Allometry, sexual dimorphism, and sexual trait elaboration in the birds-of-paradise

Rensch's rule (RR) is a widespread macroevolutionary pattern describing a positive association between male-biased dimorphism and species size. Applied to sexual size dimorphism, RR is often associated with sexual selection, as larger body sizes may benefit males in competition and courtship. Moreover, the presence of RR in sexual traits further indicates that males reap relative performance benefits beyond large body size alone. Here we describe patterns of elaboration, variation, and sexual dimorphism in tail length in the birds-of-paradise (Aves: Paradisaeidae), which exhibit an extreme diversity in tail lengths, ranging from short-tailed species to the longest-tailed passeriform birds. We found that body size followed RR in polygynous, but not monogamous species, in accordance with the sexual selection hypothesis. However, we found no evidence of RR in tail length, indicating similar evolutionary allometries between males and females. Evolutionary allometries of male and female traits were both strongly positive among long-tailed species, suggesting that the lack of RR results from phenotypic correlations between the sexes, rather than constraints on ornament exaggeration. Our study represents the first integrative test of RR in an ornamental morphological trait and evidences how different aspects of dimorphism interact in a group with a hyperdiverse courtship trait.