the rains. In temperate regions, the young are born in the spring. Females have an estrous cycle of about 21 days, are receptive for 2-3 days, and generally produce one litter annually. The gestation period is 100-140 days. The number of young per litter is 1-12, usually 4-8. Unlike the young of most ungulates, piglets are born in a nest and remain there following birth. They are weaned after about 3-4 months and may leave the mother prior to birth of the next litter, but young females often remain longer. Sexual maturity may be attained by as early as 8-10 months, but females usually do not mate until around 18 months. Males generally are not able to compete successfully for mating privileges until they reach full size, at around 5 years (Fradrich 1974; Grzimek 1975; Henry 1968; Lekagul and McNeely 1977). Average longevity is about 10 years, but some pigs have lived up to 27 years.

Wild pigs have been extensively hunted by people for use as food, for sport, and because they sometimes are destructive to crops. Their size and sharp tusks make them potentially dangerous antagonists, but they usually do not attack unless molested. Wild S. scrofa was exterminated long ago in many parts of its original range, including the British Isles, Scandinavia, and Egypt, but has been reintroduced in Scandinavia (Lever 1985). In the period 1965-75 there was a dramatic increase in wild boar populations across Europe, with the numbers being taken by hunters approximately doubling and reaching 100,000 per year in Russia; subsequently populations have stabilized (Sáez-Royuela and Tellería 1986). Wild boars also have been deliberately released by human agency, mainly for purposes of sport hunting, in certain other parts of the world. There is sometimes difficulty, however, in determining whether the free-ranging pigs in these areas represent originally wild stock or are the descendants of domestic animals (Corbet 1978; Grzimek 1975; Laurie and Hill 1954; Wood and Barrett 1979).

Escaped domestic pigs certainly have formed large feral populations in many regions, including Central and South America, the West Indies, Australia, New Zealand, Indonesia, the Andaman Islands, New Guinea (but see below), Hawaii, the Galapagos, and many other oceanic islands (Lever 1985; Oliver 1984). They are generally considered detrimental in these areas, especially on islands, where they have been responsible for the destruction of many species of native animals and plants through either direct predation or habitat disruption. Pavlov (in Strahan 1983) referred to the feral pig as the most significant mammalian pest of agriculture in Australia and as a reservoir of many diseases.

According to Lekagul and McNeely (1977), domestication of  S. scrofa took place in China around 4900 b.c. and may have occurred as early as 10,000 ;R5b.c. in Thailand. Many breeds have since been developed, especially in Europe. Pigs are valuable to agricultural economies, because they mature sooner than do other domestic ungulates, have larger litters, and can feed on human refuse. They thus have contributed to the spread of human populations to New Guinea and the Pacific islands. Several British breeds now are considered rare and are the subject of conservation efforts (Hall 1989).

The first pigs in the United States were those brought by the Polynesians to Hawaii around ;R5a.d. 1000 and those introduced by the Spanish to the Southeast in the early sixteenth century. Several valuable farm breeds of S. scrofa were developed in the United States, but large feral populations also became established. European wild boars were introduced in several places for purposes of sport hunting, and these interbred with the feral animals already present. Free-ranging pigs now occur from Texas to Florida and the Carolinas, throughout California, on eight of the major Hawaiian Islands, and on Puerto Rico and the Virgin Islands. They are valued as game animals in some areas, and over 100,000 are taken annually by sport hunters. Many persons, however, consider feral pigs to be detrimental to agriculture, forestry, and native wildlife (Singer 1981; Wood and Barrett 1979).

Groves (1981a) determined that some of the feral and domestic pigs of the East Indies are not S. scrofa but S. celebensis. He hypothesized that S. celebensis was domesticated initially on Sulawesi and then brought to the Moluccas, Flores, Timor, Halmahera, and some nearby small islands. In addition, the feral pig populations of New Guinea, Ceram, and some small islands in the Moluccas, which are to a large extent genetically continuous with the domestic pig populations in the region, appear to have resulted from hybridization between introduced stocks of  S. celebensis and S. scrofa.

The pygmy hog (S. salvanius) is classified as endangered by the IUCN and the USDI and is on appendix 1 of the CITES. Its range now appears to be restricted to northwestern Assam, where an estimated 100-150 individuals survive. It has declined mainly because of the modification and usurpation of its limited habitat by people. It is protected by law in India but is illegally hunted. The Visayan warty pig (S. barbatus cebifrons), designated vulnerable by the IUCN and formerly found throughout the central Philippine archipelago, has become uncommon through human destruction of most of its rainforest habitat and excessive hunting (Cox 1987). The same problems were thought to have led to the extinction of the Javan pig (S. verrucosus), which is restricted to a region having one of the world's densest human populations. A recent survey showed that it still survives in several parts of Java but that it is jeopardized not only by human persecution but also by hybridization with the more numerous S. scrofa (Blouch 1988). The IUCN now gives the vulnerable designation to S. verrucosus and also to S. scrofa riukiuanus, of the Ryukyu Islands, and S. barbatus, of the Malay Peninsula, Rhio Archipelago, Sumatra, and Bangka. Oliver (1984) indicated that S. b. oi is jeopardized through illegal importation of pork by Singapore, while the main threat to S. scrofa riukiuanus is excessive hunting for local consumption. He noted that the latter is a true endemic wild pig, not an introduced feral population as sometimes claimed.

Retrieved by Leopoldo Costa (January, 2002) from http://www.press.jhu.edu/books/walker.html. The online edition of 'Walker's Mammals of the World' is no longer available. Adapted and illustrated to be posted.

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