Weight lifting and health status in the black wheatear

Abstract

Male black wheatears Oenanthe leucura demonstrate an exaggerated sexual display by carrying many heavy stones (on average 3.1 kg per season for this 35-g passerine) to cavities inside caves before the start of each clutch. The energetic cost of this display is mainly determined by the size of stones, which gives rise to the largest power output of a muscle ever recorded for any species (400 W/kg), rather than the total number of stones carried, which only amounts to 1% of daily energy expenditure for a black wheatear male. We tested whether stone carrying reflected male health status by removing two primaries from a group of males, but not from controls. Experimental manipulation did not significantly affect stone carrying. Similarly, male health status measured as hematocrit, leukocyte concentration, and T-cell response to an injection with phytohemagglutinin (a lectin) was not affected by experimental manipulation. Male black wheatears that carried very heavy stones, each on average more than a quarter of their own body mass, had a stronger T-cell response to injection with a lectin than males that carried light stones. Furthermore, males that carried heavy stones had a reduced hematocrit level, which may have arisen from the effects of the extremely heavy work load. Rate of stone carrying and mass of stones was not significantly related to the abundance of two species of ectoparasites. Male black wheatears with high T-cell responses were significantly more likely to survive than males with low responses, but other measures of male performance did not differ significantly between survivors and nonsurvivors. The male stone-carrying display thus reflects male health status as demonstrated by a component of immunocompetence.