Pig Suffering

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Animals can't tell us how they feel, so researchers have tried to work out physical indicators of stress or frustration. However, each type of measurement has its limitations, so a combination of measurements will give the most reliable picture of the animal's welfare. The following are some ways used to assess stress and possible suffering.

  1. Stress hormones in the blood . Physical or emotional stimulation has been shown to cause increased levels of hormones such as cortisol, corticosterone and ACTH to be released into the bloodstream. Levels of these hormones rise sharply following the pain of tailing, mulesing and castration in lambs ( 1 ), and the stress of being restrained in a nose sling in pigs ( 2 ).

  2. Natural opioids . The same factors which stimulate the release of stress hormones can also stimulate the release of natural opioids such as endorphins , which are thought to have analgesic effects. The maximum endorphin binding capacity is higher in the brains of tethered sows than in group penned sows ( 3 ). Tethered sows have a stronger opioid response to the stress of nose sling restraint than loose housed animals ( 4 ).

    Higher levels of opioids may explain why confined sows are more often apathetic and less responsive to stimuli than group housed animals ( 5-6 ). Regarding this unresponsiveness, the Scientific Veterinary Committee of the European Commission has said:
    " ... the sows may well be depressed in the clinical sense and poor welfare is indicated " ( 7 ).

  3. Immune function . High levels of stress hormones suppress the immune system, so stressed animals produce fewer antibodies in response to infection. For example, tethered pigs produced fewer antibodies than group housed pigs in response to injected bacteria ( 8 ). Among sows in individual stalls, those with the highest cortisol levels produced the fewest antibodies to a tetanus toxoid challenge ( 9 ).

  4. Growth rate . A reduced growth rate may indicate that young animals are stressed. However, a high growth rate doesn't necessarily indicate good welfare. For example, the fastest growing meat chickens are most likely to die of a heart attack.

  5. Inappropriate behaviour . When they are stressed or frustrated, animals may express their natural behaviours in ways that are detrimental. For example, pigs become more aggressive when frustrated in experiments where expected food rewards are withheld ( 10-11 ). The strong foraging instinct in pigs is often redirected to nibbling the ears and tails of pen mates ( 12 ). Included in the category of inappropriate behaviour is also vacuum behaviour , when a natural behaviour is carried out in an inappropriate environment. For example, battery hens try to dust bathe on wire floors, and sows about to farrow try to excavate a nest on concrete floors. Such behaviours suggest that important needs are not being met.

  6. Stereotyped behaviour . Stereotyped behaviour is an unchanging sequence of movements with no obvious function, such as pacing back and forth, circling or waving the head, or running the mouth along the bars of a cage. Stereotyped behaviour doesn't occur in free ranging animals, and it development suggests an inadequate environment that involves frustration, unavoidable fear or stress, and/or lack of stimulation ( 13 ). For example, there is less stereotyped behaviour when sows have straw bedding as an outlet for their urge to forage ( 14-15 ).

    Stereotyped behaviour appears to stimulate the release of endorphins, which may have a calming effect on animals. This has lead some researchers to claim that animals carrying out stereotyped behaviour are not suffering. However, a leading UK researcher has compared stereotyped behaviour to obsessive-compulsive neuroses in humans: ( 16 )
    "They (neuroses) have been reported to have anxiety-reducing effects, a very similar finding to the stress-reducing effects of stereotypies in pigs. The parallel is sufficiently close to warrant the view that sows performing stereotyped behaviours are not so much ‘coping’ as suffering clinical neuroses."

  7. Preference tests . The closest we can get to asking animals what they want is in preference tests, where they choose between two or more options, or have to work increasingly hard to reach a desired goal. For example, pigs will press a key many times to get access to earth in which they can root ( 6 ).

Dry sows

During their pregnancies, up to half of the breeding sows in Australia are still kept in individual stalls where they can't walk or turn around, Most of the remainder are kept in group pens, where they can at least move and interact with other sows, but the pens are still bare, with nothing to occupy the sow's strong urge to forage.

Many studies have found that stereotyped behaviour is much more common in individual stalls than in group pens ( 17-19 ). Some confined sows spend a large part of their day in repetitive pointless behaviour - in one study it was an average of 55% of an 8-hour observation period ( 20 ). These behaviours include bar biting, tongue rolling and vacuum chewing with nothing in the mouth.

Confinement is one factor that increases stereotyped behaviour, lack of fodder is another. Stereotyped behaviour is much less common when sows have straw to root and chew ( 14-15 ).

Studies of stress hormones don't always show a difference between sows in individual stalls and those in group pens. However, when there is a difference, it is pigs in individual stalls that show higher stress levels. For example, in one study pigs in tether stalls not only had higher stress hormone levels than pigs in group pens, but this difference increased over time from the first pregnancy to the second ( 8 ). The tethered pigs also produced fewer antibodies in response to an injection of bacteria.

Stress can also affect reproduction. Several studies have shown that a lower percentage of successful pregnancies are achieved in sows in individual stalls than in group pens ( 21-23 ).

Farrowing sows

It is estimated that 95% of sows in Australia are still confined in farrowing from just before the birth of their piglets until weaning ( 24 ). In this bare and extremely confined space the sow can hardly move and has no material to satisfy her strong instinct to build a nest.

Many studies have shown that sows are very restless in the 24 hours before they give birth ( 25-27 ). They paw the concrete floor and root inappropriate objects such as the food trough. They may be so persistent in these activities that they end up with a bleeding nose ( 25 ), or as another group of researchers observed ( 26 ):
"... most sows substitute vigorous activity for nest-building a few hours before the onset of farrowing. Such activity often leaves the sow with lacerations, contusions and abrasions and sometimes even leads to apparent exhaustion." "

The fact that nesting is carried out as a vacuum activity shows that it is a very strong instinct. It is triggered by a rise in the hormone prolactin 24 hours before the piglets are born ( 28-30 ). Not surprisingly, the frustration of this strong instinct is stressful. Sows in farrowing crates have higher levels of the stress hormone cortisol in the 24 hours before farrowing than sows in straw pens ( 28) .

Sows will work quite hard to be able to move around on the day before farrowing. In one study, the sows had to lift a lever with their snout one extra time each time they left a small pen. On the day before farrowing they lifted the lever many times in order to be able to walk around a larger pen ( 31 ).

When given a choice between a soil or a concrete floor, all sows scraped a hollow in the soil and farrowed there ( 32 ). When given a choice between pens with pre-formed nests and pens with loose straw, all sows chose the loose straw ( 33 ). The day before farrowing sows pressed a panel to gain access to a pen with straw as often as a pen with food, even when the panel was adjusted to require considerable effort to open ( 34 ). Manipulating nesting material seems to be important to the sows in its own right, and a pre-formed nest doesn't satisfy this need.

Growing pigs

On intensive farms, growing pigs are kept in crowded group pens. Each pig has very little space, and the pens are bare, with nothing for active and inquisitive young animals to do.

In these crowded conditions, compared to more spacious pens:

Tail biting can be a problem on intensive farms. As South Australian researcher Graham Pope has concluded ( 42 ):
" ... tail biting is now one of the most commonly encountered and entrenched disorders of growing pigs, and that its incidence has tended to increase in parallel with our efforts to intensify modern pig production. "

Tail biting has been linked to the absence of suitable material for the young pigs to root and gnaw. Biting starts as redirected nibbling, and is relatively gentle.. However, if by accident a wound is created, the tail is attacked much more violently. A well-known Dutch researcher has described this relationship as follows ( 12 ):
" Without straw the animals will look for something else to gnaw. In a pen of steel, asbestos or concrete only the partners qualify for this. Ears and tails are easiest to bite. Ears, however, are more sensitive and a bite there can be understood as aggression. So the tail remains a target. It is first played about with and taken into the mouth sideways. At a certain moment, more or less by accident, it is bitten. A small bleeding wound is caused. This wound irritates the bitten animal so that it starts slashing its tail about. This encourages further biting. Several animals start chasing the wounded pig intensively. The tail is violently gripped lengthwise with the incisors and canine teeth, while pieces of tissue are ripped off. The tail stump becomes frayed and is most painful for the wounded animal. "

Several studies have confirmed that tail biting is dramatically reduced when pigs have material such as straw or spent mushroom compost to root and chew ( 43-47 ). In the Dutch study above, increases in ammonia in air increased the incidence of tail biting as pigs became more irritable, but tail biting occurred in only 2 of 13 pens with straw, compared to 11 out of 12 pens without straw. Significantly, tail biting is not a problem in straw-bedded Ecoshelters.

The Scientific Veterinary Committee of the European Commission has stated:
" ... tail biting is an indication of an inadequate environment and indicates that welfare is poor in the animal carrying out the biting " ( 48 ).

The available evidence suggests that growing pigs are frustrated in the crowded conditions and barren pens that are typical of intensive farms.

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