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Diversity of Living Pigs
The family Suidae (pigs) is one of a number of families in the mammal order Artiodactyla (even-toed ungulates). Meijaard et al. (2011) recognized 12 species in the Suidae, although they note that clarifying the status of some of these will require further work, especially in the Philippines. One or more species are found throughout most of the Old World except for Australia, northern Africa, and far northern Eurasia (feral pigs are found even more widely, including in North America and on many Pacific islands). Various species can be found from near-desert conditions to closed forest in tropical, subtropical, and boreal regions. The most unusual-looking pigs are no doubt the babirusas, which have long, spindly legs, a nearly naked body, and, most strikingly, upper canines which (in males) grow up through the snout and continue to grow in a backward-curving spiral. The function of these remarkable, greatly enlarged canines remains unclear. The babirusas share a number of other unusual characteristics as well that have raised questions about whether they actually fall within the Suidae, but this question has not yet been resolved.
A distinctive feature of pigs is the unusual snout, with terminal nostrils which can be closed; the snout can be moved by the pig without moving its head. Most pig species are highly omnivorous, feeding even on carrion, but plant material typically accounts for a large proportion of the diet. With the exception of the Forest Hog and babirusas, pigs use their canines and flexible snout disk to root through soil looking for food and dig up roots and bulbs. Babirusas specialize on fruit and Forest Hogs feed more on grasses than any other pig species; Forest Hog piglets are fond of fresh elephant dung.
With the exception of the babirusas, which appear to have retained the foregut fermentation characteristic of ruminants, most pigs use hindgut fermentation in the caecum to digest cellulose, but their cellulose digestion is inefficient and pigs consequently must spend much of their time feeding.
All pigs use mud wallows when they have an opportunity to do so. Wallowing serves to cool the animal them and to protect it from ectoparasites (external parasites) and biting flies. Pigs nearly always rub themselves against a tree after wallowing.
Pigs are generally considered to be among the most intelligent mammals, after humans and other great apes, dolphins, and elephants. Their intelligence has been likened to that of a three-year-old child.
Pigs have a remarkable variety of scent glands, used in scent-marking, which are distributed over various parts of their bodies.
Pigs are the only ungulates (hoofed mammals) to commonly produce large litters of offspring (up to half a dozen or even more than a dozen offspring at once).
Pigs and Humans
There are an estimated two billion domestic pigs on Earth (domestic pigs are derived mainly from the Eurasian Wild Pig and the Sulawesi Warty Pig). Pork accounts for a large proportion of global meat production (41% in 2007). In addition to eating pigs and, in some places, keeping them as pets, humans have used them to detect and dig for underground truffles. Available evidence suggests that pigs may have been domesticated independently in Europe, Asia Minor, the Far East (including Japan), and various parts of Southeast Asia. In many traditional human societies around the world, pigs have tremendous cultural importance. Feral pigs wreak environmental havoc in many regions where they occur. On the other hand, populations of most of the world's pig species are declining, some critically. Particularly vulnerable are those species ocurring in small and isolated populations in archipelagoes such as the Philippines, Indonesia, and Japan.
(Meijaard et al. 2011 and references therein)