15.4 n&v 703 NR

the study, the second-year females’ choice of mate showed no effect of bower treatment, but they did in year 2, when even more decorations were added. An age-biased effect is not restricted to female responses to bower decorations, but also applies to male display intensity. The more intense the male display during the second stage of courtship, the more likely were older females to visit a male in the third stage. But that did not apply to the two younger cohorts. Coleman et al. conclude that selection generated by age-biased variation in female choice is responsible for the evolution of complex courtship in these bowerbirds. The fear that the male’s display instils in younger females seems to drive the age-biased differences in courtship preference. Clearly, the further elaboration of bower decorations to the degree used in the manipulations would be under stronger selection from younger rather than older females. In this study, however, it is not clear if older females would generate selection to maintain the current level of bower decorations. Perhaps there is a lower threshold for this component of the display in older than in younger females; if so, older females alone could generate the selection needed to maintain this complex display. The fact that second-year females were not influenced by augmented bower decorations in year 1 of the study,but were in year 2 when more decorations were used, suggests that thresholds for an effect might change continuously with age. This age class could also offer a target group for research to understand the relative value of decorations and male display intensity for females of different ages. A larger question concerns the evolutionary pattern of bowerbird displays. The complexity of bowers, decorations and courtship behaviour varies greatly among different species of bowerbird. Can these broad macroevolutionary patterns among species be explained by the same microevolutionary process suggested by the findings of Coleman et al.? Studies of fish, frogs, cockroaches and humans have shown that female mating preferences can change as a function of female age, size and reproductive status. Such variation in preferences can contribute to the maintenance of variation in male traits. Young female guppies, for example, prefer males with more orange pigmentation, whereas older females are agnostic about colour and seem to prefer courtship vigour; larger female cricket frogs prefer lower-frequency mating calls, but smaller females prefer higher-frequency calls. What is unusual in the case of satin bowerbirds is that the variation in female mating preferences more clearly contributes to the evolution of multi-component and news and views