Inferring the origins of state-dependent courtship traits

We examine a simple model of state-dependent (indicator) traits that focuse s on their evolutionary origins as courtship signals. A necessary condition for the initial evolution of signals was found: the marginal female prefer ence for minimal traits must exceed a certain threshold, where that thresho ld is proportional to the marginal male fitness costs for minimal traits. W e interpret a positive threshold as implying a need for preexisting sensory bias in order to overcome the threshold if indicator signals are to start to evolve. We extend the model to allow for the possibility that signal cos ts and female preferences may vary over evolutionary time. If there is inde pendent information on the way that signaling costs have evolved, then one may use measurements of contemporary female preferences to make inferences concerning the presence of the ancestral threshold. It is the marginal fema le preferences for minimal male traits that are important, whereas reconstr ucting ancestral origins from measurement of average size signals is not in formative. Our analyses suggest two foci for future studies: measurement of the marginal response of contemporary females to minimal male signals and reconstruction of how signaling costs have changed over evolutionary time.