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Biological Basis of the Behavior of Sick Animals
B E N J A M I N L. H A R T
Department o f Physiological Sciences, School o f Veterinao, Medicine University o f California, Davis, CA 95616
R e c e i v e d 1 F e b r u a r y 1988 HART, B. L. Biological basisof the behavior of sick animals. NEUROSCI BIOBEHAV REV 12(2) 123-137, 1988.The most commonly recognized behavioral patterns of animals and people at the onset of tebrile infectiousdiseases are lethargy, depression, anorexia, and reduction in grooming. Findings from recent lines of research are reviewed to formulate the perspective that the behavior of sick animals and people is not a maladaptiveresponse or the effect of debilitation, but rather an organized, evolved behavioral strategy to facilitate the role of fever in combating viral and bacterial infections. The sick individual is viewed as being at a life or death juncture and its behavior is an all-out effort to overcome the disease. Fever lnterleukin-1 Anorexia Sleep Depression Disease
ALMOST everything that is written about thespeciestypical behavioral patterns of animals, including discussions of adaptive significance, ultimate causation, proximate causation, and physiological determinants, involves the behavior of healthy animals. The behavior of sick animals is not mentioned in the context of discussion of the function of behavior, presumably on the assumption that the behavior is not particularly adaptive and is theresult of debilitation from a disease process along with secondary effects such as inability to obtain food and water. Animals that are acutely ill with systemic protozoan, bacterial or viral infections are typically described as depressed and lethargic with little interest in eating food and drinking water. A little later in the course of a disease they commonly show signs of dehydration alongwith indications that they have lost interest in grooming since they develop rough hair coats. These behavioral signs generally accompany a fever response and, together with the occurrence of fever, are recognized by animal handlers and veterinarians as signs that an animal is sick or is becoming sick with an infectious disease. This picture of the lethargic, depressed, anorexic, and febrileindividual is not specific to any particular animal species, but is seen in humans and a variety of animals, and the behavioral signs are seen with a variety of systemic diseases as well as with some more localized infections. Some recent research, especially that demonstrating the role of fever in facilitating a person's or an animal's ability to combat viral and bacterial infections, has led to thedevelopment of a perspective presented here that the behavior of a sick individual is not a maladaptive and undesirable effect of illness but rather a highly organized behavioral strategy that is at times critical to the survival of an individual if it were living in the wild state. In this paper 1 will present an overview of the possible adaptive and functional values of the behavior typical ofsick animals and people. Some aspects of the perspective have been briefly outlined before [72,73]. AI-
though the emphasis is on animals, the concepts are relevant to human biobehavioral science and could be of use in understanding behavioral aspects of human illness. Perhaps because our view of infectious disease is so shaped by an orientation on the importance of vaccinations, chemotherapeutictreatment, and supportive therapy, we tend to overlook the fact that animals and people have been exposed to, and have survived, the effects of disease-causing organisms through millions of years of evolutionary history. The coevolution of the host-pathogen or host-parasite relationship is by now well established in evolutionary biology [59,138]. Indeed exceedingly complex nonspecific and...