Pathogen resistance and immunocompetence covary with social status in house finches (<i>Carpodacus mexicanus</i>)

Abstract

Summary Social status has striking effects on individual physiology, but the extent to which it influences pathogen resistance has rarely been explored. Here we examine how social status within groups of wintering house finches ( Carpodacus mexicanus ) relates to immunocompetence and susceptibility to an infectious disease. We assayed cellular and humoral components of the immune response, then quantified symptom severity in response to experimental infection with Mycoplasma gallisepticum , a naturally occurring bacterial pathogen that causes periodic epidemics in social aggregations of house finches across North America. Socially dominant house finches mounted stronger immune responses to phytohaemagglutinin but weaker antibody titres to sheep red blood cells. Dominant males, but not females, showed significantly lower symptom severity and proportion of time symptomatic when inoculated with M. gallisepticum . Individual condition was not associated with social status before or after infection, but infection with M. gallisepticum created disparities in resource access across social status. Infected dominant birds of both sexes fed for significantly more time intervals than infected subordinate birds following infection. Changes in resource access may partly underlie observed relationships between social status and disease severity, but the detected sex‐specific interactions suggest a role for endocrine–immune relationships that warrant further study. Overall, our results indicate that social behaviour can influence both immunocompetence and resistance to an infectious disease of broad impact in the wild, highlighting the importance of considering the social context of individuals in studies of disease in natural populations.