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This chapter shows how free-living animals respond to attack, either by predators or parasites. It is clear that infection, injury, population density, and competition trigger facultative and presumably adaptive behavioral and physiological responses such as sickness behavior, dispersal, and irruptive migration. These responses, however, are modified by the individual’s social status and coping style (reactive or proactive). Most of these kinds of investigations have been conducted at least partially in the field, and therefore represent facultative responses under naturalistic conditions. Nonetheless, the sheer diversity and complexity of these responses and their relationships to age, gender, social status etc. present an almost daunting prospect for the future to tease apart common mechanisms and underlying themes that explain how life on Earth copes with a frequently capricious environment.

Parasites both affect and are affected by the behavior of birds. Moreover, specific avian behaviors can either suppress or augment parasite spread, depending on context and parasite transmission mode. The chapter focuses on major categories of behavior important for the key steps of the parasite transmission process: first, the likelihood of exposure to parasites; second, host susceptibility to infection and degree of infectiousness reached once exposed; and third, the likelihood of spreading infection to other hosts or vectors. The chapter begins by discussing behaviors that birds use to minimize exposure to parasites (e.g., preening and other ‘antiparasite’ behaviors) and the immediate effects of infection on behavior (e.g., lethargy and other ‘sickness’ behaviors). The focus then truns to foraging and movement, which are specific behaviors that both can increase exposure to parasites and are altered by infection. Finally, the chapter considers how a suite of behaviors including social interactions, individual personality, and mating behaviors can affect and are affected by parasites in ways relevant to all three steps of the parasite transmission process. Throughout, the chapter highlights and integrates areas in which recent advances have been made or for which more data are sorely needed in avian systems, emphasizing directions for future research at the intersection of avian behavior and infectious disease ecology.

This chapter discusses insect behavioral responses to parasites. Dividing behaviors conceptually into those that occur before and after infection, we start by reviewing the evidence that insects identify and avoid potentially infectious environments to minimize negative consequences of infection. Behavioral responses following infection according to their adaptive value to either the insect host or to the parasite will then be considered. One section covers sickness behaviors proposed to benefit the host by conserving energetic resources during infection; another section discusses evidence for altered host behavior as a parasite adaptation enhancing parasite survival or transmission. The mechanistic link between behavior and immunity in insects is briefly discussed, and provide a summary of methods and techniques becoming standard to studying behavior of model insect species in the context of infection. The chapter concludes by discussing future directions in the study of insect behavioral responses to parasites.

If one thinks about how it felt the last time one came down with the flu or had a serious cold, negative emotions and bodily experiences are likely to come to mind. Feeling somewhat depressed, a bit worried or anxious, fatigued, and sore would be typical, as well as wanting to limit the number of social interactions. This chapter explains why these kinds of reactions are triggered when danger arises, especially when your body is threatened from within at the micro level, such as when bacteria or viruses are detected. These behavioral responses are collectively called sickness behavior or sickness response, and are displayed—as far as we can observe them—in very similar ways in animals and humans, to save energy and to improve survival. Even though the behavioral changes are adaptive in the short run, they are maladaptive if unabated. The chapter describes how healthy people, or even the researchers themselves, can be made purposely sick by injection of a small and non-dangerous fragment of a bacterium, resulting in a few hours of malaise during which the brain, behaviors, and the subjective health experience can be studied.