Testosterone influences basal metabolic rate in male house sparrows: a newcost of dominance signalling?

Sexually selected signals of individual dominance have profound effects on access to resources, mate choice and gene flow. However, why such signals s hould honestly reflect individual quality is poorly understood. Many such s ignals are known to develop under the influence of testosterone. We conduct ed an experiment in male house sparrows in which testosterone was manipulat ed independently during two periods: before the onset of the breeding seaso n and prior to the autumn moult. We then measured the effects of these mani pulations on basal metabolic rate and on the size of the chest bib, a sexua lly selected signal. The results demonstrate that testosterone simultaneous ly affects both signal development and basal metabolic rate in the house sp arrow (Passer domesticus). This evidence, therefore, supports a novel concl usion: that testosterone-dependent signals act as honest indicators of male quality possibly because only high-quality individuals can sustain the ene rgetic costs associated with signal development.