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Social modulation of sickness behavior and its neuroendocrine basis


When animals are suffering from an infection, they frequently exhibit symptoms such as reductions in activity, reductions in food and water intake, reductions in libido and in social interactions. Adoption of these "sickness behaviors" is thought to promote immune function by reducing energy expenditure in activities that are not essential for recovery from the infection and investing this energy in mounting an immune response. In other words, during disease, since the body has limited resources, these need to be traded-off between investment in immunity and investment in other activities. My dissertation work was focused on exploiting this trade-off idea by examining how different social contexts affect the expression of sickness behaviors in birds. Social modulation of sickness behaviors should be especially relevant when animals have an opportunity to reproduce. Hence, my work focused as well on how immune challenges affect the reproductive system and how the social environment can determine the extent to which animals invest in reproduction while sick. Finally, it was my purpose to understand whether alterations in sickness behaviors due to social context impact the immune response in ways that are costly for animals.

The work in this dissertation emphasizes the plasticity of the sickness behavior response. Here, I demonstrate that birds are able to adjust the expression of sickness behaviors when subjected to social circumstances that promote other adaptive opportunities. While the reproductive system is extensively shut down during an immune challenge, I demonstrate that this effect is reversed within 30 minutes of presentation of a potential mate. In addition, my work indicates that the social modulation of sickness behaviors comes at the cost of reduced immune defenses. In a world where infectious diseases represent one of the major causes of death, an increased understanding of the way behavior during infection is impacted by the social environment and the costs this might carry might promote better guidelines on how to proceed with infected animals (including humans). As well, a deeper knowledge of the endocrine and immune factors mediating this response has the potential to lead to better tools to treat infections. On the other hand, the results in here alert for the reality that our ability to detect sick animals might be obscured by social context, reducing our chances of controlling the spread of infectious diseases (such as the avian flu). With the added knowledge from this work, I expect that sickness behavior might be used as a new tool for learning about motivation underlying social behaviors.

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