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Sexual selection explains Rensch's rule of allometry for sexual size dimorphism.

Dale J, Dunn PO, Figuerola J, Lislevand T, Székely T, Whittingham LA - Proc. Biol. Sci. (2007)

Bottom Line: However, despite numerous recent studies, we still do not have a general explanation for this 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Behavioural Ecology and Evolutionary Genetics, Max Planck Institute for Ornithology, PO Box 1564, 82305 Starnberg (Seewiesen), Germany. dale@orn.mpg.de

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.

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Related in: MedlinePlus

Variance in allometric slopes across 182 subfamilies of birds versus (a) degree of polygamy (subfamilies with higher values have proportionally more polygynous species), (b) range in sexual size dimorphism, (c) mean standardized sexual size dimorphism and (d) degree of sexual dichromatism (subfamilies with higher values have more species where males are more colourful than females). Allometric slopes were calculated with RMA models of log male wing length regressed onto log female wing length: higher slopes correspond with stronger positive allometry for sexual size dimorphism, i.e. Rensch's rule. See table 1a for the statistics of the regression lines.
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fig2: Variance in allometric slopes across 182 subfamilies of birds versus (a) degree of polygamy (subfamilies with higher values have proportionally more polygynous species), (b) range in sexual size dimorphism, (c) mean standardized sexual size dimorphism and (d) degree of sexual dichromatism (subfamilies with higher values have more species where males are more colourful than females). Allometric slopes were calculated with RMA models of log male wing length regressed onto log female wing length: higher slopes correspond with stronger positive allometry for sexual size dimorphism, i.e. Rensch's rule. See table 1a for the statistics of the regression lines.

Mentions: To test prediction 1, we first evaluated whether polygamy predicts variance in allometry. Sexual selection on male size is expected to be stronger in polygynous groups than in monogamous or polyandrous groups, because males are under stronger intrasexual competition for mating opportunities (Webster 1992). Univariate analyses suggested that subfamilies that have increased degrees of polygyny also have much stronger positive allometry, as predicted by the sexual selection hypothesis (table 1a; figure 2a). The above pattern could be confounded, however, by correlations between polygamy and other factors related to allometry such as range of size dimorphism or overall size dimorphism (table 1a; figure 2; also see Abouheif & Fairbairn 1997; Fairbairn 1997).


Sexual selection explains Rensch's rule of allometry for sexual size dimorphism.

Dale J, Dunn PO, Figuerola J, Lislevand T, Székely T, Whittingham LA - Proc. Biol. Sci. (2007)

Variance in allometric slopes across 182 subfamilies of birds versus (a) degree of polygamy (subfamilies with higher values have proportionally more polygynous species), (b) range in sexual size dimorphism, (c) mean standardized sexual size dimorphism and (d) degree of sexual dichromatism (subfamilies with higher values have more species where males are more colourful than females). Allometric slopes were calculated with RMA models of log male wing length regressed onto log female wing length: higher slopes correspond with stronger positive allometry for sexual size dimorphism, i.e. Rensch's rule. See table 1a for the statistics of the regression lines.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2211517&req=5

fig2: Variance in allometric slopes across 182 subfamilies of birds versus (a) degree of polygamy (subfamilies with higher values have proportionally more polygynous species), (b) range in sexual size dimorphism, (c) mean standardized sexual size dimorphism and (d) degree of sexual dichromatism (subfamilies with higher values have more species where males are more colourful than females). Allometric slopes were calculated with RMA models of log male wing length regressed onto log female wing length: higher slopes correspond with stronger positive allometry for sexual size dimorphism, i.e. Rensch's rule. See table 1a for the statistics of the regression lines.
Mentions: To test prediction 1, we first evaluated whether polygamy predicts variance in allometry. Sexual selection on male size is expected to be stronger in polygynous groups than in monogamous or polyandrous groups, because males are under stronger intrasexual competition for mating opportunities (Webster 1992). Univariate analyses suggested that subfamilies that have increased degrees of polygyny also have much stronger positive allometry, as predicted by the sexual selection hypothesis (table 1a; figure 2a). The above pattern could be confounded, however, by correlations between polygamy and other factors related to allometry such as range of size dimorphism or overall size dimorphism (table 1a; figure 2; also see Abouheif & Fairbairn 1997; Fairbairn 1997).

Bottom Line: However, despite numerous recent studies, we still do not have a general explanation for this 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Behavioural Ecology and Evolutionary Genetics, Max Planck Institute for Ornithology, PO Box 1564, 82305 Starnberg (Seewiesen), Germany. dale@orn.mpg.de

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.

Show MeSH
Related in: MedlinePlus