Next to sheepdogs, pigs are the most intelligent animals on the farm. When in their natural surroundings they are social, playful, protective animals who bond with each other, make nests, relax in the sun, and cool off in mud. Pigs are known to dream, recognise their own names, learn "tricks" like sitting for a treat, and lead social lives of a complexity previously observed only in primates.

Unfortunately, when they are confined in pens indoors they can become stressed and aggressive towards their pen-mates.

Research has shown that stress in pregnant sows caused when they are mixed in unfamiliar social groups resulted in their piglets being "stress programmed" for life.

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Piglets are born with very sharp needle teeth in the corners of their mouths. During suckling, these teeth can cause severe damage to the sows' udders and cuts to the faces of other piglets in the fight to access the best teats. In the worst cases farmers may be advised by their vets to remove the tips of the four offending needle teeth with a pair of clippers to make them blunt without damaging the gums or the pulp cavities of the teeth.

Pigs have a strict social hierarchy, and when they are mixed with unknown animals they need to form this hierarchy resulting in aggression.

Whenever I had to mix different pigs I used to throw a dozen-or-so empty feed-bags into the pen. That led to a massive scrum as the pigs set about, shoulder-to-shoulder, shredding the bags. When it was all over those exhausted pigs lay down, side by side for a nap. When they awoke they all smelled the same and struggled to tell friend from foe - for a while at least.

Mixing can occur a number of times throughout a pig's life and is often necessary due to economic constraints, such as the need to free up pens for other pigs.

As there is no practical solution to the need to periodically mix pigs, over the years researchers have tried to change the pigs' environment, adapting it so they will react less aggressively when mixed, However these changes, such as adding straw, mixing the animals at night and tranquilising them - like my trick of giving them feed-bags to fight over - have not worked in the long-term.

Stress and anti-social behaviour in pigs is a big problem. Bored pigs can become very bad pigs. They chew the tails and ears of smaller ones and that can lead to outbreaks of cannibalism.

Tail-biting occurs in different scenarios, ranging from a constant minor problem to explosive outbreaks in individual batches. Sporadic outbreaks can be very difficult to manage.

Tail injuries range from mild to severe and tail-bitten pigs are likely to be in pain and distress. The wounds are a site for further infection, which can result in reduced growth performance and carcass condemnation.

Tail-docking, which usually limits the likelihood and severity of tail-biting is permitted following veterinary advice, although routine tail-docking is illegal. It may only be done where there is evidence of tail injuries and where other measures to prevent tail-biting have been tried. In the UK about 70 per cent of pigs have had their tails docked, which is lower than in most other European countries.

One of the best measures to prevent tail-biting is to bed the pens with straw to give the pigs something to root among and chew, although that's not always a silver bullet. It is also impractical for some, as straw can clog or block muck handling systems.

To overcome that, the bureaucrats in Brussels introduced laws in 2003 that made it compulsory for pigs to have toys - or "manipulative material", as they say in Eurospeak - to play with.

In the UK 62 per cent of pigs have access to substrate, most of which is straw, and 32 per cent of pigs have access to objects.

Practical farmers give their pigs hard plastic, indestructible balls, or blocks of wood to play with, Others hang chains and lengths of hard plastic for them to sniff, rattle, bite and chew.

Some are selectively breeding more sociable pigs that are "team players" with less aggression. The idea is that courteous pigs allow others easy access to feed, leading to increased daily weight gains for all their pen mates.

Not only could social performance generate higher weight gains, it could also have the added benefit of less aggression and tail-biting.