ANIMALS often settle disputes by means of conventional displays. It has been suggested that this enables the contestants to assess each other's strength without resorting to a serious fight1. If this is true then we would expect natural selection to favour displays which give reliable information about fighting potential; assessment signals that are easily mimicked by weak individuals will not be evolutionary stable2,3. Often the outcome of a contest will depend simply on who is the larger, and many displays seem to involve assessment of body size1,4. In this paper we show experimentally that male toads, Bufo bufo, settle contests for the possession of females by means of vocalisations that give a reliable signal of body size and hence of fighting ability.