European Union ruled in January of 2003 that pigs must be provided with manipulatible
material such as straw, peat moss, and mushroom compost. The reason for such a ruling was
that the European ruling body determined that exploration, rooting and manipulation
behaviors are deeply embedded in the pigs' evolutionary history and thwarting of these
behaviors by restricted movement and non-manipulatible materials is considered cruel. The
new European Union (EU) ruling is backed by an accumulation of research findings showing
that "environmental enrichment" reduces aggressive and stereotypical behaviors,
whereas boring and restrictive environments can predispose to stress, fighting and vices.
Environmental enrichment is defined as an improvement in the biological functioning of
captive animals by adding something to the environment. Environmental enrichments are
thought to decrease stereotypies by increasing foraging behavior by allowing the pigs
substrate to manipulate and decrease their time inactive.
What are Stereotypies?
Stereotypies or fixed repetitive actions are behaviors that appear when the animals are
bored or frustrated and the onlooker may regard this as an indicator of poor welfare.
Stereotypies are defined as repetitive actions that are invariable in form and serve no
obvious function. A common example of a stereotypy is the pacing behavior of large cats in
Although pork producers would like to decrease the incidence of these destructive
stereotypic behaviors in pigs, the current production environments do not allow for
increased space or complexity of the enclosures. Producers require an economic incentive
to replace current systems with an environment with more foraging opportunity. Therefore,
it is essential to investigate how increased foraging opportunities, genetics and learning
impact the development and performance of oral stereotypies in the domestic pig. It is
also necessary to assess what form of complexity or enrichment would be most appropriate,
given the natural behavior of the domestic pig and the cost to the producer. If we can
determine that increasing opportunities for foraging can effectively decrease the level of
stereotypy and determine a solution that the producer can economically institute within
his or her production system, it is possible to improve the welfare of the domestic pig.
Stereotypies and Animal Welfare
Stereotypies are one of the most intensively studied welfare issues of the domestic pig.
These behaviors are often described as abnormal behavior in part because they arise under
circumstances in which animals are thought to be "bored" or
"frustrated". Some examples of stereotypies in pigs are pacing, bar, biting,
vacuum chewing (chewing when nothing is present), and chain chewing. The concern over
stereotypies is that these behaviors might serve as indicators of poor welfare and so
there have been recent studies to link stereotypic behavior and stress. Stereotypies
usually develop in situations characterized by restriction of movement (limited space),
and lack of stimulation. In recent years, there has been growing evidence that
stereotypies in pigs are specifically related to heightened feeding motivation due to feed
restriction. Animals whose feed intake is restricted commonly develop oral stereotypies.
Oral stereotypies result from both behavioral restriction and feeding restriction.
Pregnant sows are commonly restricted to 60% of their normal ad libitum intake of a
standard concentrate diet. This diet represents an extreme restriction in the diet of the
Foraging and Stereotypies
The increased feeing motivation that brings about stereotypies could be because of the
nutrient component of the diet or the lack of foraging substrate. In a study of the effect
of food deprivation on the expression of foraging behavior, researchers found that food
restriction resulted in an increase in time pigs spent rooting the ground and a decrease
in the time spent lying down. These are behaviors that are strongly correlated to
stereotypic behavior. The foraging behavior of the pig in a natural environment can
consume the majority of their daytime activity. Thus pigs spend a large proportion of
their time in exploration and foraging. Given the large impact foraging has on the daytime
activities of the pig, it is possible to assume that stereotypies reflect foraging
motivation. However, other factors such as learning and heredity may also be important
factors in the performance of stereotypies.
Are Stereotypies Learned?
Learning involves the shaping of a behavior such that the animal becomes more efficient at
that behavior. In some sense, stereotypies are an extreme form of shaping behavior.
Stereotypies arise through sensitization (or heightening awareness), where presence of a
strong stimulus such as the sound of the feed cart, results in an enhancement of a reflex
response, such as anticipatory mouth movements, to that stimulus. In the formation of
feeding stereotypies, feed stimuli (feed cart, hunger) positively sensitize biting. The
latency of the response decreases and the number of the responses increase with continued
exposure to these stimuli. In this way, stereotypies can be considered to have a learned
component. More research is needed to determine if management practices can be modified to
reduce the opportunity to learn these abnormal behaviors.
Are Stereotypies Genetic?
There are individual differences in the expression of stereotypy and so it is possible
that some genetic component must also be involved. Certain animals may be genetically
predisposed to have a higher level of arousal than others. For instance, some strains of
pigs have an inherited inability to cope with stress. These pigs are said to have PSS
(Porcine Stress Syndrome), which is a metabolic disorder engaged by stress. These pigs
become literally immobilized by fear and die. There are possibly other heritable factors
like the PSS gene that influence the development of stereotypic behavior but this has yet
to be studied. More research is needed to determine if there are heritable factors that
can be manipulated through breeding to reduce stereotypies.
Is There a Solution?
Stereotypy is highly prevalent in our captive domestic pigs. In large commercial
production systems, pigs have minimal complexity in their housing systems. The combination
of a frustrated feeding motivation combined with a lack of foraging opportunities highly
impacts this problem. Increasing foraging complexity while still maintaining appropriate
levels of energy and nutrients might help to eliminate the performance of stereotypic
behavior in pigs. Improving the genetics of the pig to reduce stereotypic behavior should
also be investigated as a possible solution to decrease these abnormal behaviors. The
management practices may also be modified to decrease the chance of animals learning to
perform this behavior. Future environmental enrichment should take foraging style,
genetics and learning of the domestic pig into consideration and, possibly then, these
enrichments can prevent stereotypic behavior.
References for More Information
Dantzer, R. 1986. Behavioral, physiological and functional aspects of stereotyped
behavior: a review and re-interpretation. Journal of Animal Science. 62:1776-1786
Stolba, A. and D.G.M. Wood-Gush. 1989. The behaviour of pigs in a semi-natural
environment. Animal Production. 48:419-425.