Context-dependent effect of social environment on immune response and sexual signalling in male zebra finches

Variation in avian immune response can be influenced by social environment. This is of particular interest in the context of immunomediated sexual behaviour because social environment may subsequently affect a bird's relative investment in immunocompetence versus sexual signalling. I tested whether the effect of social environment on immune response and sexual signalling depends on socio-sexual status using male zebra finches (Taeniopygia guttata). To do this, I manipulated social environment ('same sex' versus 'dual sex') and socio-sexual status ('high' versus 'low') of the males. I then determined what effect these manipulations had on an index of immunocompetence, namely cell-mediated immune response, and two indices of sexual signalling (bill colour and song rate). I found that social environment influenced cell-mediated immune response and sexual signalling in low-status males. These males had lower immune responses and increased sexual signalling in the dual-sex environment compared with the same-sex environment. In contrast, high-status males had similar immune responses and sexual signalling regardless of social environment. These results suggest that social environment can influence immune response and sexual signalling; however, the nature of this effect was context-dependent, with low-status males more affected than high-status males.