Social Behavior of Swine
Pigs used in agriculture are descendants of the wild boar (Sus scrofa). Although pigs have been domesticated and selectively bred for production, many of their wild behavior patterns are still expressed. Pigs’ natural behavior repertoire, including social behavior, can be observed in both feral pigs and domestic pigs when they are kept in seminatural environments.
Wild boars are omnivores that live in forests in small family groups called sounders. Under natural conditions, females form sounders consisting of four to six related sows (depending on resource and space availability) and their offspring from the previous 1–2 years.
Young males leave the sounder when they reach sexual maturity to live in small bachelor groups or as solitary individuals. By comparison, young gilts either remain with the group or form new sounders. The territory of each family group ranges from 100 to 2,500 hectares (approximately 250–6,200 acres) and varies by food availability and season. Therefore, encounters with unfamiliar pigs are rare.
Wild pigs generally have a diurnal activity pattern. They spend approximately 50% of their time during the day foraging. Individuals in a sounder return to a common place at night for communal resting, generally in areas with dense overhead cover, when available.
Within a sounder, a linear hierarchy is established, with older and larger sows usually dominating younger, smaller individuals. This order is stable over time; therefore, aggressive behaviors are rarely observed. Physical aggression toward other pigs typically occurs only when unfamiliar animals come into contact or when there is competition for a highly valued resource, as with males fighting over females for breeding.
Sows in a sounder breed at the same time (approximately twice a year), and all sows become subordinate to the boar that joins their social group. Estrus is induced by photoperiod, pheromones, the sight of a boar, and the weaning of piglets (for sows).
The boar exhibits flehmen (curling the upper lip back, closing the nostrils, and inhaling with an open mouth) when detecting the pheromones of a female in heat. Courtship relies on a combination of vocalizations (a courting song consisting of rhythmic grunts) and nuzzling the female’s head, shoulders, flank, and anogenital area. Boars usually urinate several times and produce thick saliva rich in pheromones. A receptive female exhibits lordosis (tonic immobility with stiffened legs and arched back), allowing the boar to mount her.
Before farrowing, sows isolate themselves from their social group and build a nest in which they can give birth and nurse their piglets. Sows leave the nest and introduce their litter to the sounder 1–2 weeks after giving birth. Communal nesting and maternal behavior are common within a sounder; one sow will remain with the piglets while the others forage.
Piglets are precocial and locate the udder within hours after birth. Within a few days, the piglets form a teat order whereby the heavier piglets usually suckle on the teats closer to the sow’s head, which produce more milk, giving them further nutritional advantage, as evidenced by their greater weight gains. This teat order remains stable throughout the lactation period, until weaning at approximately 3–4 months old. Other social behaviors observed among piglets are social play and huddling to help maintain body temperature.
Pearls & Pitfalls
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Sows typically wean their piglets after 2–4.5 months; on commercial farms, however, piglets are usually weaned at approximately 20–35 days old. Multiple litters are usually mixed into one social group, causing a marked increase in aggression. Once social stability has been regained and pigs have become familiar with each other, they resume social play until sexual maturity. However, a preference for playing with littermates persists.
Social play in pigs is referred to as "rough-and-tumble play," consisting of behaviors that would be used in fights, but in a more relaxed and inhibited expression. Pigs initiate play bouts often by tossing their head and quickly pivoting, before chasing, circling, shoving, and biting shoulders and ears.
Communication in pigs relies mainly on vocalizations and olfactory and visual signals. More than 20 distinct vocalizations have been identified in pigs, including grunting.
Grunting is the most common sound, often heard while the pig is rooting for food or during positive social interactions. Short, repetitive grunts signal excitement; longer grunts are typically used as contact calls. Sows also grunt at the beginning of a nursing bout to call the piglets.
Other common vocalizations are squeals and screams (signaling distress) and barks (used as a threat, to indicate alarm, and during play).
Olfactory signals are also highly developed in pigs. When introducing a new pig into an existing group, the resident pigs investigate the newcomer by nosing it. Visual signals are not as well developed, and pigs can form social ranks when they are blindfolded, relying solely on vocal and olfactory cues.
Body postures can also be used as a form of communication between animals. Unlike most pigs in modern swine production, which have short, docked tails, pigs in nature have full-length tails, which they use for signaling. Tail position and movements can convey information; eg, a curled or wagging tail often indicates arousal, and a drooping tail signals fear or distress.
The extensive repertoire of behaviors performed by wild boars is observed in both feral pigs and domesticated pigs in production when they are given the opportunity for behavioral expression.
Intensive animal production systems are characterized by great numbers of individuals housed in highly confined environments on a few farms, and they rely strongly on automation and technology. These unnatural housing systems put strong pressures on the social behavior of pigs, leading to severely heightened aggression, decreased reproductive performance, and stereotypical behaviors.
Behavior Problems of Swine
Intensive management systems for swine can hinder pigs’ ability to perform certain behaviors and impose stress that causes behavior problems (abnormal behaviors).
Pigs are often housed in highly confined and barren environments, mixed with unfamiliar individuals, and must compete for resources such as feed or space. Consequently, commonly reported behavior problems include heightened aggression between conspecifics within a group, decreased reproductive performance, and stereotypical behaviors.
Abnormal behaviors are typically observed either under intensive management systems and not in the wild, or in the wild but in an abnormal context in confinement as pigs seek to cope with environmental stressors.
Aggression and Fighting in Pigs
In natural settings, aggression between pigs is rarely observed beyond the first week of life, when piglets establish the teat order. In intensive rearing systems, however, aggression between pigs is common.
Aggression caused by mixing unfamiliar individuals is a major welfare concern during intensive production of growing pigs and sows.
Mixing is frequent after weaning, when the piglets are separated from the sow. During growing and finishing stages, pigs can be regrouped multiple times to meet spacing and feed requirements.
Intensive production systems usually isolate gilts and sows during both gestation and reproduction; nonetheless, aggression has been recorded between individuals in neighboring crates.
Group sow housing has become more common in swine production. Although group housing of sows has many benefits for sow welfare, aggression between sows is frequently observed when new individuals are introduced to the group pen. Under more extensive management, regrouping based on reproductive stage is also common.
Mixing during transportation and at slaughter is also very common.
In growing pigs, producers often form homogeneous groups of individuals with similar weights; however, this practice slows the establishment of a new social order. Because pigs have similar competitive abilities, fights last longer. After losing a fight, pigs are also more likely to retaliate, resulting in an overall increase in the number of aggressive behaviors.
Fights usually adhere to the following behavior sequence: 1–2 minutes of nosing and sniffing each other, often while vocalizing, until the fight breaks out into biting and shoving. Fights generally end when one of the pigs retreats or displays submission to the other.
Over time, fights are gradually replaced by threats, avoidance, and withdrawal behaviors until the hierarchy is formed, usually within a few days after the introduction of new animals.
Limited space at the feed trough or limited amounts of concentrated feed can cause aggression even in stable social groups. When females are in estrus, boars might fight and become very vocal; boars will strut shoulder to shoulder, champ their jaws (producing pheromone-rich saliva), and then finally face each other and attack. Serious injuries can result, especially among boars that still have their tusks.
Under intensive management, boars are generally housed individually, away from sows and other males; therefore, instances of fighting are highly unlikely. Breed and genetics play a role in the prevalence of aggression in pigs: Large Whites tend to be more aggressive than Hampshires, which tend to be more aggressive than Durocs. Leaner breeds tend to be more aggressive when handled than are those with more body fat.
Skin lesions result from biting during fights between pigs. Injuries sustained from fighting have an immediate negative impact on the welfare of pigs. Aggression can also hinder weight gain and affect reproductive performance.
During estrus, sows might show severe aggression toward newly introduced individuals. Research has found that submissive sows show the least estrous behavior, have smaller litters, and eventually lose body condition, because they are less successful in gaining access to feed in competitive systems (eg, floor feeding, nongated stalls). Injuries and stress related to aggression can decrease meat quality and require cuttings on the carcass.
The primary way to decrease aggression is to keep pigs in stable social groups. When mixing is unavoidable, aggression is most efficiently decreased using a combination of multiple approaches. Early-life socialization by allowing multiple litters to interact as they would under natural conditions can help decrease aggression at weaning.
Aggression can be decreased by providing adequate space for the display of avoidance and retreat behaviors, by including shelters or dividers in the pen, and by mixing subgroups with preestablished relationships. Progressive introduction of existing groups by first housing them in adjacent pens can also decrease aggression. Maternal or boar pheromones, as well as mixing at sunset or dimming the lights, have also been used successfully to decrease aggression.
Feeding diets high in tryptophan (a precursor for serotonin) or administering tranquilizing drugs (eg, azaperone or amperozide) can mitigate aggression and stress. Electronic sow feeders where individuals enter a stall to receive their feed can help decrease aggression due to competition for feed.
Tail Biting in Pigs
Tail biting in pigs, which involves one pig biting or chewing on the tail of another pig and causing open wounds, occurs mostly in confined environments.
Boredom, competition for food, and the inability to perform species-specific oral behaviors all drive tail biting in pigs. Tail biting can be a direct aggressive behavior or begin as gentle manipulation before escalating. Once the behavior appears in a pen, if the first few biting pigs are not removed promptly, it spreads rapidly.
It has been suggested that tail biting spreads through a pen as a result of social facilitation or stimulus enhancement (in which the bitten tail stimulates further investigation, resulting in an increase in the behavior’s prevalence). Tail-biting injuries are likely to become infected and result in culling; cuttings in the carcass also might be required.
The most efficient way to decrease tail biting is to provide opportunities to express natural rooting and nosing behaviors. Providing more space and a rooting substrate, such as light clean straw, has been successful in preventing outbreaks.
Feed type and delivery have also been found to affect tail biting: floor-feeding of pellets, compared with trough-feeding of liquid meals, increases the risk of tail biting. Environmental enrichments such as rubber toys or metal chains are efficient as a short-term solution because they are novel; however, pigs’ interest wears off over time.
Tail docking, though commonly used to address tail-biting behavior, focuses on the sign rather than on the underlying causes. Biting behavior might then be redirected toward the ears or the vulva.
Sexual and Maternal Behavior of Pigs
Reproductive Performance
Poor libido in boars is characterized by a decreased interest in mating, often leading to decreased reproductive performance. Boars raised without the presence of other pigs show lower sexual performance. Boars can be used to detect estrus in females; however, some gilts and sows experience a "silent estrus" in which they do not show behavioral signs of heat when they are in estrus.
Risk factors for poor libido and silent estrus are stress, inappropriate nutrition (both deficiencies and overfeeding), and suboptimal housing conditions. These problems can be mitigated by providing pigs with adequate nutrition, an environment that allows the expression of natural behavior, and opportunities for social interactions.
Regular exposure to sows in estrus helps maintain the libido of boars. Exposing a boar to an aggressive female, however, can result in low sex drive.
Piglet Crushing
Piglet crushing is one of the leading causes of neonatal death. Crushing occurs as the sow lies down or rolls over and inadvertently traps a piglet under her. The weakest piglets in a litter are the most at risk of being crushed, because their mobility is low (ie, they are not able to move away fast enough if they see the sow beginning to lie down).
Piglet crushing can be mitigated through breeding. Some sows are more reactive than others to piglet screams, getting up faster and therefore decreasing the number of deaths by crushing. Breeding for litters with fewer but larger and stronger piglets can also improve the mobility of piglets, making them less susceptible to crushing.
Piglet crushing can also be mitigated by improvements in the physical environment. Nonslip floors, sloping walls, and provision of bedding material can decrease crushing events. Bedding material can be provided before farrowing so that the sow can perform nesting behavior.
Sufficient space for the sow to perform prelying behaviors (in which she roots around to alert the piglets to move away before she lies down) also decreases crushing. In addition, a warm creep area (eg, provided by a heat lamp) can encourage piglets to rest away from the sow. Sow lameness also leads to uncontrolled lying behavior (eg, flopping), putting piglets at greater risk of being crushed.
Savaging
Occurring mainly in gilts or after farrowing complications, savaging is an aggressive behavior, directed at piglets, that results in injuries or even death. Usually the sow barks to warn piglets walking by her head and then later attacks them, biting them to death.
Savaging accounts for 4% of piglet deaths and affects approximately 18% of litters (1). It is most common immediately after parturition, when the sow is stressed and fearful of the piglets or of contact with humans.
Savaging appears to have a genetic component, but heritability might vary with the breed. The environment, the sow's experience, and its personality can all affect the risk of savaging; the cause for savaging behavior is yet to be determined.
Stereotypical Behaviors of Pigs
Stereotypical behaviors (stereotypies) are repetitive actions with no apparent purpose. They are often observed when the environment does not provide for the animal’s needs and reflects a state of poor welfare. Opportunities to express species-specific behaviors must be provided to decrease the incidence of stereotypies.
In juvenile pigs, the most common stereotypy is belly nosing, a behavior that stems from a strong suckling motivation, because weaning occurs much earlier in intensive production systems than under natural conditions.
Belly nosing is characterized by repeated movements of the snout on the belly of another piglet. This behavior can be mitigated by providing outdoor housing or providing bedding such as straw in indoor environments.
Ear biting or tail biting of a social partner (see Tail Biting in Pigs), if done repetitively and obsessively, constitutes a stereotypy. Ropes and balls in the environment temporarily lower the frequency of these behaviors. However, the initial novelty of these enrichments quickly wears off, thus decreasing their effectiveness, even when they are rotated regularly between multiple items.
Nonnutritive oral stereotypies are frequently observed in sows. Examples include the following:
bar biting (chewing on the bars of the pen)
head waving (side-to-side tilting movement of the head)
sham or vacuum chewing (chewing motions performed without food or substrate)
tongue rolling (extension and curling of the tongue outside the mouth)
rubbing the snout against a barren surface
polydipsia (ingesting high amounts of water)
In sows, these oral stereotypies are attributed to severe restriction of the animals' movement in conventional systems. The behaviors are much less frequent in loose housing systems.
Sows are generally fed highly concentrated diets once per day, often presented as pellets, that satisfy their nutritional requirements. However, such feeding schedules do not provide the opportunity for foraging behaviors such as rooting and grazing, which pigs would spend up to 10 hours a day performing under natural conditions. Providing sows with a high-fiber diet can increase the time they spend eating and thus decrease oral stereotypies.
Key Points
Pigs are highly intelligent and frequently understimulated in their environment.
Proper management of social groups is important to decrease pigs' stress.
Providing opportunities to perform species-specific behaviors in swine production can help decrease behavior problems.
For More Information
Spinka M. Behaviour of pigs. In: Jensen P, ed. The Ethology of Domestic Animals: An Introductory Text. 3rd ed. CABI; 2017:214-227.
References
Cronin GM, Smith JA. Effects of accommodation type and straw bedding around parturition and during lactation on the behaviour of primiparous sows and survival and growth of piglets to weaning. Appl Anim Behav Sci. 1992;33(2-3):191-208. doi:10.1016/S0168-1591(05)80007-3
