Figure 1.

A simple model of costly signalling. Plots show the optimal quality-dependent signal size s* (a) and the associated cost paid by honest signallers (b). If a given signal size is equally costly for individuals of any quality (k = 0), then it is not adaptive for individuals to advertise their quality. However, if a given signal size is differentially costly for low-quality individuals (k > 0), then individuals will honestly signal their quality, with high-quality individuals producing the larger signals. As the magnitude of the differential cost increases in this model, s* tends to increase, whereas the cost of producing the optimal signal tends to decline. The outcome of the joint evolution of signal size and quality dependence, from figure 2, is shown by the red circles (for k = 0) and blue squares (for k = 5).