Background

Origin

The feral pig in Australia is a descendant of various breeds of the domestic pig, Sus scrofa.

Pigs arrived with the first fleet in 1788 and were the only stock that thrived in the early years of settlement. By 1795, free-ranging pigs were such a nuisance that an order was issued allowing landholders to shoot any pig found on their property. Nonetheless, pigs were generally kept in an unrestrained, semi-feral state until the 1860’s when property fencing started to become widespread. These conditions allowed feral populations to establish in many areas.

By the 1880's feral pigs were widespread throughout much of NSW. Extended periods of above-average rainfall allowed pigs to disperse along water courses into areas where they had not previously been introduced, particularly into western regions of the state. Historical distribution maps suggest that pig populations have continued to spread throughout the country since the 1950's.

Distribution

Today, feral pigs are widely distributed in NSW, Queensland, the Northern Territory and the Australian Capital Territory. Their natural spread and persistence in the landscape has been assisted by illegal, deliberate releases and translocations. Isolated populations also occur in Victoria, South Australia and Western Australia, and on Flinders Island in Bass Strait. Many of these populations are continuing to expand.

In NSW feral pigs are found in most regions west of the Great Dividing Range; including the tablelands. High densities of feral pigs are found in western NSW floodplains and ephemeral wetlands as well as the northern slopes and plains.

Ecology and biology

Habitat

Feral pigs are remarkably adaptable and are able to thrive in a wide range of environmental conditions and take advantage of diverse food resources. They occur in tropical and temperate rainforests, monsoon forests, paperbark swamps, open floodplains, marshes, coastal fringes, semi-arid floodplains, dry woodlands, subalpine grasslands and agricultural systems such as cereal and cotton crops. However, pigs have limited ability to reduce their body temperature. Because of this, they require regular access to free water and shelter, particularly in hot conditions.

Diet

Feral pigs are opportunistic omnivores – their nutritional needs can be met from a wide range of food sources. However, they are not very efficient grazers so they require higher quality feed than ruminants such as cattle, sheep and goats. Sows, in particular, require a consistent high-protein food source, such as animal material or green pick, for successful breeding and rearing of young. Nutrient- and energy-rich foods such as succulent green vegetation, fruit, grain, bulbs, corms, fungi and animal material are often consumed preferentially when they are available.

Home ranges and movement

Not surprisingly for an animal that exploits such a wide range of habitats, home range and movement patterns vary widely across different regions and environmental conditions. A pig’s home range can be considered as the area travelled by an individual during its normal activities of foraging, mating and caring for young. Home range size is largely determined by habitat type, food supply, the size of individual animals and population density. Daily home range sizes are often quite small, although long-term home ranges may be much larger. Mature males tend to have a larger home range than sows in the same population. Pigs in semi-arid or sub-alpine areas tend to have larger daily home ranges than those in areas where food resources are more abundant.

Feral pigs are largely sedentary; it is a common misconception that they make frequent long-distance movements between different habitats. During hot weather, days may be spent in one area and nights spent feeding in another nearby. In some habitats there is a seasonal trend of movement between specific areas, depending on the current food supply. However, their ability to exploit a wide range of food sources often allows pigs to adapt to changing conditions and avoid having to move to other locations.

Feral pigs generally restrict their activity to cooler parts of the day, even when the weather is not especially hot. Very hot weather can greatly restrict their movements, resulting in lower food intake, reduced mating opportunities and reduced visibility in the landscape to casual observers.

Pigs often use trails to travel between regularly visited areas, such as from shelter to food supply or water. They also use sheep and cattle pads to and from water. Trails that are frequently used by pigs can often be identified by the presence of tracks, dung and trees or logs that have been rubbed or tusked by pigs. Trails such as these can serve as markers to other pigs, indicating routes to food or water.

Social structure

Sows and piglets generally run together as a group. Immature males and females may also stay with the group until they reach maturity, or they may run as a juvenile group until they mate. At about 18 months males tend to become more solitary, re-joining a group only for mating or to feed on localised food sources.

Group sizes vary depending on the season and habitat. In forested areas of south-west WA, group sizes rarely exceed 12, whereas in more open country up to 40 or 50 pigs may form a mob. In times of severe food and water shortage, large groups of 100 or more may gather around the remaining waterholes.

Reproduction

Feral pig breeding is heavily influenced by the amount and type of feed that is available. Pigs are able to breed throughout the year in many areas, but breeding may be distinctly seasonal in others where food is limited. Breeding success depends on the availability of nutrients, in particular energy and protein. Peak birthing periods usually coincide with the seasonal abundance of food in different habitats.

Female feral pigs reach sexual maturity once they reach a weight of 25 to 30 kg, which normally occurs between 7 and 12 months depending on conditions. Males become sexually mature around 18 months. Average litter sizes are usually about four to six piglets. The abundant high quality feed allows pigs to increase their reproductive output. It reduces the age at which sows can start breeding, reduces the interval between breeding events, and increases litter sizes. Conversely, during poor conditions, pigs are able to reduce their breeding activity and invest energy in survival instead of reproduction. The result of this flexible reproductive strategy is that pig populations can persist in the landscape through extended unfavourable conditions, and then increase rapidly when conditions improve again, through increased reproductive output and greater survival and recruitment of young pigs into the breeding population.

Mortality

Mortality of feral pigs from the foetal stage to weaning is generally high but can be highly variable depending on local conditions. Most pigs born will never reach their first birthday. Mortality is due to factors such as adverse weather conditions, accidental suffocation by sows, loss of contact, predation and starvation. Adult mortality can vary from 15% to 50%. In most areas, few feral pigs are likely to survive beyond five years.

Starvation can affect pigs of all ages: lactation of sows can cease if protein levels are not adequate and excessive tooth wear in older pigs can interfere with eating. Malnutrition also leaves feral pigs more susceptible to parasites and diseases.

Feral pigs occupy a wide range of habitat types in Australia (Photos Andrew Bengsen and Troy Crittle).

Impact of feral pigs

Agricultural impact

Feral pigs prey on newborn lambs and goats. Crop destruction through feeding and trampling reduces yields in summer and winter grain crops, sugarcane, fruit and vegetable. Fences and water sources can be damaged and dams and waterholes fouled through wallowing and defecation. Feral pigs also compete with livestock for pasture and damage pasture through up-rooting vegetation.

Environmental impact

Feral pigs disturb natural environments through rooting up soils, native grasses and forest litter, consuming a range of native plants and fouling freshwater systems. Feral pigs also eat a range of native animals including, earthworms, beetles, centipedes, amphipods, snails, frogs, lizards, snakes, turtles and their eggs and small ground-nesting birds and their eggs. Environmental impacts of feral pigs are so serious that they are listed as a Key Threatening Process under the NSW Threatened Species Conservation Act 1995. At the time of writing, 53 threatened species, populations or communities were listed as threatened by predation, habitat degradation, competition or disease transmission from feral pigs.

Disease impact

Feral pigs can be hosts or vectors of a number of endemic parasites and diseases, some of which can affect other animals or people. Livestock health can be significantly affected by:

• leptospirosis
• porcine brucellosis
• melioidosis
• tuberculosis
• sparganosis
• porcine parvovirus
• Murray Valley encephalitis and other arboviruses.

For example, leptospirosis can cause illness, abortion and death in sheep and cattle. Surveys in the Central West region of New South Wales showed that about three quarters of feral pig mobs tested for the presence of one of the main bacteria that cause leptospirosis had been infected at some stage, and about 30% of those mobs included individuals that were actively shedding bacteria that could be picked up by sheep or cattle. About 60% of sheep flocks and 50% of cattle herds also tested positive for exposure to the bacterium.

A number of worm species also carried by feral pigs can affect livestock.

Feral pig with sheep carcass (photo by Peter O'Brien)

Human health can be affected by:

• leptospirosis, through contact with the urine of affected feral pigs
• porcine brucellosis, through handling raw feral pig meat
• tuberculosis and sparganosis, through eating inadequately cooked feral pig meat.

Feral pigs are also susceptible to and can be hosts or vectors of, a number of exotic parasites and diseases. Feral pigs would also be capable of carrying and spreading a number of exotic diseases and parasites if these were to enter Australia. These include:

• foot-and-mouth disease
• swine vesicular disease
• African swine fever
• Aujeszky's disease
• trichinosis
• classical swine fever.

Social impact

Feral pigs can impact the well-being of individuals and the community. Negative impacts include human-disease spread, damage-related stress and damage to visual amenity.

However feral pig hunting can generate positive social impacts through the contribution of hunting-based tourism to local communities and hunting as a recreational activity.

Recognition and signs

Feral pig damage around a freshwater lagoon (Photo Jim Mitchell)

There are a number of signs to indicate feral pig activity and abundance in an area. Regular sightings of pigs and abundant fresh sign (tracks and scat) normally means high numbers of feral pigs; some sightings of pigs and obvious fresh sign indicates medium numbers of feral pigs; no or few sightings of pigs and very little fresh sign often indicates low numbers of pigs.

Camera traps are a useful way of detecting the presence of feral pigs and can give an indication of group size and activity patterns. If used intensively and systematically as part of a carefully designed survey, camera traps can be used to estimate local changes in feral pig numbers. Pigs can also be detected by using spotlight observation at night, although they do not have reflective eye shine, and by aerial survey. Trained dogs can be used to detect feral pigs at low densities or in thick cover.

During daylight, many feral pigs shelter in deep cover and are rarely seen. The presence and number of pigs are more reliably evaluated by observing signs of their activity and impact. An experienced observer can rapidly estimate the presence and coarse relative abundance of feral pigs by carefully examining these signs.

Rooting: Feral pigs use their snouts and teeth to dig for underground food, including small animals and tubers, particularly where the soil is soft or after rain. The result varies from selective uprooting of specific types of plants to the creation of extensive areas resembling ploughed paddocks. The distribution of rooting areas is a reliable guide to the location of pigs at night.

Crop damage: Feral pigs damage crops by eating them, by trampling and bedding in them, and by uprooting seed and seedlings. Significant damage over a short period of time can be seen during crop planting, especially in legume and pulse crops such as chickpea and faba beans. Damage is also common at the end of the growing season, as crops mature and protein content of grain increases; improving palatability.

Fence damage: Pigs will push through fences, usually enlarging an existing gap under the bottom wire; these holes are then used by other animals. Mud or coarse bristly hair on the wire or post indicates feral pigs.

Pads: Pigs often create pads when travelling in single file to frequently used food and water sources. Pads can be a reliable sign of feral pigs and are often an effective way of identifying feral pig bedding and feeding areas.

Tracks: Feral pigs leave hoof-prints in any soft surface. Their tracks have a distinctive square shape and can often be differentiated from other cloven hooved animals by the presence of two widely-separated dew claws.

Faeces: Pigs defecate on and off pads. The size, shape and consistency of the scat varies with age and diet, but it is typically 3 to 6 cm wide, 7 to 22 cm long and fairly well formed. Close examination will reveal finely chewed plant matter, grain and occasional bone fragments, pig bristles, wool or other hair.

Tusk-marks: Adult boars slash the trunks of growing trees with their tusks, leaving a distinctive pattern of cut-marks. The trees selected for cutting and rubbing are often next to pads and near water. Because boars stand on toes and reach up when tusk-marking, the height of the mark can be a guide to the size of the pig. Marking may serve to notify other boars of the marker’s presence and size.

Nests: Just before farrowing, sows make nests from the available vegetation, which they uproot and carry by mouth. If long, grassy vegetation is plentiful, the nest can be very large – up to 3 m by 1.5 m and 1 m high, with a domed roof. For the first 1 to 5 days of life, the piglets stay in the nest and the sow is usually also inside or nearby. In cold temperatures, boars and non-farrowing sows may also build and use nests.

Wallows: Pigs wallow by lying in moist or wet areas, often near permanent water. Wallowing may help to control the animal’s temperature and protect it against insects and external parasites. Wallows are distinctive oval depressions in mud and can show how recently and for how long pigs have been in the area.

Mud-rubs: After wallowing, pigs often rub their heads, shoulders and sides on nearby vertical objects such as tree trunks and fence posts. The result is a distinctive muddy rub site at pig height. Thick coarse hairs are often found embedded in mud-rubs.

Distribution map 2009

Click map for larger view (PDF, 256.4 KB)

Distribution map 2016

Click map for larger view (PDF, 441.4 KB)

Feral pig distribution and abundance 2020

Larger image on a new page is available to view

Feral pig distribution change from 2016 to 2020

Larger image on a new page is available to view

Other publications

• Feral pig control
  
• PestSmart Feral pigs
  
• FeralPigScan