Pigs, Peccaries and Hippos Status Survey and Action Plan (1993)

Chapter 5.10

Introduced and Feral Pigs: Problems, Policy and Priorities

William L. R. Oliver and I. Lehr Brisbin.

Policy and Action Plan Summary

Wild pigs or their domestic and feral derivatives have been widely distributed by man as a source of food, and naturalized populations have become established, often in large numbers, on all continents except Antarctica and on a great many oceanic islands. The overwhelming majority of naturalized populations are regional variants or derivatives of the Eurasian wild pig, Sus scrofa, although the Sulawesi warty pig, S. celebensis, has also been domesticated and introduced in some areas. The resulting diversity of native, naturalized, domestic and hybrid forms has produced patterns of distribution and interrelationships of great taxonomic confusion, particularly in the Indonesian and Papuan Archipelagos. The origins of some of these populations are obscure, though many are associated with the earliest phases of human expansion, exploration and colonization. Some are certainly thousands of years old, but the majority are much more recent and are of little immediate scientific and anthropogenic interest.

However, almost all of these naturalized populations are of relevance to conservation interests. There are a variety of reasons for this, some of which are ambivalent. For example, the babirusa, Babyrousa b. babyrussa is frequently cited as one of the main target species for conservation interest in Buru and the Sula Islands, where they have apparently been introduced. Moreover, these populations may represent an otherwise extinct form from southern Sulawesi. On the other hand, the similarly introduced bushpigs, Potamochoerus larvatus ssp., of Madagascar and Mayotte, are also genetically isolated and distinct from their mainland conspecifics, but these animals are generally regarded as environmental pests. Many of the variously derived forms of Sus are of potential importance as genetic resources, with possibilities for the further domestication of one of the most important sources of animal protein. Many of the most ancient forms are also of considerable anthropological interest in terms of the ethnic origins and cultural integrity of some surviving tribal societies, whose antecedents may have originally introduced these animals. In those hunting societies, which do not practice domestication, wild pigs often represent a basic economic resource, whereas many other naturalized populations are of only marginal economic importance, except to recreational or commercial game meat interests. However, given their evident adaptability, their widely eclectic omnivorous diet and the fact that they have a higher reproductive rate than any other ungulates, naturalized pigs generally have a profound and invariably negative impact on the ecosystems to which they been introduced. They disrupt natural successional sequences, out-compete or predate native species, and they are frequently implicated in the extinction and endangerment of endemic plants and animals - particularly on oceanic islands. In many areas they also cause serious damage to agriculture, pasture and forestry, and they can act as reservoirs for various pathogenic organisms transmissible to humans and other livestock.

For these reasons, it is recognized that, with a few notable exceptions, naturalized pigs, whether of unmodified wild type or of variously derived feral or hybrid forms, should be treated as exotic pests and strictly controlled or eradicated as appropriate. Nonetheless, it is also felt that a small number of these naturalized populations are of sufficient scientific importance to merit conservation attention. In each case, these populations are: 1) threatened throughout their known range; 2) of relatively ancient origin (i.e. introduced at least several hundred years ago); 3) likely to have been genetically isolated for most or all of the period since their original introduction; and 4) have genetic characters of potential importance for further domestication and/or are of particular socio-economic significance to surviving tribal societies. Consequently, it is also suggested these few introduced populations should be conserved in-situ, unless it is subsequently revealed that their continued existence is in clear conflict with the survival or ecological integrity of other threatened (endemic) taxa or communities.

The single highest conservation priority amongst these few populations is the Buru and Sulu Islands' babirusa (B. b. babyrussa), although the (mostly) ancient domestic and feral pigs of S. celebensis origin on Simeulue and certain other Indonesian islands, and the feral pigs of S. scrofa derivation on the Andamans and some of the Nicobar Islands, are also of particular genetic and anthropogenic significance, and merit special efforts to conserve them - preferably in-situ. The more recent, but nonetheless distinct, dwarf pigs of Ossabaw Island, S. E. USA, are also of some genetic importance and should be conserved by means compatible with the ecological integrity of this island - if necessary, ex-situ.

Introduction

Interactions with people from earliest times to the present day have significantly altered the genetic, distributional and ecological characteristics of many of the world's pig populations. These interactions have produced two forms of non-native, free-living or 'naturalized' pig populations, which have successfully colonized many different habitats on all continents except Antarctica, and many oceanic islands. These naturalized forms include both 'introduced' populations, which are defined herein as essentially unmodified wild forms which have simply been transported and re-established in a free-ranging state outside of their original range, and 'feral' populations whose genetic structure has been altered through a process of domestication, before being re-established in a free-ranging state. Amongst the suiformes, two species of peccaries (Tayassu spp.), the babirusa (Babyrousa babyrussa), a warthog (Phacochoerus africanus), and the bushpig (Potamochoerus larvatus) are all represented by free-ranging 'introduced' populations, but only two species, the Eurasian wild pig (Sus scrofa) and the Sulawesi warty pig (S. celebensis) have been truly domesticated and are represented by both 'introduced' and 'feral' forms (Fig. 17).

Unfortunately, however, it is not always possible to distinguish between those naturalized populations which are introduced and those which are truly feral, or even between native and naturalized populations in some cases. Often the evidence is purely circumstantial, e.g. whether or not self-introduction is considered to be physically possible and/or whether there are any other large mammals in the same locality. In the Andaman Islands, for example, an aberrant dwarf wild pig, S. s. andamanensis, was long thought to be endemic, whilst the locally abundant spotted deer (Cervus axis) and a variety of other animals were recognized as exotics since their later introduction had been documented. In fact, the taxon 'andamanensis' is technically invalid as these pigs are now known to be feral (Oliver, 1984a), though their remains date back to the earliest midden deposits of the original negrito settlers, by whom they were presumably introduced. Moreover, it has recently been shown that there are actually two quite distinct, and apparently stable, feral pig morphotypes in the Andamans, i.e. long-snouted and short-snouted forms (Abdulali, 1962). However they both remain poorly known and their derivation is far from certain. Similarly, in Corsica and Sardinia, wild pigs (S. s. meridionalis) and wild cats (Felis sylvestris ssp.) have simply been introduced, whilst the wild mouflon ('Ovis musimon') are feral, albeit as barely-modified relics of a species not long under domestication (Groves, 1989).

The otherwise problematical presence of bushpigs, P. larvatus, on Mayote (Comoro Islands) and Madagascar, and babirusa, B. b. babyrussa, on Buru and the Sula Islands, are also most easily explained in terms of human introductions (Ansell, 1971; Grubb, this vol., section 4.1; Groves and Grubb, this vol., section 5.1). This has sometimes been refuted on the basis that there is also no evidence to suggest that either species has ever been domesticated, although this argument is rather weak in view of the quite widespread introduction of unmodified wild pigs elsewhere (see later text; Fig. 17). Moreover, there are a number of accounts of tame bushpigs (Simoons, 1953; Epstein, 1971) and babirusa (Groves, 1980) being kept as pets by local people, and Dammerman (1929) has even described how local rulers in Sulawesi formerly kept and bred babirusa for use as diplomatic gifts.

Link to Fig. 17: Approximate distribution of introduced and feral pig and peccary populations (excepting P. africanus and B. babyrussa).

Taxonomy and Distribution of Naturalized Pigs

Nowhere are the various native, introduced, feral, domestic and hybrid pig populations more diverse or of more ancient origin than in the biogeographic area known to botanists as Malesia. The pigs of the Indonesian and Papuan archipelagos in particular, include an astonishing array of forms, whose taxonomic and anthropogenic affinities are only now being unraveled. In perhaps the most extreme case, Groves (1981) provided convincing evidence that the second species of Sus to be described, 'S. papuensis' of New Guinea, was not only a feral population but that it was almost certainly a hybrid of two independently domesticated species, S. scrofa and S. celebensis, which were introduced to New Guinea, either separately or as a hybrid stock. Similar hybrids also occur as ferals in the Moluccas, the most likely source of origin of the New Guinea pigs, and from where both S. scrofa- and S. celebensis-type pigs are found in feral states on other islands in this group. Similarly, the so-called 'S. timorensis' of Timor and 'S. mimus' of Simeulue Island (off N. W. Sumatra) are both highly modified introduced forms of S. celebensis, whereas 'S. andamanensis' and 'S. nicobaricus' of the nearby Andaman and Nicobar Islands (Bay of Bengal) are both highly modified forms of S. scrofa.

The origins of many of these naturalized pig populations are associated with the earliest phases of human expansion and dispersal. As settling peoples spread from South-east Asia into Melanesia, and thence into Polynesia, they carried pigs with them. Pigs were introduced to New Guinea at least 6,000 years ago (White, 1972; but also see Oliver et al., this vol., Section 5.9) and human settlers and their pigs reached Fiji by 1,300 years B.C. and had spread into most of Polynesia by about 1,000 B.C.. By 1,000 A.D. pigs had been introduced throughout much of Oceania, including the Hawaiian Islands.

Elsewhere, the introduction of pigs can often be traced back to the first European navigators and the colonial settlers who followed them. Christopher Columbus introduced pigs into the West Indies in 1493, and Spanish settlers brought the first pigs to Florida in 1539 (Belden and Frankenberger, 1977). By the end of the 16th Century, Spanish colonial settlements were well established in Mexico, parts of Central America and the West Indies, Peru and Chile, and the Portuguese had founded settlements in Brazil. The widespread practice of allowing domestic pigs to range freely, inevitably led to the early establishment of feral populations, and their descendants are now found in at least sixteen U.S. states and in most Central and South American countries. In Argentina and in the U.S., some of these feral populations have interbred with the descendants of Eurasian wild pigs, S. s. scrofa, which escaped from game ranches where they had been introduced for hunting in the early 1900's (Mayer and Brisbin, 1991). In parts of their range in the Americas, the naturalized pigs are 'sympatric' with the native peccaries, Tayassu tajacu and T. pecari, which have also been introduced to Cuba for hunting purposes (Figs 1, 2 and 17).

The European expansion into the Indian, Pacific and Southern Oceans also contributed greatly to the spread of these animals. Pigs carried for food or for trade were marooned on oceanic islands for the benefit of later voyagers. The Portuguese navigator, Pero Mascarenhas, released "hogs, goats and fowls" on Mauritius in 1512, although this island was not actually colonized until 1683 (Hachisuka, 1953). De Surville carried the first pigs to New Zealand in 1769 (Clarke and Dzieciolowski, 1991), and a boar and two sows were released by Captain Cook at Queen Charlotte Sound, South Island in 1773. These rapidly increased in numbers, and the New Zealand population was supplemented by later additions, e.g. by Governor King of New South Wales who presented ten pigs to the Maoris at the Bay of Plenty, North Island, in 1793 (Hutton and Drummond, 1904). Pigs had evidently been introduced to Australia by that time, probably on a number of occasions dating from the founding of the first European settlement at Sydney in 1788, though there is some evidence of their earlier introduction into the Cape York Peninsular, via the Torres Strait, by aboriginal traders from the Western Province of Papua New Guinea (Baldwin, 1983). In any event, feral pigs are now widely distributed in all states, including Tasmania, although populations are concentrated in the wetter areas of the Northern Territory and throughout the eastern states of Queensland and New South Wales (Tisdell, 1982).

From the late 18th century to the early 20th century, pigs were also introduced into many other locations in the Pacific and Southern Oceans. Lever (1985) listed such populations as occurring, at intervals, throughout the western Pacific and as far east as the Galapagos Islands, Ecuador, and the Juan Fernandez Isles, Chile. Pigs were first introduced into Tristan da Cunha some time before 1810 and onto numerous other widely separate locations in the South Atlantic Ocean, including the Falkland Islands (where they have since died out or have been eradicated) and in the French Antarctic Territories of Amsterdam Island, Ile Saint Paul and (the later named) Ile aux Cochon in the Crozet Islands in the south Indian Ocean. In quoting the narratives of 19th century seal hunters, Ross (1847) reported that in 1840 'Pig Island' was: "so overrun with these animals that you can hardly land for them"; and that: ..."in less than six years they had increased in an almost incredible manner, although great numbers are killed every year by the sealers, not only for their present subsistence, but salted down for supplies on their voyages to and from the Cape". In fact, these animals had evidently been introduced many years before 1834 (as implied by Ross) as they were recorded as being "very numerous and very ferocious", by Goodridge (1834) who was shipwrecked on the Crozet Islands in 1820. The subsequent history of this population, which has since disappeared, is unknown, although Gressit (1970) suggested that they may have been deliberately exterminated by later visitors on account of their ferocity.

Most if not all of these pigs were of S. scrofa-type and the human-induced expansion in the range of this species, and its domestic and feral derivatives, has resulted in its becoming the most abundant and widely distributed of all large mammals. The only major areas where these animals do not occur are the northern United States and Canada, the polar regions and most of continental Africa south of the Sahara. Their failure to become established as a naturalized form in the former areas can be attributed to extreme seasonal climatic conditions, although the evidence is less clear in sub-Saharan Africa, particularly since domestic pigs of scrofa-type are widely distributed there (Epstein, 1971). However, this region broadly equates with the distribution of the bushpigs, Potamochoerus spp, which are the analogues of Sus in this region. It is possible that Sus is competitively disadvantaged in this situation and/or poorly adapted to local conditions, though both introduced and feral populations of scrofa-type pigs have become established in parts of Sudan and the Republic of South Africa (Botha, 1989; Lever, 1985). The unfortunate recent introduction of European wild pigs, S. s. scrofa, and a variety of other animals (including both species of Tayassu peccaries), into the Wonga-Wongue Presidential Hunting Reserve in Gabon is of relevance in this context, as these animals are not only reported to be surviving but to have hybridized with native river hogs, P. porcus (Nicoll and Langrand, 1986). A similar cross between a male bushpig, P. larvatus, and an escaped domestic sow is also cited by Skinner and Smithers (1990), though it is not known if these hybrids are fertile.

Habitat, Ecology and Behavior

The importance of wild pigs as a basic resource lies as much with their remarkable adaptability as to the fact that they are one of the most prolific suppliers of meat for human consumption. They are clearly tolerant of regional climatic conditions ranging from sub-Antarctic to tropical and, given their highly omnivorous diet, they are also able to exploit a wide array of available habitats. In Jamaica, feral pigs presently survive in conditions ranging from xeric scrub-forest, salt marsh and mangrove woodland at sea-level to wet montane forest at elevations exceeding 2,000 m; these habitats also representing local climatic extremes in terms of average seasonal temperatures and annual rainfall (Oliver, 1984b).

S. scrofa-type pigs are also able to reproduce at a much higher rate than any other ungulates. They generally attain sexual maturity at age six months to one year, and they are relatively long-lived. Under favorable conditions, domestic and feral sows are polyestrous and often produce two litters of up to eight or more young per year. Dzieciolowski et al. (1992) recorded a mean litter size of only 6.2 amongst feral pigs in New Zealand, but have noted a third pregnancy within 12 months and that captive feral sows were capable of producing 3 litters in a 14 month period. Hunting pressure and infant mortality are the most critical factors in determining population growth, though Wood and Barrett (1979) estimated that (unless controlled) the feral pig population in the U.S. could be expected to increase by a factor of 33% per annum, even with a 90% mortality of young pigs, given a conservative average of two litters of five young each per year. Tisdell (1982) felt that this would be an underestimate if applied to Australia, and suggested that the feral pig population could be expected to increase by up to 60% per annum given the comparative remoteness of some areas and the general absence of effective predators apart from man. Either way, these rates are considerably in excess of that normally attained by native S. scrofa, which is generally limited to a single breeding season, whereas its descendant ferals often breed year-round in more equable climes.

In these circumstances, their potential for successful colonization is obviously phenomenal. Wild pigs first introduced into New Zealand in 1769, had become so well established soon after the turn of the century that they were considered a scourge by sheep farmers. By the 1860's, they had ..."accumulated in such vast numbers in uninhabited valleys that experienced pig hunters took out contracts for their suppression" (Hutton and Drummond, 1904). In the United States, it has been estimated that the total number of wild pigs was between 0.5 and 1 million animals in Texas alone (Jackson, 1964), and these animals (including introduced Eurasian wild pigs and their hybrid derivatives) now range over sixteen states (Mayer and Brisbin, 1991). These are harvested in some states where they have game status. In Florida, for example, an estimated 77,500 animals were taken in the 1976-77 hunting season, whilst 32,100 were taken in California during the same period (Wood and Barrett, 1979). In New South Wales, Tisdell (1982) has estimated that amateur hunters account for at least 150,000 feral pigs per annum, and that the kill rate for Australia as a whole is in the region of 0.5 million pigs per annum. Even so, this is probably only a fraction of the potential sustainable harvest of the Australian feral population, which has been estimated at between 5 and 10 million animals. A substantial international trade in the meat of animals killed in New South Wales and south-west Queensland has been therefore been developed to meet shortfalls in the gourmet demands for wild pork in Japan. In 1984, for example, this trade resulted in the export to Japan of more than 2 million kg of Australian feral pig meat, with a value at the point of export of AU$10 million (Takahashi and Tisdell, 1989).

The widespread introduction and naturalization of these animals has had a generally disruptive and negative impact on agriculture, forestry and, particularly, native wildlife. This has been especially true on oceanic islands with a high rate of endemism and where specialized forms have evolved in the absence of major predators and competitors. The degradation and extinction of faunal and floral elements in these fragile communities has often been attributed to direct or indirect destruction by pigs and diverse other exotic species (see below). Their impact on the ecology of continental habitats and native species is frequently less overt, at least in terms of attributable extinctions, but may nonetheless be significant and is invariably deleterious.

In the United States, feral pigs are regarded as livestock or game species by different agencies and interests, but the undoubted consensus of opinion is that they are a nuisance if not a major environmental pest. The U.S. Forest Service, for example, regards feral pigs as undesirable because they destroy pine seedlings and compete with native species for the mast crop. They also damage ditch lines, riverbanks, forest roads and recreation areas, and they can be dangerous to the public (Belden and Frankenberger, 1977). Similar biological and economic considerations have led the U.S. Fish and Wildlife Service to treat feral pigs as exotic pests and to carry out active control programs on many federal lands (Mayer and Brisbin, 1991). This agency has further identified pig damage to include: disruption of natural plant communities and successional sequences; damage to agricultural crops planted for wildfowl; predation upon various small vertebrates and the eggs and young of ground-nesting birds; and damage to fencing and water control structures (Thompson, 1977; Springer, 1977).

In temperate climates, feral pig diets vary seasonally, with the pattern described by Wood and Roark (1980) being typical of studied populations: fruits and mast, primarily acorns, being the most common food items in the autumn and winter, with new grasses becoming predominant in the diet in the spring and roots, tubers, various herbs and foliage becoming the most important foods during the summer months. In warmer and less seasonal climates, diets probably include a combination of all of these foodstuffs on a year-round basis, according to local availability. Nonetheless, their ability to eat practically anything not only results in direct competition with a wide variety of native wildlife species, but is also reflected in their being effective, if opportunistic, predators. Stomach content analyses of feral pigs in the U.S. have yielded remains of various birds, rodents, rabbits, fawns, piglets, terrapins, snakes, frogs, salamanders, small fish and a large number of invertebrates. The extent to which larger-bodied species are predated or eaten as carrion is not clear, although feral pigs have been observed feeding on the carcasses of adult deer, fawns, cattle and other pigs. On Auckland Island, off New Zealand, feral pigs are known to scavenge dead penguins, sea lions and even beached whales (C. Clarke, pers. comm.). In Argentina, feral pigs have even been reported to attack and kill newborn cattle (R. Lourival, pers. comm.), whilst in Australia these animals are now well known for their depredations on flocks of lambing ewes and bounties are paid for their control. Pavlov and Hone (1982), for instance, observed lambs to be killed in 10 of 42 observed chases, and that one individually identifiable boar was seen to kill five lambs during one lambing season.

As indicated in the latter example, such behavior tends to be confined to a few offending individuals in the population, and the consumption of vertebrate prey may be a relatively rare event in most feral pigs whose diet, under normal conditions, consists mostly of plant material in all seasons (Springer, 1977). Wood and Barrett (1979) also felt that wild pigs were essentially opportunistic predators, but that they could still pose a serious threat to some animal populations, particularly in concentrated nesting areas. Thus Merton (1977) records that feral pigs were so destructive to nesting seabirds on biologically important islands in New Zealand that they had to be eradicated by poisoning and hunting with trained dog packs. On Mona Island, Puerto Rico, Wiewandt (1977) found that predation by feral pigs of the nests of the threatened ground iguana (Cyclura stejnegeri) averaged 25% per annum, but rose to 100% in excessively dry years. Predation by feral pigs is also regarded as one of the principal threats to the nesting sites of various threatened species in the Galapagos Islands, including dark-rumped petrels (Pherodrama phaeopygia), green turtles (Chelonia mydas) and giant tortoises (Geochelone elephantopus ssp.). These pigs are also known to predate young giant tortoises (up to a weight of 25 lbs = 11.3 kg.), which has delayed restocking programs until hatchlings have achieved a good size (MacFarland and Reeder, 1975). Similarly, predation by introduced bushpigs, P. larvatus, is regarded by Juvik et al. (1981) as the primary threat to the endangered Malagasy tortoise, G. yniphora, and current plans to protect the most important remaining population site of this species in N. W. Madagascar will have to include measures to exclude these pigs (L. Durrell, pers. comm.). In the Mascarene Islands, feral pigs, together with a host of other introduced species, are implicated in the direct and widespread extinction and endangerment of most of the endemic avifauna, some of the herpetofauna, and have facilitated the spread of various exotic plants, such as guava (Psidium spp.) and brambles (Rubus spp.), which are transforming the little remaining native vegetation (C. Jones and W. Owadally, pers. comm.).

Position Statements on Introduced and Feral Pigs

In view of the essentially negative impact of almost all naturalized pig populations on their environments, the Pigs and Peccaries Specialist Group considers that:

1. Wild pigs or peccaries of any species or subspecies (or their domestic or feral derivatives) should never be deliberately released to range freely outside their known, recent and original distribution, and that all possible efforts should be made to prevent the accidental naturalization of domestic or wild populations of these animals.

The only, presently conceivable, exception to this might be in the event that such an 'introduction' was considered essential to the future survival prospects of a wild species or subspecies that was extinct or seriously threatened throughout its original known range; and: a) neither of the preferred alternatives of 'reintroduction' or 'translocation' were possible or practicable, and b) available information was sufficient to indicate that the any such introduction would not prejudice the survival prospects any other threatened organisms/communities which were native to the proposed introduction site.

2. That all existing naturalized populations should be regarded as exotic pests which should be controlled, reduced in numbers or eradicated wherever possible and appropriate; unless and excepting those (genetically isolated and threatened) populations which are considered to be of sufficient importance to warrant their continued in-situ conservation based on one or more of the following criteria:

a. Representation of otherwise extinct or endangered taxa – i.e. the population(s) is/are known (or considered likely) to represent the sole or otherwise critically important remnant(s) of a scientifically valid wild taxon which is endangered or extinct throughout its original known range.

b. Anthropogenic or socio-economic significance – i.e. the population(s) has/have particular religious or other cultural significance to local people and/or represent an essential basic economic (subsistence, rather than commercial or recreational) resource.

c. Unique genetic importance – i.e. the population(s) is/are known (or considered likely) to possess a unique genetic trait or traits of potential resource value for future husbandry or other legitimate scientific purposes. Evidence should exist to indicate that any such population has had an extended history of existence in the naturalized state (usually at least several hundreds of years), during which time it may be expected to have become adapted to local environmental conditions and should have been essentially free from genetic contamination through hybridization with other native, naturalized or domesticated forms.

It is also suggested that any threatened naturalized populations which fulfill criteria A) and/or B) (above) should be conserved in-situ, unless it is demonstrated that their continued presence is in conflict with other threatened, native taxa and communities; whereas naturalized populations which only fulfill criterion C) (above) should be conserved in-situ only if it can be demonstrated that their continued presence is not in conflict with other native taxa and communities.

Threats to the Survival of Selected Priority Populations

In accordance with the criteria and priorities stated above, the following introduced or feral populations are known to be threatened and are considered to be of sufficient importance to merit particular conservation attention:

1. The Buru and Sula Islands' babirusa, Babyrousa babyrussa babyrussa.

Status category: 5 (endangered). As previously stated, these animals, which are otherwise known as the 'golden' or 'hairy' babirusa on account of their unusually long (<5 cm) yellowish pelage, are known only from Buru and the Sula Islands of Taliabu and Mangole (Fig. 16). The available, if mostly circumstantial, evidence suggests that these populations were introduced, possibly from southern Sulawesi where the species is now extinct (Groves, 1980). Very little is known of their status on either Taliabu or Mangole, although recent reports and remains have confirmed their continued existence on these islands. If babirusa ever occurred on the only other large island in the Sula group, Sulabesi, they are now almost certainly extinct as this island has been virtually deforested. Their present status on Buru is also poorly known. Reports obtained in 1990 indicated that this population had declined in recent years, but that there were still 'reasonable' numbers in some parts of the island. A large area in the west of the island has been designated for protection, but logging in the north and the resettlement of immigrant people from Java in the east, are putting severe pressure on remaining habitat in these areas (also see Macdonald, this vol., section 5.8).

2. The Andaman and Nicobar Islands' feral pigs, 'ex-Sus scrofa'.

Status categories: Andaman Islands: 5 (endangered); Nicobar Islands ? (indeterminate). The wild pigs of the Andaman and Nicobar Islands comprise a small series of genetically isolated populations which, in terms of their diminutive size (35-40 kg) and various other diagnostic characters, are distinct from all other regional populations of S. scrofa derivation. The Andaman pigs have also diverged into at least two distinct morphotypes, i.e. 'long-snouted' and 'short-snouted' forms, which both occur on Little Andaman Island, though it is not known whether these are genetically dimorphic or sympatric. Unfortunately, however, it is doubtful if many of the original Nicobar Islands' populations still exist as pure-bred forms following importations of modern domesticates and the genetic continuity of the domestic and feral populations in the principal inhabited islands, where domestic sows are often mated to wild boars (Mathur, 1967). However, these populations are of extreme importance to the cultures and economies of these people (Mathur, 1967; Oliver, 1984a; Oliver et al., this vol., section 5.9), and it is possible that remnant, purebred populations of ancestral type survive on some of the smaller, uninhabited islands (R. Whitaker, pers. comm.).

Both the Andaman and Nicobar populations were generally assumed to be endemic, and they are accorded full legal protection by their Schedule 1 categorization under the terms of the Indian Wild Life (Protection) Act, 1972. In fact, both are certainly feral, though midden deposits in the Andamans indicate that they were introduced at least 2,000 years ago (P. Dutta, pers. comm.). During this period these pigs have evidently evolved as an integral component of these biologically important, but now seriously threatened insular ecosystems, which also include some of the most isolated tribal societies in the world. These are the Jarawa, Sentenilese and the (nearly extinct) Andamanese and Onges negritos, whose hunting cultures are linked intimately with the wild pigs, which are a primary food resource and may also have ritual and religious significance. Unfortunately, the negritos and their wild pigs are now seriously threatened by increased contacts with more recent immigrant groups, and high levels of deforestation from logging, agricultural encroachment and other developments (Whitaker, 1988). In addition, the wild pigs are subject to increasing pressure from immigrant poachers who use more efficient hunting techniques, including firearms, snares and dogs, as compared to the indigenous negritos with whom they are in direct competition for these diminishing resources. Recent proposals to promote the interests of the negritos have included suggested gifts of domestic pigs (Tewari, 1984), which could pose a further serious threat to the wild pigs through genetic contamination and/or disease (Oliver, 1984a,b).

3. Simeulue Island and other selected 'ex-Sus celebensis' ferals.

The revelation by Groves (1981) that the dwarf feral pigs of Simeulue Island (N. W. Sumatra), the so-called 'Sus mimus', are a highly modified form of S. celebensis, also provided clues to the origins of the island's human inhabitants, whose language is most closely related to Buginese or other south Sulawesi dialects. Shortly before the publication of Groves' review, MacKinnon (1981) reported that the Simeulue pigs were threatened, and urged that their taxonomic status be clarified with a view to the establishment of a suitable reserve in the event that they were found to be a distinct species. Although this is not the case, the Simeulue pigs are still of considerable scientific, especially anthropogenic, interest and they are of undoubted socio-economic importance to the present-day Simeuluese, whose forefathers were presumably responsible for their introduction. Therefore, despite their feral status, these pigs have been cited as one of the reasons for the creation of the proposed Pulau Simeulue Game Reserve (26,750 ha), which will also protect an endemic subspecies of macaque (Macaca fascicularis fuscus), and an important endemic avifauna and entomofauna (WCMC, 1991).

Other populations of wild pigs referred to as feral (and/or domestic) S. celebensis are known from the neighbouring island of Nias, and from Halmahera and Buru (Moluccas), and Flores, Timor, Lendu, Roti and Sawa (Lesser Sunda Islands (Groves, 1981; MacKinnon, 1981; Macdonald, this vol., section 5.7; Fig. 15). However, available data is insufficient to assess the present and possible future management needs of most these populations, or their socio-economic significance to local people.

4. Ossabaw Island feral pigs, 'ex-S. scrofa'.

Unlike all of the aforementioned examples, the dwarf feral pigs of Ossabaw Island, S. E. USA, are of far more recent origin (i.e. <400 years) and they are of little or no ethnic or socio-economic significance. However, in view of their genetic isolation (Smith et al., 1980) and their accessibility for detailed study, they are of considerable interest to basic and applied research in a number of fields, including reproductive biology, biochemistry and endocrinology, as has been detailed elsewhere (Brisbin, 1989; Mayer and Brisbin, 1991; Miller and Hedrick, 1991).

However, Ossabaw Island is now part of the State of Georgia's 'Heritage Trust Program', and the continued existence of these animals as a free-ranging population must be contingent on their numbers being managed to ensure that they do not pose a threat to any of the island's native ecosystems and species, especially the nesting sites of loggerhead turtles (Caretta caretta). Present control measures should therefore be intensified with a view to the removal of these animals from all ecologically sensitive areas or to some other location. Currently (1991), between 50-100 captive Ossabaw pigs are held by private owners and public institutions in the U.S., though this stock is derived from only 14 founders (Mayer and Brisbin, 1991), and a studbook is maintained by the American Minor Breed Conservancy (Heise and Christman, 1989).

With the exception of the Ossabaw Island pigs, all of these populations are threatened by habitat destruction and, to varying degrees, by increased hunting pressure and/or hybridization and disease contagion from more recently imported domesticates. Available information suggests that they are each of sufficient biological or anthropological importance to warrant special efforts being made to protect representative populations in-situ and, indeed, that the existence of some of these populations has often been cited as a reason for the establishment of protected areas within their respective ranges, but that existing or proposed reserves within these ranges are inadequate, or inadequately protected, at the present time. By comparison, the Ossabaw pig is not threatened at present, but its future is uncertain unless the population is confined or managed to minimize its impact on the ecology of this island.

Within the context of this review, it is clear that the Buru and Sula Islands' B. b. babyrussa, merits particular concern in that it is not only recognized as a distinct taxon, but that available data indicate that it also represents an otherwise extinct form of a globally threatened species of considerable interest and conservation concern. All of the other, currently prioritized forms are of relatively ancient derivation and now exist as pure-bred feral and/or domestic populations which, by virtue of their small body size and various other characters, are of potential importance as genetic resources for the further domestication of one of man's most important sources of animal protein. All of the S. E. Asian populations of Sus are also of particular socio-economic importance to surviving tribal societies. These considerations involve a range of issues of relevance to a wide variety of scientific disciplines, including anthropology and archaeozoology, biogeography, animal genetics and husbandry, and conservation biology, as well as basic and applied research interests in the physiological and biomedical sciences. However, further research is required on almost all of these topics before the resource potential and future management needs of these populations can be properly assessed and appropriate conservation measures implemented.

Conservation Measures Proposed:

An Action Plan for Selected Populations

As previously emphasized, the overwhelming majority of naturalized populations of pigs should be regarded as exotic pests, which should be managed on a 'damage limitation' basis and strictly controlled, reduced in numbers or eradicated, wherever possible or appropriate. Those (few) selected populations described above (or others not yet identified which may meet the criteria suggested herein) should therefore be regarded as exceptions which prove this rule. Conservation measures directed towards these few populations should, where possible, take into account any cultural and economic requirements of legitimate ethnic groups, and these populations should continue to be managed in-situ, providing their continued presence is not in conflict with the survival prospects of other threatened and native species or natural communities. In the latter event, the protection of threatened native species and communities should take precedence over those of threatened naturalized populations which should, if possible, be relocated to less sensitive environments and/or managed as captive populations. In accordance with these criteria the following actions are recommended for the aforementioned selected populations:

Priority Projects:

1. Buru and Sula Islands' babirusa, B. b. babyrussa.

Field surveys of Buru, Taliabu and Sulabesi (Sula Islands) are required, perhaps as a matter of some urgency, in order to determine the current distribution and conservation status of this taxon, and to explore options and possible locations for longer-term field studies. Data on hunting and other threats, such as logging and agricultural encroachment, should be collected and analyzed with a view to an assessment of their magnitude and the development of recommendations for the enhanced protection of representative populations on each island. Strong support should be given for the early establishment of the proposed reserves on Buru and Taliabu, and the possibility of establishing a similar reserve on Mangole should be investigated. The neighbouring island of Sulabesi should be visited to ascertain whether any babirusa survive there. The possibility of setting-up a properly structured captive-breeding program should be investigated, given that the available data suggest that this is the most threatened subspecies of babirusa, as well as the least known and most distinct morphologically. (Also see Macdonald this vol., Section 5.8).

2. Andaman and Nicobar Islands' wild pigs.

High priority should be given to a joint zoological - anthropological survey of all islands known to support populations of wild pigs, with a view to: a) ascertaining their present (and, where possible, recent) distribution and status; b) the nature and relative degree of threats to their survival; c) conducting interviews with hunters and tribal leaders concerning hunting methods, numbers of animals killed, their economic importance, traditional beliefs and cultural practices associated with wild (and/or domestic) pigs, and collection of anecdotal data on the biology and ecology of the wild pigs; d) collection of hard and soft tissue specimens from hunter-killed animals for analyses of their regional genetic variation and systematic affinities, and e) the development of longer-term studies of the ecology and behavior of these animals in selected (comparative) locations and, especially, their importance to the culture and economies of the various tribal groups of these islands. Recommendations for the enhanced future protection of representative populations of wild pigs, based on these findings, should be given high priority, but any such recommendations must take the legitimate rights and needs of the original human inhabitants into account, as well as the possible negative impact of these pigs upon other threatened endemic species.

3. Simeulue Island pig and other feral S. celebensis.

Field studies and surveys are required on Simeulue, Nias and other selected locations, especially in the Moluccas (e.g. Halmahera) and the Lesser Sunda Islands (e.g. Timor, Flores, Lendu, Roti and Savor), in order to clarify the distribution, status and systematic affinities of the various domestic and feral populations of S. celebensis origin in these areas. Particular emphasis should also be placed on anthropological components of these surveys, which should include investigations of possible relationships between the ethnic origins of local tribal groups and the distribution patterns amongst wild pigs of varying derivation, the socio-economic importance of these animals to these people, and their cultural traditions with respect to the hunting and husbanding of pigs and other livestock. These pigs are also of interest in genetic resource terms, and the introduction of 'improved' breeds should be actively discouraged in all areas where purebred, ancient domestic or feral populations survive. Support should be given for the early establishment of protected areas within the ranges of these forms, but management plans should also address the possibility that feral pigs may need to be actively managed to control population numbers in well-protected sites and/or remove these animals from ecologically sensitive areas.

4. Ossabaw Island feral pig.

Although this population is of sufficient importance to merit efforts being made to conserve it, it should not be allowed to range freely in any areas where its presence may impact negatively on the island's native ecosystems and species. Further research is required on the impact of these pigs on these communities, but they should certainly be excluded from all ecologically sensitive areas. This could be achieved by setting up fenced exclusion zones or confining a reduced population of pigs in a least-sensitive area. Unfortunately, Ossabaw pigs are also known to carry pseudorabies and vessicular stomatitis, which may negate the possibility of translocating a wild (sub)population to the mainland and/or acquiring additional founders for the existing captive population. Current studies of the epidemiology of vessicular stomatitis on the Island, and the testing of procedures for the use of a vaccine to reduce the prevalence of pseudorabies in these animals should therefore be continued.

Acknowledgements

Our thanks are extended to the many authors whose original works have been ruthlessly plagiarized, especially those of Colin Groves and John MacKinnon whose lucid accounts of previously confused topics merit particular acknowledgement. We are grateful to Paul Vercammen and Peter Cuypers for preparing the range map, and to Dr. John Mayer, Colin Clarke, Dr. Juliet Clutton-Brock and Dr. Alastair Macdonald for their useful comments on earlier drafts of this text. The support provided by the Jersey Wildlife Preservation Trust and the University of Georgia (under contract No. DEAC09-76SR00-819 to the United States Department of Energy) during the preparation of this review is thankfully acknowledged.

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