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

Sexually selected traits include not only some of the most elaborate phenotypes in nature, but also some of the most diverse and sexually dimorphic. Understanding the interspecific allometric patterns of such traits is key to understanding their evolution. Rensch's rule (RR) represents a widespread macroevolutionary pattern that describes a positive association between male-biased dimorphism and species size. When applied to sexual size dimorphism (SSD), the presence of 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 find that RR in body size is followed by polygynous, but not monogamous species, pointing to sexual selection as a driver of RR. Surprisingly, tail length does not follow RR, indicating similar evolutionary allometries between males and females. However, evolutionary allometries of male and female traits were both strongly positive among long-tailed species, indicating the lack of RR is likely the result of correlations between male and female phenotypes, and not due to 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. Future research on Rensch's rule might benefit from evaluating patterns of allometry in both SSD and dimorphism in secondary sexual characters, as well as from the examination of sex specific allometries.

Universidad Nacional Mayor de San Marcos
Universität Wien