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 animal will look for food, when tired, animals will find a place to rest and sleep, when frightened, an animal will try and escape.
Consequences of stress
Stress is a part of daily life and
not in itself a bad thing, however, when stress overwhelms the body’s ability to compensate, the animal experiences distress, which can result in a variety of pathological states and ultimately reduce its “fitness”3.
For example, a pig in a hot environment may move into
the shade, lie down on the cool ground and drink additional water in order to modulate its internal temperature. However, if these behaviours, together with internal processes such as vasodilation, are unable to control the body temperature, the pig may start to experience heat stress, which can ultimately lead to death.
While experiencing these physical symptoms, the pig’s brain is also capable of experiencing emotions such as anxiety, fear and frustration, which ultimately leads to poor welfare for the pig.
Emotional states such as fear, frustration, and anxiety are also capable of inducing a stress response in the body.
Stressors
It’s easy to understand what stressors humans may experience,
but how can we know what a pig may experience as a stressor? This is one of the many questions that the field of animal welfare science tries to answer.
Stressors can be things which influence biological functioning, such as lack of food or extreme temperatures, situations that induce feelings of fear such
as restraint, any stimulus that results in pain, or interactions
with conspecifics or individuals of another species. Just as different aspects can influence how an animal responds to a stressor, these same factors can determine if an animal even perceives something as a stressor.
For example, pigs handled in a calm and gentle manner by people may not perceive the presence of
a person in the environment to be a stressor, whereas a pig, which
has been frightened or hurt by a person, may perceive the presence of that person in the environment as highly stressful4. However, it
is safe to say that in general, the natural history of a species will give us a good indication of what that animal may find stressful.
A very important factor that
can determine whether or not something is perceived to be stressful or results in distress is
the predictability and control associated with that stimulus. With pigs, for example, inconsistent handling (a mixture of pleasant and
unpleasant handling) was found to be just as stressful as unpleasant handling5.
Coming up: In the next article on stress, we follow the life of a pig born on a farm and unpack the various ways in which stress impacts her life, growth, health, welfare, and productivity.
References
1. Moberg, G. & Mench, J. The Biology of Animal Stress: Basic Principles and Implications
for Animal welfare. The
Biology of animal Stress: basic principles and implications for Animal Welfare (CABI, 2000). doi:10.1079/9780851993591.0000 2. McEwen,B.Stressandthe Individual: Mechanisms Leading to Disease. Arch. Intern. Med. 153, 2093–2101 (1993).
3. Broom,D.M.&Johnson,K.G. Stress and Animal Welfare: Key Issues in the Biology of Humans and Other Animals. (Springer).
4. Mason,G.&Mendl,M.Whyis there no simple way of Measuring Animal Welfare? Anim. Welf. 2, 301–319 (1993).
5. Hemsworth, P. H., Barnett, J.
L. & Hansen, C. The Influence of Inconsistent Handling by Humans on the Behaviour , Growth and Corticosteroids of Young Pigs. Appl. Anim. b 17, 245–252 (1987).
Figure 1: A model of the response to stress (adapted from Moberg, 2000)
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