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November 2, 2009

More about Pigs

The underestimated animal

The Humane Society of the United States

pig with knowing look

Christian Lohman/iStockphoto

Pigs are highly intelligent, curious animals who engage in complex tasks and form elaborate, cooperative social groups.

Their uncanny physiological and behavioral similarities to humans have given pigs a mysterious and often mythical quality that lends itself to folklore and fables.

Pigs were once considered wicked and dirty, but science has helped to shed light on the depths of their remarkable cognitive abilities and to extend a greater appreciation for these often maligned and misunderstood animals.

Ecologists, zoologists and naturalists now remark on their impressive ability to survive and adapt to different environments around the world.

David Low, an esteemed British academic, described the wild boar as a "noble" animal,[1] while renowned writers and scholars have acknowledged pigs' "unforgettable characters and obvious intelligence."[2]

Sir Winston Churchill, Britain's famed prime minister, was fond of pigs and reportedly said, "Cats look down on you; dogs look up to you; but pigs look you in the eye as equals."[3]

There's a lot that most people don't know about pigs, namely that they are:

Related to giraffes—a tall order

Pigs, who are sometimes referred to as swine or hogs,[4] are part of the order Artiodactyla, or even-toed ungulates. They share this order with cattle, sheep, goats, camels, deer, giraffes and hippopotamuses.[5]

Pigs belong to the family Suidae, which includes the warthog (Phacochoerus) and bushpig (Potamochoerus) of Africa, the babirusa (Babyrousa) of Indonesia and the Eurasian wild boar (Sus scrofa), who is the ancestor of all domestic pigs.[6],[7]

The collared peccary (Pecari tajacu), who are native to the southwestern United States, closely resemble pigs although they belong to the family Tayassuidae.[8]

Early arrivals to the Americas

Among the world's earliest domesticated animals, pigs were first domesticated in Europe and Asia approximately 10,000 years ago.[9],[10]

Polynesians may have introduced domestic pigs to Hawaii sometime during the first millennium,[11] although it is generally believed that when Christopher Columbus landed in Cuba in 1493, he brought to North America the first domestic pigs—pigs who subsequently spread throughout the Spanish West Indies (Caribbean).[12]

In 1539, the Spanish explorers brought the first pigs to the mainland when they settled in Florida. The pigs were released into the wild and they spread throughout the region that is now the southeastern United States.[13],[14],[15] Throughout the 17th and 18th centuries, the colonists brought pigs to America and raised them as free-roaming animals; those who escaped into the wild became feral.[16] During the 20th century, European wild boar were brought to the United States as a "game" animal for hunting. Some of these animals escaped and mated with free-ranging domestic and feral pigs.[17],[18]

Consequently, today's feral pigs are a combination of descendents of escaped farmed pigs, wild boar, and hybrids of the two.[19],[20]

Everywhere but Antarctica

Several centuries ago, the wild boar became extinct in Britain as a result of hunting and habitat destruction,[21] but they have since been reintroduced in England and are thriving in many parts of Europe, Africa, and Asia.[22],[23] Today, in fact, wild boar and pigs are found on every continent except Antarctica.[24],[25]

Feral pigs, especially, thrive in temperate and tropical areas, such as Australia, New Zealand, and Indonesia.[26] In the United States, feral pig populations have dispersed throughout many southern states including Florida, Texas, and California,[27],[28],[29] where the limited ground frost abets their diet of roots and tubers.[30] Heavy snowfalls and frozen ground may limit their ability to survive in colder regions[31],[32] but, like humans, pigs are able to adapt to a wide variety of environments and circumstances,[33],[34] and their range has steadily expanded northward, mainly within the Midwest, to include at least 39 states.[35],[36]

It is possible that the interbreeding with wild boars, who are more rugged and cold hardy than the domestic pig, has helped wild pigs to thrive in northern, less temperate regions.[37]

Feral pigs and wild boar live in a wide range of habitats but primarily dwell in forests and woodlands, where the trees and vegetation provide shelter and copious supplies of their preferred foods, acorns and beechnuts.[38] The animals dig out beds under the cover of oak trees and shrubs,[39] and prefer swamps, marshes, and other areas with year-round access to surface water and moist ground for wallowing.[40] Their home ranges vary widely, from 1-25 km2 (247-6,170 acres),[41],[42] depending on the seasonal variations of food and water sources, as well as other factors including age, gender, and human encroachment.[43],[44]

In times of scarcity, such as the winter and early spring months in some regions, ranges might expand as the animals search for new feeding grounds.[45],[46],[47] Pigs and wild boar are able to travel great distances, perhaps as much as 15 km (9.3 miles) in a single night.[48] They are seldom territorial, often sharing common spaces with pigs of neighboring family groups in their home ranges.[49]

Omnivorous oinkers

Many ungulates, such as deer and giraffes, have developed specialized digestive systems for their herbivorous diets. Pigs, however, are more generalized—evolutionarily, halfway between carnivores and plant-eaters. According to Juliet Clutton-Brock, the distinguished author and animal scholar, "Pigs are much more like dogs and people than they are like cattle or sheep and in many respects their behaviour seems to be intermediate between that of the carnivores and the more highly evolved artiodactyls."[50] Additionally, unlike most other ungulates who give birth to single or occasionally twin offspring, pigs give birth to litters like many species of carnivore.[51]

They adapt their diet to the seasonal availability of edible plant foods in their home ranges.[52],[53] In spring, they may graze in open grassland and marshland for grass and herbage, such as clover; during summer, as the land dries up, they eat more roots, bulbs, tubers and wild oats; and from autumn through winter, they forage woodlands for berries and nuts, including acorns,[54],[55],[56] which are an important part of the pig's diet.[57]

In fact, in medieval Britain, domestic pigs were allowed to forage in the forests for acorns and other nuts, a practice known as pannage.[58] Although it is no longer common, a few farmers in the New Forest of England are encouraged to keep up the tradition to help clear the ground of acorns and beechnuts, which are poisonous to the resident ponies and cattle.[59]

Although pigs subsist primarily on plant matter,[60],[61],[62] they are omnivores and supplement their diets with earthworms, insects, amphibians, reptiles, and rodents, and can consume carrion and may engage in scavenger behavior as well.[63],[64],[65] The total amount of animal and insect matter in wild pigs' diets is usually less than 4% annually, although wild boar may consume more in certain habitats.[66],[67]

Some pigs develop highly individualized tastes, showing a behavioral preference for specific foods such as mushrooms.[68]

Still wild after all these years

Although selective breeding by the pig industry has altered the appearance and physiology of domestic pigs, comparative studies show that their behavioral characteristics are fundamentally the same as those of the wild boar.[69],[70]

For example, the maternal behavior of domestic sows (female pigs), such as nest building, is seemingly innate and has not changed much despite domestication and artificial selection for such production-related traits important to the pig industry as efficient feed conversion or greater litter size; indeed, such natural behavior is suited to life in the wild.[71]

David Wood-Gush and Alex Stolba, scientists at the University of Edinburgh in Scotland, observed domestic pigs in a semi-natural enclosure over several years and concluded that, "The social behaviour of the domestic pig seems to resemble, in all important respects, that of the European wild boar, Sus scrofa, when the domestic form is allowed to live in semi-natural conditions."[72]

Strong but sensitive, snoutwise

Like a cat's whiskers, a pig's snout provides the animal with heightened senses to navigate and interact with the environment, and is especially designed for rooting in the ground in search of food.[73] Under natural conditions, pigs may spend 75% of their daily activity engaged in rooting and foraging,[74] for up to 6-8 hours a day.[75]

The nasal disc on a pig's snout, while rigid enough to be used for digging, has numerous sensory receptors.[76] In addition to being useful as a fine and powerful tool for manipulating objects, the extensive innervation in the snout provides pigs with an extremely well-developed sense of smell.[77],[78]

Pigs can smell roots and tubers that are deep underground—a unique skill that has been exploited since the ancient Babylonian period to find truffles, a subterranean fungus that grows around the roots of oak trees and is highly prized by gourmet chefs.[79]

Indeed, olfactory signals play an important role in communication, and pigs can communicate by releasing chemicals in their saliva and urine called pheromones.[80] In an impressive example of symbiosis and interspecies communication, truffles developed the capacity to produce an allomone that mimics boar testosterone. The sow, upon finding the scent, uses her special snout to dig out the truffle, thus disseminating its fungal spores and enjoying a treat for her efforts.[81]

Pigs can also use odor from urine and the facial glands to identify other pigs,[82],[83] and even pigs who are unable to see are able to recognize other individuals in their group, indicating the strength of their other senses.[84],[85] Olfaction plays an important role in a sow's maternal behavior, for example, by helping to form strong bonds with her offspring.[86]

Pigs also communicate by scent-marking prominent features in their home ranges.[87] As they are not very territorial, however, the purpose of scent-marking might be to establish group cohesion, rather than to mark territory.[88]

Happy to chill out

Pigs are naturally very hygienic animals and designate discrete sites for defecating and urinating far from their sleeping and feeding areas.[89],[90],[91] Near their nests, pigs also have rubbing sites, usually trees, which they use to aid in grooming when other pigs are not present to assist them.[92],[93]

They regularly bathe in water and wallow in mud in order to prevent heat stress.[94] The mud covering their bodies also offers protection from flies and helps to prevent sunburn.[95]

Despite the commonly used expression "sweating like a pig," pigs do not have sweat glands and are therefore prone to heat stress, so wallowing and engaging in other thermoregulatory behavior are very important for maintaining body temperature.[96],[97]

Pigs stay cool during warmer months by limiting most of their activity to night, and being primarily nocturnal also helps them to avoid predators.[98],[99]

On colder days, pigs may reduce their foraging activity to conserve energy[100] and be more active during the day, resting during the chillier parts of the night.[101],[102] In areas without predators or hunting pressures, however, they readily adopt more crepuscular or diurnal patterns as well.[103]

Vulnerable to hypothermia, pigs instinctively huddle together to stay warm as young piglets,[104] a behavior that can be retained through adulthood.[105]

Safer and sounder with relatives

Highly social animals, pigs live in small, matriarchal groups, known as sounders, usually comprised of 2-6 sows and their young.[106],[107] Several sounders may form loose networks of related family groups, sharing overlapping home ranges and sometimes congregating in larger herds.

Although the typical sounder comprises 6-30 members, herds of up to 100 pigs have been observed in Europe and California.[108],[109],[110] The size of the group may be limited by environmental factors such as the availability of food and other resources to sustain a population within an area.[111]

Within the sounder, two sows may become foraging and sleeping partners, and other intra-group bonds and special relationships can form among littermates as well.[112] The siblings often maintain these ties into adulthood.[113]

Juvenile males stay with their family groups until the dominant males in the area force the younger males to leave, at around 7-18 months of age, when the sows come into estrus. The juvenile males may then join a small "bachelor" group with other young adults before becoming more solitary as they age.[114],[115],[116],[117]

Pigs belong to relatively stable social hierarchies, which play an important role in self-regulating behavior and maintaining group harmony.

Whereas unfamiliar pigs who are intensively confined and crowded in industrial operations will engage in aggressive, agonistic behavior, in the wild, pigs are naturally gregarious animals and the members of a sounder maintain close contact at all times, often engaging in such social behavior as huddling and grooming.[118],[119] They sleep in communal nests that adults help to maintain by adding fresh bedding materials such as branches and grass.[120],[121]

Pigs greet one another vociferously and by making contact with their snouts.[122] While foraging or exploring, they stay in contact through constant vocalization and are always aware of each other's actions, often learning by watching others' behavior.[123],[124],[125]

Awareness of who's who

Social recognition, or the ability to identify familiar individuals, is a key to forming these stable relationships.[126]

Pigs have strong capacities for identity recognition and social organization, and recognize other pigs primarily by their powerful sense of olfaction,[127] much as humans rely on visual cues to distinguish between friends and strangers.

Pigs also utilize visual cues for social recognition, but with their powerful and heightened olfactory and auditory senses, they have other ways of communicating that are beyond a human's capacity for perception.

For example, pigs have well-developed hearing and vocal abilities. Their large, pricked ears act like radar dishes and can localize sound to a threshold of just 4 degrees, making them, like cats, one of the most accurate localizers of sound in the animal kingdom.[128] They can distinguish between very subtle differences in tone and are known to use more than 20 different types of vocal noises to communicate during feeding, courtship, exploring, and other social activities.[129] With these vocalizations, which include grunts, squeals, and growls, pigs are able to communicate to others such information as their location, mood, well-being, and intent.[130],[131]

Social organization, starting with the teat

Sows reach puberty at 5-8 months of age.[132] European wild boars typically begin breeding when they are 18-20 months old[133] and normally produce only one litter per year.[134]

In the wild, pregnant sows separate themselves from the group 1-2 days before farrowing, or giving birth, and begin to search for a suitable nest site.[135] The sow is particular about her nest site and may change the location a few times.[136] Often, she prefers a dry, wooded spot in a secluded area that is adequately sheltered by branches and other cover.[137],[138],[139] She may travel far from her home range to find a site that is sufficiently isolated and protected. Here, the sow constructs a comfortable nest by digging a hollow in the earth, insulating it with grass and twigs, and lining it with branches.[140],[141],[142]

Like dogs and cats, pigs are polytocous, or litter-bearing animals, and the average litter size is 4-7 piglets.[143],[144] The young are well-developed at birth, or precocial, which is rare among polytocous mammals.[145] They are among the most precocious newborns of all ungulate species, but share the delicate metabolisms and weak constitutions of the polytocous mammals.[146],[147],[148] They can see and hear at birth, and start to walk within a few minutes of being born.[149]

Piglets quickly find their mother's teats, often with some gentle nudging and encouragement from the sow, to nurse.[150],[151] A teat order (similar to a "pecking order"), the beginning stage of social organization in the life of a pig, is formed on the first day, giving each piglet his or her own specific teat for the rest of the nursing period.[152],[153],[154]

The sow and her piglets use various vocalizations to initiate nursing, and newborn piglets will recognize and respond to their mother's voice when she calls them to suckle.[155],[156] Piglets can distinguish between their own mother's voice and that of other sows.

A team of British scientists found that piglets who were only one-day old responded to recordings of their mother's calls but ignored similar sounds from other sows.[157]

Family friendly

While the sow stays in the nest with her litter, isolated from the sounder for about 1-2 weeks, she is very protective, and this period of exclusive contact with her piglets enables the development of strong bonds.[158],[159] One or two days after giving birth, the sow begins to leave on short foraging trips, though staying close to the nest and her young.

Exploratory behavior, such as rooting, develops within the first few days of life,[160] and the piglets soon begin to follow the sow on short excursions away from the nest.[161],[162] When separated from their mother, piglets call her with distinctive vocalizations, and the sow responds urgently by vocalizing in return. Piglets with greater needs than their littermates can communicate their heightened distress to the sow, who will show a much stronger response to their calls.[163]

The piglets increasingly spend more time and venture greater distances away from the nest and, with their mother, abandon the nest where they were born after 7-14 days to join the rest of the group.[164],[165] The following weeks are a period of high social activity for the piglets as they interact with the other sows and their young.

The piglets begin to play, an important and natural activity, within the second week of life and engage in such group activities as chasing and frolicking, as well as individual play like rooting and using their mouths to examine novel objects—activities that continue into adulthood.[166]

Within the sounder, if the other sows are also nursing litters, the mother pigs may share maternal duties, leaving each sow more time to forage alone.[167],[168] Adoption and allosuckling, or nursing another sow's piglets, are not uncommon behaviors, and demonstrate a tendency towards communal dynamics.[169],[170]

At about 8 weeks of age, the piglets are fully integrated in the group, although the social bonds among siblings remain strong.[171] Weaning is a slow and gradual process, and the piglets continue to suckle until 14-17 weeks of age.[172],[173],[174]

Wild boars have been observed to live 11-25 years in the wild;[175],[176] however, due to hunting, the average lifespan of wild boars and feral pigs is about 4 years.[177],[178]

Pigs raised for meat in North America are usually slaughtered at about 6 months of age, when they are still juveniles.[179]

Smarter than your dog?

Pigs are widely known to be highly inquisitive, with considerable learning and problem-solving abilities.[180],[181] They easily learn to operate levers and switches to obtain food and water, and to adjust ambient temperature to their liking.[182]

Pigs have also been observed to work in collaboration to free themselves from their pens.[183]

According to Donald Broom, Professor of Animal Welfare at University of Cambridge Veterinary School, who has been conducting mirror reflection tests with pigs: "Pigs have the cognitive ability to be quite sophisticated. Even more so than dogs and certainly [more so than] three-year-olds."[184]

After teaching pigs to control a special joystick with their snouts, researchers at Pennsylvania State University found that pigs could learn to play simple matching games by moving the cursor around a computer screen. The pigs demonstrated a similar capacity as primates for learning the task.[185]

Like chimpanzees, pigs can exploit the knowledge of conspecifics by following them to a secret or hidden food site. This suggests that pigs might possess high level social cognitive abilities, such as visual perspective taking, which is the ability to assume what the other sees and to adjust one's own behavior accordingly.[186],[187] In turn, pigs might avoid going near the hidden food site when they are being followed by a dominant pig.[188]

Studies on this topic suggest that pigs might possess a degree of theory of mind, which is the ability to presume the intentions of others' behavior.[189],[190] Michael Mendl, one of the researchers involved in the studies, believes the findings suggest that "pigs can compete with each other in quite complex and 'cerebral' ways."[191]

Surprising: minding our theories

The late Lyall Watson, a famed ethologist and anthropologist, believed that pigs consistently demonstrate the ability to think and reason, but only that "there is still not enough documentary evidence to make such a conclusion scientifically acceptable."[192] However, the value of any such scientific tests of consciousness will be limited so long as we insist on defining animal intelligence across different species according to human needs and humanly recognizable traits.

As the editorial staff of The New York Times wrote in 2006:

We keep probing the animal world for signs of intelligence—as we define it—and we're always surprised when we discover it. This suggests that something is fundamentally wrong with our assumptions. There is every reason to value other life-forms as much for their difference from us as for their similarity, and to act accordingly. That may be the only intelligence test worthy of the name.[193]

On the other hand, although scientists must resist anthropomorphism in the study of animal psychology, it would clearly be wrong to deny that other species do possess mental abilities equatable to those of humans.[194],[195]

Many scientists and naturalists who have observed and studied pigs believe that their intelligence, sociability and consciousness set them apart from other domesticated animals and perhaps place them in the same category as elephants, dolphins and great apes.[196]

Pigs share many similarities with humans with regard to emotions and cognitive states, and increasing scientific inquiry into the true nature of these animals continues to recognize their substantial mental abilities and sociable nature, as well as their capacity to experience pain, pleasure, fear and joy.


References

[1] Low D. 1846. On the Domesticated Animals of the British Islands (London: Longman, Brown, Green, & Longmans, p. 403). http://openlibrary.org/b/OL7114743M. Accessed April 7, 2009.

[2] Kristof ND. 2008. A Farm Boy Reflects. The New York Times, July 31. www.nytimes.com/2008/07/31/opinion/31kristof.html. Accessed April 7, 2009.

[3] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books, p. 16).

[4] Mayer JJ and Brisbin IL Jr. 1991. Wild Pigs in the United States: Their History, Comparative Morphology, and Current Status (Athens, GA: The University of Georgia Press, p. 5).

[5] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[6] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books).

[7] Sonder B. 1998. Pigs & Wild Boars (New York, NY: Todtri Productions Limited, pp. 11-12).

[8] U.S. Department of Agriculture Animal and Plant Health Inspection Service. 2005. Feral/wild pigs: potential problems for farmers and hunters. Agriculture Information Bulletin No. 799. www.aphis.usda.gov/publications/wildlife_damage/content/printable_version/feral pigs.pdf. Accessed April 7, 2009.

[9] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[10] Jensen P. 1988. Maternal behaviour of free-ranging domestic pigs. I. Results of a three-year study. Swedish University of Agricultural Sciences.

[11] Nogueira SSC, Nogueira-Filho SLG, Bassford M, Silvius K, and Fragoso JMV. 2007. Feral pigs in Hawai'i: using behavior and ecology to refine control techniques. Applied Animal Behaviour Science 108(1-2):1-11.

[12] Mayer JJ and Brisbin IL Jr. 1991. Wild Pigs in the United States: Their History, Comparative Morphology, and Current Status (Athens, GA: The University of Georgia Press, pp. 7-8).

[13] Mayer JJ and Brisbin IL Jr. 1991. Wild Pigs in the United States: Their History, Comparative Morphology, and Current Status (Athens, GA: The University of Georgia Press, p. 21).

[14] Wood GW and Barrett RH. 1979. Status of wild pigs in the United States. Wildlife Society Bulletin 7(4):237-46.

[15] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books, p. 108).

[16] Hanson RP and Karstad L. 1959. Feral swine in the southeastern United States. The Journal of Wildlife Management 23(1):64-74.

[17] Singer FJ. 1981. Wild pig populations in the national parks. Environmental Management 5(3):263-70.

[18] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[19] U.S. Department of Agriculture, Animal and Plant Health Inspection Service. 2005. Feral/wild pigs: potential problems for farmers and hunters. Agriculture Information Bulletin No. 799. www.aphis.usda.gov/publications/wildlife_damage/content/printable_version/feral pigs.pdf. Accessed April 7, 2009.

[20] Mayer JJ and Brisbin IL Jr. 1991. Wild Pigs in the United States: Their History, Comparative Morphology, and Current Status (Athens, GA: The University of Georgia Press, p. 2).

[21] Howells O and Edwards-Jones G. 1997. A feasibility study of reintroducing wild boar Sus scrofa to Scotland: are existing woodlands large enough to support minimum viable populations. Biological Conservation 81(1-2):77-89.

[22] Goulding MJ, Smith G, and Baker S. 1998. Current status and potential impact of wild boar (Sus scrofa) in the English countryside: a risk assessment. Report to Conservation Management Division C, MAFF.

[23] Jensen P. 1988. Maternal behaviour of free-ranging domestic pigs. I. Results of a three-year study. Swedish University of Agricultural Sciences.

[24] Waithman JD, Sweitzer RA, Vuren DV, et al. 1999. Range expansion, population sizes, and management of wild pigs in California. The Journal of Wildlife Management 63(1):298-308.

[25] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books, pp. 8-9).

[26] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[27] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[28] U.S. Department of Agriculture, Animal and Plant Health Inspection Service. 2005. Feral/wild pigs: potential problems for farmers and hunters. Agriculture Information Bulletin No. 799. www.aphis.usda.gov/publications/wildlife_damage/content/printable_version/feral pigs.pdf. Accessed April 7, 2009.

[29] Baber DW and Coblentz BE. 1986. Density, home range, habitat use, and reproduction in feral pigs on Santa Catalina Island. Journal of Mammalogy 67( 3):512-25.

[30] Hanson RP and Karstad L. 1959. Feral swine in the southeastern United States. The Journal of Wildlife Management 23(1): 64-74.

[31] Yalden D. 2001. The return of the prodigal swine. Biologist 48(6):259-62.

[32] Barrett RH and Birmingham GH. 1994. Wild Pigs. Prevention and control of wildlife damage.

[33] Boitani L, Mattei L, Nonis D, and Corsi F. 1994. Spatial and activity patterns of wild boars in Tuscany, Italy. Journal of Mammalogy 75(3):600-12.

[34] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[35] U.S. Department of Agriculture, Animal and Plant Health Inspection Service. 2005. Feral/wild pigs: potential problems for farmers and hunters. Agriculture Information Bulletin No. 799. www.aphis.usda.gov/publications/wildlife_damage/content/printable_version/feral pigs.pdf. Accessed April 7, 2009.

[36] Gipson PS, Hlavachick B, and Berger T. 1998. Range expansion by wild hogs across the central United States. Wildlife Society Bulletin 26(2):279-86.

[37] Waithman JD, Sweitzer RA, Vuren DV, et al. 1999. Range expansion, population sizes, and management of wild pigs in California. The Journal of Wildlife Management 63(1):298-308.

[38] Leaper R, Massei G, Gorman ML, and Aspinall R. 1999. The feasibility of reintroducing wild boar (Sus scrofa) to Scotland. Mammal Review 29(4):239-59.

[39] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[40] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[41] Council of Europe. 2004. Recommendation concerning pigs. The Standing Committee of the European Convention for the Protection of Animals kept for Farming Purposes. www.coe.int/t/e/legal_affairs/legal_co-operation/biological_safety,_use_of_animals/farming/Rec pigs rev E 2004.asp. Accessed April 7, 2009.

[42] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[43] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[44] Leaper R, Massei G, Gorman ML, and Aspinall R. 1999. The feasibility of reintroducing wild boar (Sus scrofa) to Scotland. Mammal Review 29(4):239-59.

[45] Kurz JC and Marchinton RL. 1972. Radiotelemetry studies of feral hogs in South Carolina. The Journal of Wildlife Management 36(4):1240-8.

[46] Singer FJ. 1981. Wild pig populations in the national parks. Environmental Management 5(3):263-70.

[47] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[48] Leaper R, Massei G, Gorman ML, and Aspinall R. 1999. The feasibility of reintroducing wild boar (Sus scrofa) to Scotland. Mammal Review 29(4):239-59.

[49] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books, p. 25).

[50] Clutton-Brock J. 1987. Pigs. In: A Natural History of Domesticated Mammals (London, U.K.: British Museum (Natural History), p. 73).

[51] Fraser D. 1984. The role of behavior in swine production: a review of research. Applied Animal Ethology 11:317-39.

[52] Nogueira SSC, Nogueira-Filho SLG, Bassford M, Silvius K, and Fragoso JMV. 2007. Feral pigs in Hawai'i: using behavior and ecology to refine control techniques. Applied Animal Behaviour Science 108(1-2):1-11.

[53] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[54] Graves HB. 1984. Behavior and ecology of wild and feral swine (Sus scrofa). Journal of Animal Science 58(2):482-92.

[55] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[56] Wood GW and Roark DN. 1980. Food habits of feral hogs in coastal South Carolina. The Journal of Wildlife Management 44(2):506-11.

[57] Barrett RH. 1978. The feral hog at Dye Creek Ranch, California. Hilgardia 46(9):283-355.

[58] Watson L. 2004. The Whole Hog: Exploring the Extraordinary Potential of Pigs (Washington, DC: Smithsonian Books, p. 118).

[59]Tyzack A. 2007. Pannage season. Country Life, February 7. www.countrylife.co.uk/countryside/pursuits/article/109076/Pannage_Season.html. Accessed April 7, 2009.

[60] Goulding MJ, Smith G, and Baker S. 1998. Current status and potential impact of wild boar (Sus scrofa) in the English countryside: a risk assessment. Report to Conservation Management Division C, MAFF.

[61] Howells O and Edwards-Jones G. 1997. A feasibility study of reintroducing wild boar Sus scrofa to Scotland: are existing woodlands large enough to support minimum viable populations. Biological Conservation 81(1-2):77-89.

[62] Leaper R, Massei G, Gorman ML, and Aspinall R. 1999. The feasibility of reintroducing wild boar (Sus scrofa) to Scotland. Mammal Review 29(4):239-59.

[63] Hanson RP and Karstad L. 1959. Feral swine in the southeastern United States. The Journal of Wildlife Management 23(1):64-74.

[64] Wood GW and Roark DN. 1980. Food habits of feral hogs in coastal South Carolina. The Journal of Wildlife Management 44(2):506-11.

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