Sickness and behaviour in animals: a motivational perspective
Introduction
Animals acutely sick from systemic protozoan, bacterial or viral infections, commonly display a whole set of non-specific symptoms. Among the most reliable symptoms are changes in body temperature, increase in slow wave sleep, uneasiness, loss of interest for daily activities and reduction in food intake [1], [2]. For most physicians and veterinarians, these non-specific symptoms, and in particular the behavioural changes, are traditionally interpreted as the inability to achieve normal activities resulting from a debilitation of the general physiological state of the sick individual.
This conception of infectious diseases has led to neglect the fact that throughout their evolutionary history, all species have been exposed to and have successfully resisted the noxious effects of various invading micro-organisms and are the products of an “arms race” [3], [4]. Indeed, for the most part, the scientific literature dealing with behavioural adaptation, including ultimate and proximate causation, mainly focuses attention on healthy individuals. In their natural environment, animals are challenged by numerous pathogens that can originate infectious or inflammatory illnesses. It seems therefore reasonable to hypothesise that animals would have various physiological and behavioural mechanisms to fight illness and promote recovery.
Nevertheless, considering the host-pathogen relationship, various studies have pointed out the emergence of immune strategies in mammals in order to fight infection [5], [6]. Moreover, the large amount of data indicating a dense cross-talk between neuroendocrine and immune systems suggests the evolution of non-immune (e.g. behavioural) strategies to serve the same purpose. This supposition is reinforced by the fact [2] that sick animals are submitted to a situation in which their integrity is heavily compromised, either through the bypass of their immune resources or by over-delaying the expression of basic life-supporting behaviours.
However, despite highlighting the bi-directional immune-neuroendocrine communications, the study of behavioural patterns in sick animals has not received much attention, presumably because of its triviality. Almost all behavioural studies in the field of the neurobiology of cytokines use behaviour as a dependent variable to determine which cytokines are neurologically active, and what underlying receptor mechanisms may be involved. The approach that is proposed here is different. It considers sickness behaviour as an independent variable that deserves to be studied as such, in order to determine the way it is organised and regulated at the behavioural level. The understanding of sickness behaviour per se should provide basic knowledge on a little studied natural animal behaviour.
Our alternative hypothesis is that the behavioural modifications accompanying the course of infection or inflammation (e.g. lethargy, adipsia, hypophagia, and social disinterest), reflect the expression of motivational reorganisation, specifically devoted to counter the pathogenic micro-organisms. On this view, motivation is defined as a central state that orientates the perception and actions of the sick animal [7].
Section snippets
Host response to infection and inflammation
Response of the organism to disturbances of its homeostasis during viral, bacterial, or parasitic infection, and tissue injury is called acute phase response [8]. It consists of both a local and systemic reaction. The local reaction is dominated by inflammation. It involves many different cell types, including phagocytic cells, lymphocytes and endothelial cells. Interactions between these different cell types are mediated by soluble factors known as cytokines. The systemic reaction is mediated
Sickness behaviour as an adaptive response to infection and inflammation
Hart [2] convincingly argued that the behavioural features of sick animals are not maladaptive responses or side-effects of the pathogen-induced debilitation. Rather, he proposed that these changes form a co-ordinated ensemble termed sickness behaviour. This concept refers to a behavioural strategy intended to support the metabolic and physiological changes that occur in the infected organism and which help to fight the pathogen. For example, cytokine-induced hypophagia can be interpreted as a
Conclusion
The results briefly reviewed here support the theoretical position of sickness as sustaining a motivational system. Thus, the data show that sick animals cannot be considered as incapacitated and that they can express complex behaviours. Moreover, we have seen that this behavioural expression is flexible and depends on changes in the situation. This flexibility can be interpreted in motivational terms. The sick animal develops new behavioural priorities geared to increase the efficiency of its
Acknowledgements
The preparation of this manuscript has been supported by Foundation IPSEN.
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