Immune challenge changes social behavior in the guppy (Poecilia reticulata)

Increased exposure to pathogens is often considered to be one of the most significant costs of group living. As a result, animals typically avoid close association with individuals who manifest symptoms of disease. The question remains, however, whether avoidance behaviors are mediated by effects relating specifically to the disease itself, or through recognition of more general sickness behaviors that emerge from the activity of the immune system in response to infection. Here we examined the effects of an immune challenge, induced through exposure to bacterial lipopolysaccharides (LPS), on individual and social behavior in female guppies (Poecilia reticulata). We found that in a binary choice presentation, healthy focal individuals showed a strong preference to associate with stimulus shoals of saline-injected fish rather than stimulus shoals of LPS-injected fish. In a subsequent examination of group-level dynamics, shoals of LPS-injected females were more dispersed than similar shoals comprised of control fish and showed less coherent collective behavior despite the lack of any obvious difference in general activity between LPS and control fish. We discuss these findings in relation to the need for social animals to mitigate the risk of disease that is associated with living in proximity to conspecifics.