Pigs and Stress

We live in an age where the term “stress” tends to be misused when applied to animals or ourselves. The animal liberationists claim that there are certain situations that are the extreme of stress – cruelty. However many of their opinions are formed by anthropomorphism, i.e. “I wouldn’t like to live like that therefore neither does the animal. No doubt, if they could, pigs would express similar concerns when viewing humans who tend to be fully clothed. Such a situation would be lethal to the pig, which has poor thermo-regulatory ability in hot conditions.

Stress must be defined physiologically to be measurable and comparable between events. Measuring the levels of adrenaline and cortisone in the blood stream normally does this.

Anatomy and Physiology

 The ancestor of the modern pig evolved on the plains and bushlands of Africa, spreading north and east over the ages. It does however retain many artefacts of its ancestry.

Pigs are social animals, used to living in family groups with a set hierarchy, this is still seen today among wild pigs such as the African Warthog. When an animal is born into such a group the main struggle for status in the hierarchy will be among its littermates and this is generally established at an early age.

When strange pigs are mixed there is an immediate hierarchical struggle accompanied by excessive fighting and mounting behaviour. Generally smell is the determinant of “strangeness”

Being ungulates (walk on two toes) pigs tend to have limited stereoscopic vision, essential for determining the exact distance of a threat, but excellent wide-range monocular vision.

Figure 2: Visionary characteristics of the pig

When the pig detects any unexpected movement in the monocular area of its vision its first instinct is to run. Once a safe distance is achieved it, like all plains animals, will stop and view the threat using stereoscopic vision to determine distance.

Using stereoscopic vision the pig is inquisitive; it will stop to investigate any change of texture under-foot, the depth of drains under gratings and any change of light. Understanding these facts highlights how easy it is to panic pigs and equally, the secret to moving pigs using yard design and the fact that they will keep a 30cm distance between themselves and a moving solid surface (backing board).

They also have an aversion to climbing. It has been demonstrated by research that inclines greater then 15^o  will cause the animals to stop and investigate the surface before ascending or descending.

Like all plains animals the pig finds safety in numbers and if one panics, they all panic, releasing a surge of adrenaline into the bloodstream releasing glycogen (muscle sugar) from the liver.

The modern pig, descended from European ancestors adapted to temperate climates, has little difficulty in cool conditions but can rapidly over-heat when it becomes hot. In the wild they will stay warm by clustering in groups and laying down fat reserves. In hot weather they seek the shade, and if possible, muddy areas in which to wallow.

The pig has sweat glands only on its nose and if it cannot find a behavioural solution to over-heating enters a period of prolonged stress.

Briefly we have explained some of the factors that lead to stress (release of “fight or flight” hormones into the bloodstream) in pigs.

·                 Panic, or other short-term stress leads to large glycogen release for a short period of time. This is the precursor of Pale, Soft, Exudative Pork (PSE).

·                 Long term stress, such as overheating, leads to a prolonged glycogen release and is the precursor of Dark, Firm, Dry Pork (DFD).

The former condition leads to poor eating quality in the form of dry, tough meat. If it is cured the cure will not remain in the tissue giving a patchy appearance, poor taste and may even lead to poor preservation and “bone taint”.

The latter condition leads to meat with high ultimate (post rigor-mortis) pH. The meat is dark, dry to the touch and very firm. The firmness arises because this meat has excellent water holding capacity but because of the high pH bacteria will grow rapidly. DFD may provide excellent eating but it has very poor keeping quality.