Exaggeration of display characteristics enhances detection of visual signals

Many animal signals have been produced by selection operating upon behavioural precursors that lack signal function, a process known as 'ritualization'. When the precursor remains in the repertoire, comparisons of its structure with that of the derived signal permit inference of the forces responsible. Selection for signal efficacy and recognition produce characteristic changes (e.g., increases in intensity and duration, and repetition). The tidbitting display of male fowl, Gallus gallus, has long been considered an example of ritualization. However, the changes associated with this evolutionary process have never been characterized, nor has there been any test of whether the signal is perceived as distinct from its putative precursor (self-feeding). To address this, we first analysed video recordings of males feeding and tidbitting. We found a significant increase in the intensity, duration and repetition of specific movements during the tidbitting display compared to feeding. These results suggest that tidbitting is a ritualized form of self-feeding behaviour, but the critical test requires assessment of receiver response. If females recognized the tidbitting signal as distinct from male feeding behaviour, we predicted that females would food search significantly more in response to tidbitting. The results of high-definition video playbacks support this hypothesis. This is the first empirical demonstration of the perceptual and functional predictions made by a classic ethological hypothesis.