About this pest
Distinctive features: Smaller and more muscular than domestic pigs, and they have a longer snout that is flattened at the end. Their forequarters are larger than their hindquarters. The tail is straight with a bushy tip (rather than curly like a domestic pig). Their back is also narrower than that of a domestic pig (‘razorback’).
Size: Males (boars) weigh 45-205 kg; females (sows) weigh 32-114 kg. Boar body length ranges between 1100mm and 2280mm, and their height ranges between 550mm and 960mm. Sow body length ranges between 1140mm and 1500mm, and their height ranges between 430mm and 600mm.
Droppings: Dark flattened, oval pellets, joined together in a cylinder shape.
Footprints: Rounded, cloven hoof prints. Marks from the dew claws may show behind and to the side of the main digits.
Kill signs: Crushed shells of large land snails (Powelliphanta spp.), bird skins turned inside out, lambs turned inside out with skins attached to hocks.
Vegetation damage: Rooted up pasture, bracken, or forest floor. Tussock, fern, or sedge turned over.
Eye shine: Hardly any.
Distribution: Widespread in both North Island and South Island, and currently occur on 13 offshore islands. They were eradicated from Stewart Island by 1965.
WHY ARE PIGS PESTS?
Feral pigs dig in the ground with their nose to search for food (rooting). The extent of rooting can be quite severe, and in some areas where numbers of feral pigs are high, they can uproot most of the ground surface, as well as lower stature vegetation. This can affect the forest ecosystem because plants and invertebrates (such as earthworms) are being eaten. Pigs can also compete for food with native animals and pig rooting could damage kiwi feeding areas. Soil disturbance can encourage regeneration of beech tree species, but constant disturbance prevents this. Although the full extent to which feral pigs are damaging the New Zealand environment is unclear, pig rooting has the potential to impact long-term ecosystem processes such as nutrient cycling and the composition of plant species in forests. Feral pigs are also a significant predator of New Zealand’s native land snails (Powelliphanta spp.), earthworms can comprise a significant proportion of feral pig diet (from 10-26% dry weight), and it is believed that control of pigs is critical for the long-term survival of mainland populations of the threatened carabid Megadromus speciosus. Pigs also eat other native wildlife, such as ground nesting birds and their eggs, and lizards and frogs, although adverse effects have not been well quantified in New Zealand. In the U.S.A they have been identified as a predator of lizards and amphibians.
The ecological impacts of pigs have been more obvious and pronounced on offshore islands where there are less changes caused by humans. On the Auckland Islands, feral pigs caused severe damage to endemic large-leaved mega-herbs and to seabird colonies. Their densities there have now been greatly reduced through culling, but they continue to impact the tussock (Chionochloa antarctica) and if they continue unchecked, may eliminate it from low-altitude parts of the islands. On Aorangi Island (in the Poor Knights group), forest regeneration and re-colonisation by seabirds which used to live there was documented following the eradication of feral pigs.
Feral pigs can damage pasture and crops and kill lambs and cast sheep. They have also been known to dig up and eat the roots of young trees on forestry plantations. These impacts are usually quite localised, and are worse on farms surrounded by native forest or scrub. Feral pigs can also carry bovine tuberculosis, but are thought to be spill-over hosts i.e. the disease will not persist in feral pig populations alone if the disease is eradicated from possums in the area (possums are considered a reservoir host).
Pig droppings are dark, flattened, oval-like pellets joined together in a cylinder. If the pigs have been feeding on coarse vegetation, then the cylinders are un-segmented i.e. individual pellets will not be obvious. Cylinders are irregular in shape and size, but are typically 25–50 mm in width and 70–120 mm in length. Pig droppings are initially dark, but become grey with time.
Can be confused with:
Fresh pig droppings may look similar to intact sheep droppings. Deer droppings can occur in cylinders too, but the individual pellets making up the cylinders look like little round balls (the size depends on the species, with the largest about 20 mm wide) with a dimple on one end and a small point on the other end.
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Footprints and Tracks
Pigs have cloven hooves with two bilaterally symmetrical toes (called cleaves). The dew claws (lateral toes) are frequently evident because they are much lower down than those of deer and other ungulate species. When dew claw prints are present, they are behind and to the side of the cleaves. The cleaves are blunter than those of deer and goats, and in general the footprint is wider and rounder. Toe indentations are often splayed away from the central line. Hoof prints are about 50–80mm in length and 40–50 mm in width.
Can be confused with:
Pig footprints can be confused with those of deer, goats, and sheep. However, sheep hoof prints tend to be more pointed at the front, goat hoof prints are more kidney shaped (due to the curve on the inside of the toe), and deer hoof prints are longer and narrower. When deer dew claw prints are present, they are directly behind the main hooves, rather than behind and to the side.
Trails and Runs
Feral pigs can create distinctive trails by trampling through vegetation. Soil can be eroded from the exposed and compacted ground along these routes.
Can be confused with:
Other ungulate (hooved animals), including farm animals such as cattle, can also create trails although feral pig trails are likely to be associated with areas disturbed by their characteristic rooting.
Feral pigs take mud baths in wet depressions in the ground to rid themselves of lice and other ectoparasites (unwanted bugs living on their skin). Pig wallows can be found in swamps, gullies, and alongside creeks. Wallows are used year round, but more intensively in summer during hot periods.
Can be confused with:
Wallows used by pigs can be confused with those used by deer. Look for other evidence such as droppings and hoof prints.
Dens and nests
Feral pigs build and use farrowing nests (for giving birth to young) and loafing beds (adult resting sites), although these can be difficult to tell apart. Examples include depressions in the ground lined with grass, bracken, or fern, and flattened areas within dense bracken. They may be located at the base of a tree or underneath a log or bank.
Can be confused with:
Farrowing nests and loafing beds may be confused with resting sites within thick cover used by deer and feral goats. However, deer and feral goats are not known to line resting sites with vegetation.
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Understorey (less than 3m)
Rooting (digging up the soil with their noses) by feral pigs can do an enormous amount of damage to the ground surface. Rooting is a foraging behaviour (food gathering), and is often undertaken systematically in deep regular patterns. It is the most common and evident sign of the presence of feral pigs. Pig rooting can sometimes be so severe that almost the entire ground surface in a given location has been uprooted. It occurs most commonly in somewhat damp areas (wet gullies, the base of hills). Pigs may also pull apart rotten logs and roll large boulders. Pig rooting has the potential to impact long-term ecosystem processes such as nutrient cycling and the composition of plant species in forests.
Pigs are omnivorous. On the two main islands of New Zealand they are known to eat fruit from native trees and sweet briar. They also are known to eat the roots of Carex appressa, Aciphylla spp., nettles (Urtica spp.), thistles (Cirsium spp.), supplejack (Ripogonum scandens), and bracken. They will also eat the base of nikau palm (Rhopalostylis sapida) fronds, roots of young palms, fungi, grasses and the base of tussock tillers.
Can be confused with:
Pig rooting can’t really be confused with anything else, except ploughing by humans!
Feral pigs will often rub on trees, logs and fence posts. They use the same site repeatedly and may travel some distance to use a favoured rubbing post. Rubbing posts often become coated with mud and pig hair, and sometimes tusk marks are evident. Pigs can kill trees if they rub against them too much, particularly if they remove all the bark.
Can be confused with:
Antler rubbing by deer could be confused with pigs rubbing, but pig rubbing will be low on the tree or fence post and mud and pig hair are likely to be present.
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Fur, feathers or scales (vertebrate)
In New Zealand, feral pigs have been known to eat ground-nesting birds and their eggs. However, there is little information available regarding characteristic signs that pigs might leave at nests they have preyed on. There are stories that pigs turn kiwi-skins inside out, but it is not clear if the pig killed the kiwi or ate it after it was found dead.
In New Zealand, feral pigs have been known to eat mice, rats, young rabbits, possum carrion, and new-born lambs. Lamb predation has very tell tale signs of skins turned inside out with just the hocks and skins remaining , but there is little information available regarding characteristic signs pigs might leave behind for other species.
Lizards and frogs
Feral pigs are known to eat lizards and frogs. There is little information available regarding characteristic signs pigs might leave behind when they devour lizards and frogs. However, it is likely given their small size that they are often eaten whole.
Could be confused with:
Remember that animal remains might have been scavenged by other animals.
Insects or snails (invertebrate)
Feral pigs sometimes dig into or tear apart rotten logs in search of invertebrates. They are known predators of New Zealand’s native land snails Powelliphanta spp. When a land snail has been eaten by a pig, its crushed shell is often found. Pigs also eat amphipods, centipedes, beetles and earthworms caught while rooting. These latter invertebrates will most often be eaten whole.
In New Zealand, feral pigs have been known to eat ground-nesting birds and their eggs. However, there is little information available regarding characteristic signs that pigs might leave at nests they have preyed on.
Could be confused with:
Remember, too, that egg remains might have been scavenged by other animals.
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Pigs can smell strongly of urine.
Pigs make grunting sounds. They also make loud squealing sounds when alarmed or when two boars are fighting. Fighting boars can sometimes be heard from a long way off, particularly in areas where the sound is carried across water e.g. around lakes edges or in locations like the Marlborough Sounds.
Feral pig hair is longer and coarser than that of domestic pigs. Places where traces might be found include: on trees where a pig has been rubbing its rump; where pigs have forced their way under fences; or embedded in the mud where a pig has been wallowing. Black is the most common colour, but other colour variations include rusty red to ginger, brown with black spots, brown and white, white with or without a black stripe, grey and smoky blue (the latter colour is common in North Canterbury).
Pigs' teeth are adapted to enable them to eat their omniverous diet (both plants and animals). They have six incisor teeth at the front of the top and the bottom jaw and a single canine tooth on each side behind the incisors, top and bottom. The bottom canines form a tusk on each side, which grows continuously and is kept sharp with friction against the upper teeth. Further back, behind the canines, are the premolars and molars, used for crushing and grinding food.
Little information is available about identifying pig bite marks but see page 24 of the Landcare Research identification guide.
Pigs’ eyes reportedly do not reflect colour at night because they lack a tapetum lucidum layer inside the eye. Species lacking eye shine are usually diurnal (active during the day and sleeping at night).
Pigs are widespread in both North Island and South Island, and currently occur on 13 offshore islands. They were eradicated from Stewart Island by 1965.
They currently occupy approximately 93,000 km2 (34%) of New Zealand.
Signs of disturbance to rotten logs could be a sign that feral pigs are present as they sometimes dig into or tear apart rotten logs in search of invertebrates.
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Feral and domestic pigs are descended from wild boar. Wild boar are native to North Africa and Eurasia, and occur as far east as Japan and Indonesia. However, they have now become extinct over much of their range, including southern Scandinavia, the Nile Valley and Britain. In general, feral pigs more closely resemble wild boar than domestic pigs do.
Feral pigs are smaller and more muscular than domestic pigs and they have a longer snout that is flattened at the end. Their forequarters are larger than their hindquarters. The tail is straight with a bushy tip (rather than curly like a domestic pig). They also have a muscular neck enabling them to root up hard ground. Their back is also narrower than that of a domestic pig (‘razorback’), and their hair is longer and coarser than domestic pigs. In New Zealand, feral pigs are most often black, but can also be rusty red, brown, brown and white, white with black spots, grey, and smoky blue.
They have upper and lower tusks. Their tusks are formed from continuously growing canine teeth. Lower tusks curve upwards, outwards and backwards, while the shorter upper tusks curve outwards and backwards.
Size and weight
Adult males (boars) weigh 45-205kg and adult females (sows) weigh 32-114kg. Boar body length ranges between 1100m and 2280mm, and their height ranges between 550mm and 960mm. Sow body length ranges between 1140mm and 1500mm, and their height ranges between 430mm and 600mm. Lower tusks are usually 150–300mm in length and form an arc of a circle that is approximately 500mm in circumference. Upper tusks are up to 90mm in length.
Pigs have poor eyesight, but acute hearing and smell. They can be active both during the day and at night, but when subjected to hunting pressure become more nocturnal or restrict their activity to dawn and dusk. Feral pigs are capable of breeding throughout the year, but their main breeding period is between April and June with most litters born between July and November. Females can produce two litters within a year, with the average litter size being six. Piglets are weaned at six weeks of age, but remain with the sow until the next litter is due.
Pigs are omnivorous and on the two main islands of New Zealand are known to eat fruits including sweet briar and fallen native fruits. They also eat the roots of Carex appressa, Aciphylla spp., nettles (Urtica spp.), thistles (Girsium spp.), supplejack (Ripogonum scandens), and bracken. They will also eat the base of nikau palm (Rhopalostylis sapida) fronds, roots of young palms, fungi, grasses and the base of tussock tillers. Animal foods include various invertebrates, frogs, lizards, freshwater eels (Anguilla sp.), ground-nesting birds and their eggs, mice, rats, sheep, young rabbits, and carrion including dead possums.
In New Zealand, feral pigs occupy areas where they can find adequate cover. This includes native and exotic forests, and thick extensive areas of scrub, bracken or gorse. They can also be found in tussock grasslands that contain patches of cover.
Distribution in New Zealand
See distribution clue.
How to get rid of pigs
Contact your local Department of Conservation or Regional Council office for advice (see Next Steps). Feral pigs can be controlled through hunting, trapping, and/or poisoning.
FIND MORE INFORMATION IN:
Chimera, C., Coleman, M.C., & Parkes, J.P. (1995). Diet of feral goats and feral pigs on Auckland Island, New Zealand. New Zealand Journal of Ecology, 19, 203-207.
Jolley, D.B., Ditchkoff, S.S., Sparklin, B.D., Hanson, L.B., Mitchell, M.S., & Grand, J.B. (2010). Estimate of herpetofauna depredation by a population of wild pigs. Journal of Mammalogy, 91,519-524.
McIlroy, J.C. (2005). Feral pig. In King, C.M., (Ed.), The handbook of New Zealand mammals (pp. 334-345). Auckland, New Zealand: Oxford University Press.
Medway, D.G. (2001). Pigs and petrels on the Poor Knights Islands. New Zealand Natural Sciences, 26, 87‑90.
Nugent, G., Gortazar, C., & Knowles, G. (2001). The epidemiology of Mycobacterium bovis in wild deer and feral pigs and their roles in the establishment and spread of bovine tuberculosis in New Zealand wildlife. New Zealand Veterinary Journal, 8, 1-42.
Pawson, S.M., & Armstrong, K.F. (2007). A morphological and molecular comparison of island and mainland populations of Megadromus speciosus (Coleoptera: Carabidae) from the Marlborough Sounds, New Zealand. New Zealand Entomologist, 30, 13-23.
Thomson, C., & Challies, C.N. (1988). Diet of feral pigs in the podocarp-tawa forests of the Urewera Ranges. New Zealand Journal of Ecology, 11, 73-78.
Walker, K. (2003). Recovery plans for Powelliphanta land snails 2003-2013. Threatened Species Recovery Plan 49. Wellington, New Zealand: Department of Conservation.