Experimentally increased social competition compromises humoral immune responses in house finches.

Although social behavior can substantially influence an individual's physiology, few studies have examined whether intraspecific competition compromises individual immunocompetence. We experimentally manipulated the intensity of social competition in captive non-breeding house finches (Carpodacus mexicanus) by supplying few (high competition) or many (low competition) feeding sites. We tested whether elevated levels of social competition caused individual changes in aggression rates, humoral immunity, body mass, and baseline and stress-induced corticosterone concentrations. We also examined whether physiological responses to social competition were related to an individual's social status. We found that house finches under high social competition had significantly higher aggression rates, lower antibody responses, and lost more body mass. Within flocks, dominant individuals mounted stronger immune responses in both competition treatments. Our statistical power to detect differences in circulating corticosterone concentrations was low, but we did not find any support for the hypothesis that corticosterone concentrations mediate immunosuppression among or within flocks: baseline and stress-induced corticosterone concentrations did not differ under high and low social competition, were unrelated to individual social status, and did not predict the extent of immunosuppression among individuals. Overall, we documented that two universal components of social behavior, intraspecific competition and social status, modulated the strength of a humoral immune response in house finches.