| Pigs are social animals and form hierarchies at social maturity. Unlike most other social mammals, however, they also form a social hierarchy at birth. This is at least partly the result of teat order—the first pair of teats is often preferred and may produce more milk. Piglets that suckle on the first pair of teats gain more weight than do other piglets. Teat order is formed within the first 48 hr, after which changes are rare. Teat order is stable by 7 days of age. Piglets
reinforce the established order early in life and will slash their companions with their teeth, which therefore are usually cut to protect other piglets and the mother. Heavy piglets (ie, the ones that suckled the first pair of teats) continue to reinforce their status through contest. The winner in piglet contests is invariably the heavier pig, which continues to have an advantage later in life. The piglet hierarchy can persist after weaning if the cohort is left together. If the
piglets are separated, other factors influence the development of social hierarchies. |
| Weaning occurs naturally at 14-17 wk but can occur in production or pen situations as early as 6-9 wk. Unfortunately, this period coincides with the period of 11-18 wk during which unpleasant experiences can delay a first pregnancy in gilts and a first successful mating in boars. Boars may be higher ranking, but the same is not true for barrows. |
| As in cattle, breed is important, and Yorkshires and Large Whites are more aggressive than Hampshires, which are more aggressive than Durocs. The standard porcine social grouping focuses on sows, piglets, and juveniles; males emigrate at ~1-2 yr of age. Pigs have labile social structures, and the extent to which the home ranges of males overlap those of females depends on season and reproduction. Sows within a social group experience synchronous estrus. If allowed to do so, sows
will combine their litters so that one sow remains with all of the piglets while the other sows forage. Upon return from foraging, the sows suckle only their own young. Piglets otherwise do not generally mingle with piglets from other litters for the first month. |
| Communal nesting behavior will occur at night, if allowed. Vegetation and stick nests are usually communally constructed in a protected area (behind a tree or barrier) or at a high point. Undoubtedly, these behaviors are adaptations to avoid predation. Piglets leave the nest permanently at ~9 days of age. Farrowing occurs year round and is more regular under warmer conditions. There are usually 2 farrowing peaks per year. |
| Overheating is a major problem in pigs, and wallowing in mud is a thermoregulatory behavior. Conditions that decrease expenditure of energy are preferred. This behavior does not represent a desire to be filthy or unclean. Pigs exhibit selective dunging behavior, ie, they choose a location to deposit their feces and faithfully return to it. Given the choice, pigs choose clean conditions in which they can also thermoregulate. |
| Postweaning sows and prepubertal gilts should be kept in the sight and smell of a boar to induce synchronous estrus (the Whitten effect). They also experience a dormitory effect (the McClintock or Fraser-Darling effect) with regard to synchrony due to the presence of other females.
Sexual behavior in pigs is almost universally associated with the “chant de coeur” or song of the heart. Courting pigs are vocal. Boars have a gape response (flehmen), and some boars can detect estrous females through olfactory means. Boars will nuzzle the head, shoulders, flank, and anogenital area of sows during courtship. If the pheromonal cues are present, boars progress to pushing on a sow to see if she will move. If she stands, she is willing to mate. Boars exhibit a unique,
pheromonally based solicitation behavior toward females: they “champ”—chewing and gnashing their teeth, producing frothy saliva that is rich in the pheromone androstenol. Androstenol is also present in preputial fluids. Courting boars mark trees with urine and saliva produced by champing. Boars are naturally slow to ejaculate (up to 30 min), which may be a correlate of their long courtship, but mate best if raised in a rich
social environment. Boars raised in isolation have decreased sexual performance later in life. |
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Rooting is a normal foraging behavior in pigs. The nose of a pig is a sensitive sensory organ, and striking a pig on the nose is discouraged. Nose rings may prevent rooting, but no studies have examined the effects that such rings may have on more subtle social and interactive behaviors. All such effects will be magnified by genetic effects on temperament. Pigs bred to have a lower fat content are more reactive and aggressive when
handled, leading to earlier slaughter. Handling differences are also known for different breeds of pigs. |
