Social context modulates sickness behavior

Sickness behaviors constitute an array of symptoms exhibited by an animal during the course of an infection, including reduced activity, reduced food and water intake, and reduced social interactions. It is hypothesized that these symptoms enable reallocation of finite energy resources to fight infection. In this way, by focusing energy on healing, available resources are being removed from other activities, potentially reducing adaptive opportunities, such as mating. Hence, to achieve increased reproductive success, animals might be able to adjust the expression of sickness behaviors to their environmental circumstances. While abiotic conditions such as temperature and season can modulate sickness behaviors, no studies in passerines have linked modulation of sickness behaviors to social settings. Here, it is demonstrated that social surroundings affect the extent to which animals exhibit symptoms of sickness. After an immune challenge, zebra finches kept in isolation markedly reduced activity, but those kept in a colony setting did not. The same trend is verified when looking at the time they spent resting. Additionally, a proinflammatory cytokine (interleukin-6) was quantified in plasma samples and all animals that had been immune challenged showed increased levels of this marker, showing that the physiological response was similar. Hence, birds in a social context were able to overcome the behavioral, but not physiological, symptoms usually associated with an inflammatory response. These findings suggest a trade-off between allowing the body to respond to an infection and taking advantage of being in a social situation.