Many animals exhibit social plasticity - changes in phenotype or behaviour in response to experience with conspecifics that change how evolutionary processes like sexual selection play out. Here, we asked whether social plasticity arising from variation in local population density in male advertisement signals and female mate preferences influences the form of sexual selection. We manipulated local density and determined whether this changed how the distribution of male signals overlapped with female preferences - the signal preference relationship. We specifically look at the shape of female mate preference functions, which, when compared to signal distributions, provide hypotheses about the form of sexual selection. We used Enchenopa binotata treehoppers, a group of plant-feeding insects that exhibit natural variation in local densities across individual host plants, populations, species and years. We measured male signal frequency and female preference functions across the density treatments. We found that male signals varied across local social groups, but not according to local density. By contrast, female preferences varied with local density - favouring higher signal frequencies in denser environments. Thus, local density changes the signal-preference relationship and, consequently, the expected form of sexual selection. We found no influence of sex ratio on the signal-preference relationship. Our findings suggest that plasticity arising from variation in local group density and composition can alter the form of sexual selection with potentially important consequences both for the maintenance of variation and for speciation.
A key question in speciation research is how ecological and sexual divergence arise and interact. We tested the hypothesis that mate choice causes local adaptation and ecological divergence using the rationale that the performance~signal trait relationship should parallel the attractiveness~signal trait relationship. We used female fecundity as a measure of ecological performance. We used a species in the Enchenopa binotata treehopper complex, wherein speciation involves adaptation to novel environments and divergence in sexual communication. We used a full‐sibling, split‐family rearing design to estimate genetic correlations (r G) between fecundity and signal traits, and compared those relationships against population‐level mate preferences for the signal traits. Animal model estimates for r G between female fecundity and male signal traits overlapped zero—rejecting the hypothesis—but could reflect sample size limitations. The magnitude of r G correlated with the strength of the mate preferences for the corresponding signal traits, especially for signal frequency, which has the strongest mate preference and the most divergence in the complex. However, signal frequencies favored by the population‐level mate preference are not associated with high fecundity. Therefore, mate preferences do not appear to have been selected to favor high‐performance genotypes. Our findings suggest that ecological and sexual divergence may arise separately, but reinforce each other, during speciation.
Finding and attracting mates can impose costs on males in terms of increased encounters with, and attraction of, predators. To decrease the likelihood of predation, males may modify mate-acquisition efforts in two main ways: they may reduce mate-searching efforts or they may reduce mate-attraction efforts. The specific behavior that males change in the presence of predator cues should depend upon the nature of risk imposed by the type of predator present in the environment. For example, sit-and-wait predators impose greater costs to males moving in search of mates. Here, we test whether cues of the presence of a sit-and-wait predator lead to a reduction in mate-searching but not mate-acquisition behavior. We used a member of the Enchenopa binotata complex of treehoppers-a clade of vibrationally communicating insects in which males fly in search of mates and produce mate-attraction signals when they land on plant stems. We tested for changes in mate-searching and signaling behaviors when silk from a web-building spider was present or absent. We found that males delayed flight when spider silk was present but only if they were actively searching for mates. These results suggest that males have been selected to reduce predation risk by adjusting how they move about their environment according to the cues of sit-and-wait predators.
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