Peer Review #2 of "Evolution of sexual dimorphism and Rensch’s rule in the beetle genus Limnebius (Hydraenidae): is sexual selection opportunistic? (v0.2)"

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.

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Evolution of sexual dimorphism in an endemic toad of the Qinghai–Tibet plateau fails to obey Rensch’s rule

Journal of Zoological Systematics & Evolutionary Research ◽

10.1111/jzs.12554 ◽

2021 ◽

Author(s):

Tong Lei Yu ◽

Gang Wang ◽

Mariana M. Vasconcellos ◽

Yu Jie Li

Keyword(s):

Sexual Dimorphism ◽

Tibet Plateau ◽

Rensch’S Rule ◽

Rensch's Rule ◽

Qinghai Tibet Plateau

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It is not always about body size: evidence of Rensch's rule in a male weapon

Biology Letters ◽

10.1098/rsbl.2021.0234 ◽

2021 ◽

Vol 17 (6) ◽

pp. 20210234

Author(s):

Glauco Machado ◽

Bruno A. Buzatto ◽

Diogo S. M. Samia

Keyword(s):

Sexual Selection ◽

Body Size ◽

Potential Role ◽

Empirical Studies ◽

Condition Dependence ◽

Rensch’S Rule ◽

Evolutionary Pattern ◽

Rensch's Rule ◽

Original Proposal

In many species, sexual dimorphism increases with body size when males are the larger sex but decreases when females are the larger sex, a macro-evolutionary pattern known as Rensch's rule (RR). Although empirical studies usually focus exclusively on body size, Rensch's original proposal included sexual differences in other traits, such as ornaments and weapons. Here, we used a clade of harvestmen to investigate whether two traits follow RR: body size and length of the fourth pair of legs (legs IV), which are used as weapons in male–male fights. We found that males were slightly smaller than females and body size did not follow RR, whereas legs IV were much longer in males and followed RR. We propose that sexual selection might be stronger on legs IV length than on body size in males, and we discuss the potential role of condition dependence in the emergence of RR.

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Evolution of sexual dimorphism and Rensch’s rule in the beetle genusLimnebius(Hydraenidae): is sexual selection opportunistic?

PeerJ ◽

10.7717/peerj.3060 ◽

2017 ◽

Vol 5 ◽

pp. e3060 ◽

Cited By ~ 4

Author(s):

Andrey Rudoy ◽

Ignacio Ribera

Keyword(s):

Sexual Selection ◽

Body Size ◽

General Pattern ◽

Strong Support ◽

Male Body ◽

Rensch’S Rule ◽

Male Body Size ◽

Rensch's Rule ◽

The Individual ◽

Male Genital

Sexual size dimorphism (SSD) is widespread among animals, with larger females usually attributed to an optimization of resources in reproduction and larger males to sexual selection. A general pattern in the evolution of SSD is Rensch’s rule, which states that SSD increases with body size in species with larger males but decreases when females are larger. We studied the evolution of SSD in the genusLimnebius(Coleoptera, Hydraenidae), measuring SSD and male genital size and complexity of ca. 80% of its 150 species and reconstructing its evolution in a molecular phylogeny with 71 species. We found strong support for a higher evolutionary lability of male body size, which had an overall positive allometry with respect to females and higher evolutionary rates measured over the individual branches of the phylogeny. Increases in SSD were associated to increases in body size, but there were some exceptions with an increase associated to changes in only one sex. Secondary sexual characters (SSC) in the external morphology of males appeared several times independently, generally on species that had already increased their size. There was an overall significant correlation between SSD, male body size and male genital size and complexity, although some lineages with complex genitalia had low SSD, and some small species with complex genitalia had no SSD. Our results suggest that the origin of the higher evolutionary variance of male body size may be due to lack of constraints rather than to sexual selection, that may start to act in species with already larger males due to random variation.

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