Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 1, 2010
Publication Date: July 18, 2010
Citation: Marchant Forde, J.N. 2010. Social Behavior in Swine and Its Impact on Welfare. In: S. D’Alliere & R. Friendship (Eds) Proceedings of the 21st International Pig Veterinary Society Congress. IPVS 2010, Vancouver, Canada. p. 36-39.
Pigs are social animals. From an evolutionary perspective, being social conveys a number of benefits, but potentially some disadvantages, especially for certain individuals within the group. The emphasis of this manuscript will be on those social behaviors that relate to formation and maintenance of social organization in swine, namely those centered on aggression and social dominance, as these are the aspects of social behavior in swine that have garnered most attention in relation to the animal’s welfare. The domestic pig is descended from the wild boar, but although they have changed greatly in terms of phenotype, their behavior, when given the opportunity, is extremely similar to their wild ancestors. The natural social organization of pigs centers on a core group or ‘sounder’ of 2-4 related sows plus their associated offspring of different sizes and ages. Within sounders, aggression is very rare. The group usually maintains a simple, linear social hierarchy, which is relatively stable over time. In contrast, aggression will be much more prevalent under commercial conditions than under natural conditions. How prevalent will be largely influenced by: 1) the degree of mixing/remixing, 2) the method of feeding, and 3) the amount and quality of space. When unacquainted pigs are mixed together, they often fight. Most fighting takes place within 2 h of mixing and by 24-48 h post-mixing, the level of aggressive interactions should be basal, and a hierarchy established. The hierarchy is then maintained by threats, avoidance and withdrawal, or short-lived aggressive interactions. Any method that might facilitate communication between individuals prior to mixing, be it olfactory, vocal or, to a degree, physical, will help the pig’s ability to assess its chances better and thereby avoid fighting or at least avoid prolonged contests at mixing. Naturally, pigs tend to synchronize feeding and actively forage for relatively low quality food for many hours during the day, with peaks in activity around dawn and dusk. Again, this is potentially very different from the commercial situation. In production, pigs will have access to high quality feed, which can meet their nutritional requirements quickly and it may only be available for an extremely limited period of time each day. Thus, access to food becomes an important resource and one that may play a major role in determining the amount of aggression being displayed within a system. For sows, feeding systems that promote competition for access, such as floor feeding, can have relatively high levels of aggression. Feeding systems that reduce competition by enclosing sows in stalls or being available ad libitum, can have relatively low aggression. The amount of space that pigs have, and the quality of that space, can also have a large impact on their behavioral repertoire, including agonistic social behavior. In general, as space allowance decreases, the total number of aggressive interactions increases. The most obvious physical impact of aggression can be injury and loss of body condition due to restricted access to feed. Aggression also activates both the sympathetic-adrenal-medullary axis and the hypothalamic-pituitary-adrenal axis, reducing immunity and upsetting reproductive status.