The role of ecological variation in driving divergence of sexual and non-sexual traits in the red-backed fairy-wren (Malurus melanocephalus)

Patterns of morphological differentiation within Manacus manacus (Aves: Pipridae) in Colombia: revisiting hypotheses of isolation and secondary contact

Determining the factors responsible for phenotypic variation within species is a fundamental task in evolutionary ecology. Isolation by distance, isolation by environment and clines along secondary contact zones between formerly isolated populations are common patterns of morphological variation. In this study, we evaluated whether patterns of morphological variation exhibit association with isolation by distance, isolation by environment or secondary contact in populations of Manacus manacus with contrasting plumage colour. We used field and museum measurements of five morphological characters from 311 individuals from 88 localities distributed across a highly heterogeneous environmental range within Colombia. Climatic variables explained a higher proportion of the variation than geographic distance among localities. We found differences in wing and culmen length between white and yellow phenotypes of M. manacus in Colombia. Overall morphology did not change in a clinal fashion in relation to the contact zone and the few traits that exhibit clinal variation suggest a distant locality for the contact zone, not in line with our expectations by secondary contact between differentiated populations, suggesting alternative mechanisms for differentiation between subspecies. Our results suggest that this species exhibits marked morphological variation associated with changes in temperature and precipitation. These associations are consistent throughout the geographic range analysed, leading us to postulate that dispersal and local adaptation jointly shape the distribution of phenotypic variation.

The couple that sings together stays together: duetting, aggression and extra-pair paternity in a promiscuous bird species

When individuals mate outside the pair bond, males should employ behaviours such as aggression or vocal displays (e.g. duetting) that help assure paternity of the offspring they care for. We tested whether male paternity was associated with aggression or duetting in the red-backed fairy-wren, a species exhibiting high rates of extra-pair paternity. During simulated territorial intrusions, aggression and duetting were variable among and repeatable within males, suggesting behavioural consistency of individuals. Males with quicker and stronger duet responses were cuckolded less often than males with slower and weaker responses. In contrast, physical aggression was not correlated with male paternity. These results suggest that either acoustic mate guarding or male-female vocal negotiations via duetting lead to increased paternity assurance, whereas physical aggression does not.

Differential rates of phenotypic introgression are associated with male behavioral responses to multiple signals

Sexual selection on multiple signals may lead to differential rates of signal introgression across hybrid zones if some signals contribute to reproductive isolation but others facilitate gene flow. Competition among males is one powerful form of sexual selection, but male behavioral responses to multiple traits have not been considered in a system where traits have introgressed differentially. Using playbacks, mounts, and a reciprocal experimental design, we tested the hypothesis that male responses to song and plumage in two subspecies of red-backed fairy-wren (Malurus melanocephalus) explain patterns of differential signal introgression (song has not introgressed, whereas plumage color has introgressed asymmetrically). We found that males of both subspecies discriminated symmetrically between subspecies' songs at a long range, but at a close range, we found that aggression was equal for both subspecies' plumage and songs. Taken together, our results suggest that male behavioral responses hinder the introgression of song, but allow for the observed asymmetrical introgression of plumage. Our results highlight how behavioral responses are a key component of signal evolution when recently divergent taxa come together, and how differential responses to multiple signals may lead to differential signal introgression and novel trait combinations.

Experimental evidence that extra-pair mating drives asymmetrical introgression of a sexual trait

Theory suggests that traits under positive selection may introgress asymmetrically across a hybrid zone, potentially driven by sexual selection. Two subspecies of the red-backed fairy-wren (Malurus melanocephalus) differ primarily in a sexual signal used in mate choice-red versus orange male back plumage colour-but phylogeographic analyses suggest asymmetrical introgression of red plumage into the genetic background of the orange subspecies. We hypothesized that this asymmetrical introgression may be facilitated by sexual selection if red males have a mating advantage over orange males. We tested this hypothesis with correlational data and a plumage manipulation experiment where we reddened the back plumage of a subset of orange males to mimic males of the red subspecies. There was no correlational evidence of a mating advantage to naturally redder males in this population. Experimentally reddened males sired a similar amount of within-pair young and lost paternity at the same rate as orange males, but they sired significantly more extra-pair young, leading to substantially higher total reproductive success. Thus, we conclude that sexual selection via extra-pair mating is a likely mechanism responsible for the asymmetrical introgression of plumage colour in this system, and is potentially driven by a sensory bias for the red plumage signal.