Behaviour is the primary way of interaction of the animal with its environment. Most physiological regulatory processes maintain homeostasis throughout coordinated behavioural and biological mechanisms. This is also the case for stress and adaptation processes (Dantzer and Mormède, 1983). Furthermore, the study of behaviour is not invasive, and can be achieved without introducing any trouble to the subjects; it usually necessitates a minimal equipment only (but experienced scientists); it has an excellent time resolution and nevertheless allows longitudinal studies; it may also by itself be a source of distress like in social and human-animal interactions or when behavioural needs cannot be satisfied. All these reasons have put forward the use of behaviour as the primary approach in welfare studies.
Behaviour is a sensitive index of the physiological status of the animal. For instance, the mere observation of the repartition of the pigs in the pen reflects the thermoregulatory status of the animals (Shao et al., 1997), and therefore integrates a number of contributing factors related to the environment (temperature, humidity, air speed, nutritional level) and the animal (physiological status, basal metabolism) through a single and simple measure. We have also cited before the example of thermoregulatory behaviour of early weaned piglets that parallels biological changes (Orgeur et al., 2001).
Behaviour has therefore been used extensively to analyze environmental needs or preferences. Some are quite obvious, as related to the ergonomy of the animal’s life, such as the disposition and design of drinking spouts and feeding troughs, the physical characteristics of the ground and the design of alleys to allow convenient displacement of the animals. Preference tests are useful but more difficult to interpret in terms of welfare, and it is necessary to measure the strength of preferences to know whether they can be said to constitute needs that may induce psychological and /or physiological disturbances when they cannot be satisfied. All these behavioural studies are important to design an optimal environment for the animals (Jensen et al., 2005).
Behaviour is also a classical symptom to the diagnosis of health problems, like the general behavioural depression accompanying fever (reduction of movements, social interactions and food intake) and known as sickness behavior (Dantzer and Kelley, 2007), or lameness indicative of locomotor problems and associated pain.
There is a lot more discussion about the significance and interpretation of so-called abnormal behaviours. Those fall into several categories.
Reduced behavioural activity / reactivity (apathy), eventually with a reduction of food and water intake is a frequent symptom in animals adapting to a novel environment.
Aggressive behaviours are primarily the expression of social behaviour of pigs. Indeed aggressive behaviours are frequent for food competition or when mixing unacquainted animals, such as at the time of weaning or when pigs from different pens are mixed before being loaded to the slaughterhouse or in the lairage area. These aggressive interactions are a frequent source of wounds and exhaustion, eventually leading to death in sensitive animals, like those carrying the stress-susceptibility allele of the ryanodine receptor. They also compromise meat quality, giving meat with a low pH, pale, soft and exsudative (Sellier, 1998; Faucitano and Geverink, in this book). Therefore mixing animals from different social groups should be as limited as possible. Outside these periods, aggressive behaviours usually occur at low frequency when there is no competition, such as for food for instance, and when group size is kept small enough to allow the expression of normal social behaviour. Aggressive interactions have been shown to increase in situations where the welfare of the animals is threatened (Beattie et al., 1995, 1996; De Jonge et al., 1996; Haskell et al., 1996). It should also be taken into consideration that large differences may occur between individuals. Genetic differences are well documented in experimental animals but little has been done in pigs (McBride et al., 1964; Torrey et al., 2001; Rhydmer and Lundeheim, in this book).
Many studies have been published about the development of abnormal behaviours in general and more specifically stereotyped behaviours, in particular with reference to various parameters of the environment, with a major influence of feeding (see reviews in Dantzer, 1986; Lawrence and Terlouw, 1993; Mason, 1991; Mason and Latham, 2004). It is generally agreed that the development of stereotyped behaviours is a sign of poor welfare, although there may be large differences in the vulnerability of individual animals to develop abnormal behaviours. An extreme form of abnormal behaviour is cannibalism that is usually directed in pigs towards the tail and ears and sometimes leaving to death (Schroder-Petersen and Simonsen, 2001).
When it comes to the study of general behavioural activity and reactivity – as opposed to the longitudinal study of the response to a given stimulus as usually done in laboratory experiments – it is necessary to disentangle the various factors influencing behavioural reactivity besides, or in interaction with the factor under study. Indeed large individual differences have been described in emotional behavioural reactivity, altogether known as temperament (Cloninger, 1994). It has been shown in laboratory animals, like in humans, that the whole range of variation is contributed to by a limited number of ‘factors’ like activity, emotionality and aggressiveness (Ramos and Mormède, 1998). This multidimensionality was also described in pigs (Lawrence et al., 1991; Jensen et al., 1995b; Thodberg et al., 1999; van Erp-van der Kooij et al., 2002) but deserves more work for a comprehensive understanding. A simple test, known as backtest, was designed to characterize individual differences in behavioural reactivity (Hessing et al., 1993). It is based on the propensity of individual animals to display tonic immobility when they are put on their back and lightly restrained. Several studies have shown that individual differences in behavioural reactivity as measured by the backtest have a number of physiological and zootechnical correlates (Hessing et al., 1994, 1995; Schrama et al., 1997; Erhard and Mendl, 1999; Bolhuis et al., 2000; Ruis et al., 2000; van Erp-van der Kooij et al., 2000; Geverink et al., 2003). Although this test is the matter of discussions about its interpretation and significance (Jensen, 1995; Jensen et al., 1995a), it has a very heuristic value to focus behavioural studies to individual differences that are now studied more comprehensively. As for the variation in neuroendocrine responses, individual differences in behavioural activity / reactivity is the result of complex interactions between the genetic make-up of the individual and environmental influences during development (McBride et al., 1964; Hemsworth et al., 1986, 1990; Hemsworth and Barnett, 1992; Beilharz et al., 1993; Désautés et al., 1997; Ramos and Mormède, 1998).
In conclusion, most behavioural changes have no simple relationship with stress and welfare, and must be interpreted in the context of their specific role in homeostatic mechanisms (including their psychophysiological dimension) and of individual variation.