Asymmetric effects of experimental manipulations of social status on individual immune response

Abstract

An individual's social environment can substantially affect its physiology, but few studies have examined how social behaviour influences individual immune responses. Here, I use a within-individual experimental approach to test whether social status within a group has a direct effect on immunity. I quantified humoral immunocompetence for male house finches, Carpodacus mexicanus, in both randomized and manipulated social environments. For initially dominant males, forced decreases in social status resulted in proportional decreases in antibody response to sheep red blood cells. However, for initially subordinate males, changes in social status did not alter antibody titres. Differences in aggressive behaviour between and within flocks suggest that this asymmetry of effects between initial dominants and subordinates may relate to the extent of aggression experienced within each social context. Aggression rates increased dramatically in flocks when initially dominant males were placed together, but did not change when initially subordinate males were combined. Furthermore, the degree of increase in defeat rate for an initially dominant male was the strongest predictor of resulting decreases in immune response. Taken together, these results suggest that physiological changes in response to aggressive interactions, and defeat in particular, probably mediate the association between social status and immunity in this system. This study represents the first experimental demonstration that changes in social status can cause corresponding changes in immunity in an outbred vertebrate population. Results of this work suggest that an individual's social environment may directly mediate its ability to resist parasites and pathogens, an important component of fitness.