Abstract
In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as ‘Rensch's rule’. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.
Highlights
To illustrate the nature of ‘Rensch’s rule’ (Rensch 1950), consider allometry for sexual size dimorphism in New World blackbirds (Icterinae)
We report that patterns of allometry in over 5300 bird species demonstrate that Rensch’s rule is driven by a correlated evolutionary change in females to directional sexual selection on males
In groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch’s rule
Summary
To illustrate the nature of ‘Rensch’s rule’ (Rensch 1950), consider allometry for sexual size dimorphism in New World blackbirds (Icterinae). Prediction 1: groups of related species in which sexual selection on size is stronger in males should demonstrate positive allometry, independently of confounding factors such as the overall degree and range of size dimorphism. (c) Comparisons of slopes Subfamily-level allometric slopes for sexual size dimorphism were calculated by regressing log male wing length onto log female wing length (e.g. figure 1b). (d) Analysis of independent variables For most independent variables entered into the models, subfamily values were assigned the mean of species scores (provided in the electronic supplementary material). This approach is routine in comparative studies conducted at higher-order taxonomic levels The significance levels for removed terms are the values in the model constructed immediately prior to their removal. c Final model: R2Z0.503, F5,94Z19.04, p!0.0001. d Multivariate model constructed with independent contrasts (standardized by the root of summed branch lengths), forced through the origin and degrees of freedom corrected by subtracting the number of polytomies (Z22) in the phylogenetic tree (Garland & Diaz-Uriarte 1999)
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