Classical sexual selection theory provides a well-supported conceptual framework for understanding the evolution and signalling function of male ornaments. It predicts that males obtain greater fitness benefits than females through multiple mating because sperm are cheaper to produce than eggs. Sexual selection should therefore lead to the evolution of male-biased secondary sexual characters. However, females of many species are also highly ornamented. The view that this is due to a correlated genetic response to selection on males was widely accepted as an explanation for female ornamentation for over 100 years and current theoretical and empirical evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs. Since few studies have explored interspecific patterns of both male and female elaboration, our understanding of the evolution of animal ornamentation remains incomplete, especially over broad taxonomic scales. Here we use a new method to quantify plumage colour of all ~6,000 species of passerine birds to determine the main evolutionary drivers of ornamental colouration in both sexes. We found that conspecific male and female colour elaboration are strongly correlated, suggesting that evolutionary changes in one sex are constrained by changes in the other sex. Both sexes are more ornamented in larger species and in species living in tropical environments. Ornamentation in females (but not males) is increased in cooperative breeders--species in which female-female competition for reproductive opportunities and other resources related to breeding may be high. Finally, strong sexual selection on males has antagonistic effects, causing an increase in male colouration but a considerably more pronounced reduction in female ornamentation. Our results indicate that although there may be genetic constraints to sexually independent colour evolution, both female and male ornamentation are strongly and often differentially related to morphological, social and life-history variables.

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The selective forces that drive the evolution of cooperation have been intensely debated. Evolutionary transitions to cooperative breeding, a complex form of cooperation, have been hypothesized to be linked to low degrees of promiscuity, which increases intragroup relatedness and the indirect (that is, kin selected) benefits of helping. However, ecological factors also promote cooperative breeding, and may be more important than relatedness in some contexts. Identifying the key evolutionary drivers of cooperative breeding therefore requires an integrated assessment of these hypotheses. Here we show, using a phylogenetic framework that explicitly evaluates mating behaviours and ecological factors, that evolutionary transitions to cooperative breeding in cichlid fishes were not associated with social monogamy. Instead, group living, biparental care and diet type directly favoured the evolution of cooperative breeding. Our results suggest that cichlid fishes exhibit an alternative path to the evolution of complex societies compared to other previously studied vertebrates, and these transitions are driven primarily by direct fitness benefits.

Signals of dominance and fighting ability (i.e. status signals) are found in a wide range of taxa and are used to settle disputes between competitive rivals. Most previous research has considered status-signal phenotype as an attribute of the individual; however, it is more likely that signal expression is an emergent property that also incorporates aspects of the social environment. Furthermore, because an individual's signal phenotype is likely to influence its social interactions, the relationships between status signals, social environment and individual quality are probably much more complex than previously appreciated. Here, we explore the dynamic relationship between social interactions and signal expression in a previously undescribed status signal, the frontal shield of the pukeko (Porphyrio porphyrio melanotus: Aves). We demonstrate that frontal shield size is a strong predictor of dominance status within social groups, even after controlling for potentially confounding variables. Then, we evaluate the relationship between social interactions and signal expression by testing whether manipulating apparent shield size influences (i) dominance interactions and (ii) future signal expression. By showing that decreasing apparent shield size causes both an increase in the amount of aggression received and a decrease in an individual's true shield size, we provide the first evidence of dynamic feedback between signal expression and social interactions. Our study provides important insight into the role of receiver-dependent (i.e. social) costs in maintaining signal honesty and demonstrates a unique approach to studying status signalling applicable to future studies on dynamic morphological signals.