Introduction

The behaviour of animals typically changes when they become sick. Although sickness is often deduced from physical symptoms, such as coughing, vomiting, and obvious injuries, behavioural symptoms are also important, and sickness is often inferred from behavioural symptoms alone. For example, when people are sick, they often do not feel like eating and may avoid many other kinds of activities. Changes in sleep patterns are also common. Such changes were recog

Table I Behavioural changes associated with infection and injury. Arrows reflect direction of change typically observed (4-: decreased; t: increased).

Behaviour Typical Change

Food intake !

Slow-wave sleep Î

Environmental exploration J,

Social exploration 1

Sexual activity I females

Cognitive function Impaired

Anhedonia (lack of interest in pleasurable stimuli) Î

nized many years ago by Hans Selye who identified a state he called the "syndrome of just being sick". He noted that sick people displayed many common features that were independent of the nature of their diseases. "They are all indisposed, they look tired, have no appetite, gradually lose weight, they do not feel like going to work, they prefer to lie down rather than to stand up" [1]. Substantial changes in behaviour, resembling those reported by humans, are also noted in sick pets, livestock and research animals. Experimental investigations have shown that infected mammals undergo behavioural changes such as decreased motor activity, decreased ingestive behaviour, decreased exploratory behaviour, impaired cognitive functioning, altered sexual behaviour, increased sleep, and changes in affective states (e.g., [2-5]; see Table I).

Such changes are induced not only by infections and other pathologies, but can be induced by compounds that stimulate the immune system, such as lipopolysaccharide (LPS, also known as endotoxin), a component of the cell walls of Gram-negative bacteria. This suggests that being sick is not a direct consequence of the infection or the tissue damage, but may be related to the activation of the immune system associated with the pathology. The suggestion is reinforced by the observation that similar patterns of behaviour can be induced by administration of certain cytokines, soluble factors (proteins or glycoproteins) released by immune cells during immune activation [4,5], Thus it is not the pathogens themselves that induce the sickness, but the response of the host's immune system, most likely involving cytokines.

Cytokines known to be behaviourally active include tumour necrosis factor-a (TNFa), inter-leukin-la (IL-la), interleukin-1P (IL-lfJ), interleukin-6, (IL-6), and the interferons (IFNa, IFNp, and IFNy). These cytokines are largely pro-inflammatory; they can activate the acute phase response, and are activated during various disease states. Cytokines are released after LPS administration as well as after the administration of pathogens and antigens. It is well documented that the pro-inflammatory cytokine, IL-1P, administered both peripherally and centrally, has behavioural effects and induces a profile of behavioural changes similar to LPS and many pathogens (see [4,5] for reviews of cytokine effects on behaviour). In addition to IL-1, other cytokines, notably IL-2, TNFa, IL-6, IFNa and IFNy may induce behavioural effects, although these cytokines are generally less potent than IL-1, and may induce different behavioural effects. Thus laboratory research has confirmed anecdotal observations of changes of behaviour induced by immune activation, and has suggested cytokines as mediators of these effects.

Medical professionals have long been aware that illness involves changes in behaviour (see the reference to Selye above [1]). Nevertheless, until recently scientists and practitioners have paid little attention to these sickness-induced changes in behaviour. The changes associated with illness were assumed to reflect debilitation of little or no benefit to the organism, and were rarely studied. In retrospect, the lack of attention given to the behavioural changes associated with ill

Table II Components of Behavioural Defense.

Aspects of behavioral defense Role in Health

Avoidance behaviours: Reflexive Avoiding materials likely to be pathogen-infected

Avoidance behaviours: Learned Avoid stimuli associated with past illness

Learn to avoid (or indulge in) activities by observing other animals Sickness behaviours Reduce energy expenditure

Avoid activities that expose animals to risk (food gathering, exploration)

Facilitate fever

Minimize contact with pathogens Immune responses induced by behaviour Conditioned immune-enhancing responses

Priming of behavioral defense by stress-induced increase in cytokines ness is surprising. Changes in behaviour in sick animals are well conserved evolutionarily. They are expressed in many species and may result from many different diseases and activators of the immune system [2,3]. It is unlikely that such changes would be so well conserved evolutionarily if they were not in some way beneficial to the organism. In a seminal review paper, Benjamin Hart argued that such behavioural changes were adaptive and discussed what he saw as their value [2]. Since this paper was published, scientists and practitioners have looked more closely at what has come to be known as 'sickness behaviour' [3], and have investigated it systematically. Most agree with Hart that sickness behaviour is a defensive strategy for the organism facilitating its return to health.

The purpose of this chapter is to assess these assertions by describing the adaptive value of behavioural changes during sickness and evaluating these changes as defensive behaviour. Before so doing, we will describe behaviours other than sickness behaviour that may be important for defence against pathogens. As indicated above, Hart argued that the behavioural changes that occur during illness are defensive and can facilitate recovery [2], However, these are not the only behaviours that fulfil a defensive function. Animals adapt their behaviour in order to minimize contact with pathogens. Such adaptations can involve reflexively avoiding pathogens or learning to avoid stimuli previously associated with pathogens. In this chapter, we will consider some examples of each of these forms of behavioural defence (Table II).

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