Pig (feral or wild)
|Common name:||pig (feral or wild)|
|Scientific name:||Sus scrofa|
This species is an established pest animal in the state of Victoria.
Europe and Asia
History of spread
The feral pig in Australia is a descendant of various breeds of Sus scrofa, the domestic pig. The pig was first introduced into Australia by the First Fleet in May 1788 when 49 pigs were brought into Sydney as a food source. Pigs were subsequently allowed to roam and by the 1880s had run wild in NSW.
Once established in the wild, populations of feral pigs rapidly increased and dispersed into suitable habitat, usually following watercourses and floodplains. Pigs spread widely and rapidly throughout the northern and eastern regions of the country, causing considerable damage wherever they became established.
Victoria, however, did not have feral pig problems until fairly recently, having first reported activity in the north of the state in 1959. It is thought that feral pigs were illegally imported into Victoria and deliberately released. Once pigs established in the floodplains bordering the Murray River, they were able to breed and disperse freely and by 1962, had become a problem in several areas.
Distribution in Victoria
The majority of feral pigs in Victoria are found in isolated populations at various locations along the Murray River and also near Mansfield, Kinglake, the Central Highlands and Grampians.
Feral pigs in Australia have a different appearance to domestic pigs. Feral pigs are smaller, leaner and more muscular than domestic pigs. Feral pigs have well developed shoulders and necks and smaller, shorter hindquarters than domestic pigs. Feral pig hair is more sparse, longer and coarser than domestic pigs. Feral pigs also have longer, larger snouts and tusks, smaller ears and much narrower backs. The tails of feral pigs are usually straight with a bushy tip rather than curly as is the case with domestic pigs.
Males and females differ in size and weight. Male feral pigs tend to be longer and taller than females, have larger heads, and are up to 10-20 kg heavier when one year old. The weight of an average adult female feral pig is 50-60 kg, with males usually weighing 80-100 kg. However this weight can vary with habitat conditions and exceptional animals have reached up to 260 kg.
Feral pigs vary in size, shape and coat colour. Differences are inherited from the various breeds which initially escaped or were released into the wild. Black is the most common coat colour for feral pigs, however a high proportion are rusty red and of mixed colours including white, light ginger, brown and white, brown with black spots, brindle and agouti (brown or black with a lighter tip). Some piglets are marked with dark longitudinal stripes, which disappear as they grow older, and some boars develop a crest or mane of stiff bristles extending from their neck down the middle of their back, which stands straight on end when the animal is enraged.
Pigs have small eyes and poor eyesight, however their senses of smell and hearing are well developed. Continuously growing canine teeth (tusks) of adult male pigs are large and protrude from the sides of the mouth. The lower tusks curve upwards and backwards, forming an arc. Their total length is up to 30 cm but up to 80 per cent is embedded in the lower jaw. The upper canines are considerably shorter; up to 9 cm long. They curve outwards and back functioning as grinders to the lower tusks.
Feral pigs restrict their activity to the cooler parts of the day at dawn and dusk and are primarily nocturnal. Feral pigs are shy animals and will avoid humans, making it easy to overlook their presence or drastically underestimate their numbers.
Feral pigs have a defined home range and regularly make use of trails between areas they use which provide shelter, food and water supplies. Under normal conditions the home range of adult males is 10-50 sq km, dry sows 10–20 sq km and lactating sows have restricted ranges of less than 5 sq km. When a feral pig population has an adequate food supply it is unlikely individuals will travel more than 5 km outside that area except when mature crops are on offer.
Pigs must have daily access to water and wallowing sites. Feral pigs may also use sheep and cattle pads to travel to and from water which they often mark by rubbing and tusking trees or logs, and wallowing in mud or dusty depressions. Both of these habits also help to reduce infection, while mud and dust helps to regulate body temperature.
Boars over 12 months of age are 10-12 kg heavier than sows and have thickened cartilaginous shield under the skin that protects the shoulders and ribs during fighting.
Mob size varies depending on the season and habitat. In forested areas a group of feral pigs will rarely exceed 12 members, while in more open country up to 40-50 pigs may form a herd. In times of drought, up to 100 feral pigs may gather around a single waterhole.
Feral pigs have high protein requirements and starvation can affect feral pigs of all ages. Lack of appropriate nutrients can affect lactating sows and hence the nourishment of their young. Excessive tooth wear in older pigs can interfere with eating.
Like domestic pigs, feral pigs require a diet high in protein and energy in order to breed successfully. Diet is particularly important during pregnancy and lactation. Pregnant sows will often relocate within their home range to meet these dietary requirements.
Feral pigs will feed on almost anything and will switch food preferences depending upon availability. Succulent green vegetation is the food item of choice, however feral pigs will eat fruit, grain, and a wide variety of animal material such as frogs, fish, reptiles, birds, small mammals and carrion. Feral pigs are known to predate on lambs and occasionally newborn calves. They will also eat underground plant material such as roots, bulbs, corms and fungi.
Shelter is important for feral pigs, in particular, shade and protection from predators. Feral pigs prefer to live in moist environments that can provide a reliable and adequate food and water supply.
In Australia feral pigs are found in a wide range of habitats that can provide these requirements, including rainforest areas, monsoon forest, paperbark swamps, open floodplains, marsh areas, semi-arid floodplains, dry woodlands and subalpine grasslands and forest.
Because feral pigs need to drink daily in hot weather they are generally not found in the dry inland areas of Australia.
The piglets of feral pigs are prone to predation by dingoes, wild dogs and large birds of prey.
Diseases and parasites
Feral pigs are generally more susceptible to disease and parasites as a result of inadequate nutrition. Feral pigs can be hosts or vectors of a number of endemic parasites and diseases, some of which can affect animals or people. For example, a number of worm species carried by feral pigs can affect livestock.
Under favourable conditions feral pigs can breed throughout the year in Australia, although breeding success depends on the quality and quantity of food available.
Sexual maturity in sows is dependent on weight rather than age, similar to domestic pigs. A sow may begin breeding from about six to eight months of age, when her weight exceeds about 25-30 kg. Sows have a 21 day oestrus cycle and a gestation period of 112-114 days. Average litter sizes vary from 5-6 piglets, but up to ten piglets can be born under good conditions.
Just before giving birth, sows will make a nest using available vegetation. The nest is usually less than 2 km from available water and piglets normally spend the first one to five days of their life inside the nest, with the sow inside or close by.
A feral pig litter is weaned after two or three months at which time mating can occur again. Under favourable conditions two litters can be weaned in a period of 12-14 months, thus feral pigs can multiply at a very fast rate. This breeding capacity allows feral pigs to quickly recover from natural setbacks or control programs, and to rapidly increase their populations in periods of favourable conditions.
A feral pig's home range is very much dependent on gender, habitat type, weather conditions and food and water availability.
Feral pigs are capable of migrating considerable distances but on a daily basis their range tends to be quite small. Home ranges vary from as little as 0.16 sq km for sows with young piglets to greater than 40 sq km for individual boars in semi-arid rangelands.
Watering points are the focus of activity for feral pig movements. In hot weather pigs may spend daytime in one area and night time spent feeding in another. In some habitats there is a seasonal trend of movement between specific areas where feral pigs will readily swap between food sources so that excessive movement is not required.
Feral pigs generally do not move very far in response to minor disturbance, including infrequent hunting by people, and usually return to their home ranges shortly afterwards. Feral pigs can shift permanently to more remote areas. For example, they can move up to 5 km away in forest if subjected to intensive or prolonged disturbance, such as large-scale hunting or other control activities.
Home ranges of individuals and groups overlap considerably. There is no evidence that feral pigs, of either gender, actively defend territories.
The biology and ecology of feral pigs are important characteristics making them highly successful pests. Their large strong bodies, snouts specifically developed for rooting up ground, omnivorous diet and adaptive activity patterns allow them to thrive in a wide range of habitats.
Mortality of feral pigs, particularly in the first two to three months of their life, is high and will vary from 10-100 per cent, depending on seasonal conditions. Mortality may be due to abortion, adverse weather conditions, accidental suffocation by sows, loss of contact with mother, predation and starvation.
Mortality of juvenile feral pigs can be high if the mother's dietary protein intake is low. Adult mortality does not vary as much with seasonal conditions, though few animals live longer than five years.
Impact on ecosystems and waterways
Feral pigs are considered an environmental pest due to their selective feeding, trampling and rooting for underground parts of plants and invertebrates, as well as predation on, competition with, or disturbance of a range of native animal species. Feral pigs are known to prey on earthworms, insects, amphibians, reptiles, ground-nesting birds, small mammals, freshwater crayfish, frogs, turtles and their eggs.
Feral pig activity also has a dramatic effect on watercourses and swamps. By wallowing and rooting around the waterline, they destroy the riparian vegetation which provides food and nesting sites for native wildlife and helps to prevent soil erosion. Water quality is also affected and their diggings may spread undesirable plant and animal species, and plant diseases in these areas.
Agricultural and economic impacts
Feral pigs cause significant damage in agricultural areas. They prey on newborn lambs, compete with livestock for pasture and can damage infrastructure such as fencing and water facilities. Feral pigs can reduce yields in grain, sugarcane, fruit and vegetable crops, through uprooting and consuming or trampling of plants, and their wallowing and defecation foul dams and waterholes. Feral pigs can also transport weeds, and their diggings provide ideal conditions for weed establishment.
Feral pigs are known to be vectors for a number of serious endemic and exotic diseases that have the potential to devastate commercial pig operations, as well as transmit to other animals and humans. Examples include foot and mouth disease virus, leptospirosis, brucellosis, melloidosis and Japanese encephalitis.
Impact on social value and health
Human health can be affected by the following: leptospirosis, through contact with the urine of infected feral pigs; porcine brucellosis, through handling raw feral pig meat; and tuberculosis and sparganosis, through eating inadequately cooked feral pig meat.
Recommended control measures
The Department of Economic Development, Jobs, Transport and Resources (DEDJTR) recommends integrated pest management using all available control measures implemented in a coordinated manner at a landscape scale.
- Animal Control Technologies 2004, Feral Pig Problems in Australia, Animal Control Technologies, viewed online 16th March 2010 at URL: http://www.animalcontrol.com.au/pig.htm
- Choquenot, D, Mxllroy, J and Korn, T 1996, Managing Vertebrate Pests: Feral Pigs, Bureau of Resource Sciences, Australian Government Publishing Service, Canberra.
- Croft, D 2007, Vertebrate Pest Control Manual: Feral pig biology and control. State of New South Wales, NSW Department of Primary Industries.
- Department of the Environment and Heritage 2004, The feral pig (Sus scrofa), fact sheet, viewed 16th March 2010.
- Invasive Animals CRC, The feral pig (Sus scofa), viewed 16th March 2010.
- Land Protection (Invasive plants and animals) 2008, Feral pigs in Queensland – distribution, ecology and impact, fact sheet, The State of Queensland, Department of Primary Industries and Fisheries.
- McGaw, C.C and Mitchell, J 1998, Pest Status Review Series: Feral Pigs (Sus scrofa) in Queensland, Department of Natural Resources and Mines, QLD
- Mitchell, J 2008, Feral Pig Control – A practical guide to pig control in Queensland, The State of Queensland Department of Primary Industries and Fisheries.
- Queensland Government Department of Natural Resources and Mines 2005, A guide to pest animal management in Queensland – Vertebrate pest manual, The State Government of Queensland, Queensland, Australia.
- Regan, K 2002, Landcare Notes: Feral pigs in Victoria, LC0306, Department of Primary Industries, State Government of Victoria.