Immunocompetence and testosterone-induced condition traits in male ruffs (Philomachus pugnax)

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Abstract The immune system is costly to operate, so we expect allocation to this function to relate to other indices of overall phenotypic condition. In captive male ruffs (Philomachus pugnax), we examine covariation between three seasonal male characteristics induced by testosterone (neck 'ruff', facial wattles, and increases in mass/size) and cell-mediated and humoral immunity. The ruff has two genetically distinct morphs of males, 'independents' and 'satellites', which differ in mating behaviour, somewhat in body size, and possibly life history strategy. Ruff length and wattle number were significantly correlated with each other and with body size, but unrelated to mass/size. Humoral immunity was weakly correlated with size, but not with secondary male traits, or with cell-mediated immunity. We did not detect differences in humoral immunity between morphs. Cell-mediated immunity was weakly correlated with ruff length, but not with wattle number, size, or mass/size. Cell-mediated immunity decreased significantly with age. Controlling for age, independents had higher cell-mediated immunity responses than satellites. The expected relationships between two measures of immune function and other measures of condition were not always present, which suggests we need more data and/or more specific predictions about how the different aspects of immune function relate to each other and to ecological variables.