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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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Standardized size dimorphism (log male wing length−log female wing length) versus male size in subfamilies with different mating systems. Polygyny is associated with positive allometry (i.e. slopes >0 in these kinds of plots), and polyandry is associated with negative allometry (i.e. slopes <0). Subfamily names are provided in the order of increasing body size. Grey points comprise the dimorphism versus size relationship for all birds, while coloured points comprise target subfamilies. Representative monogamous subfamilies (d–f) were selected to cover a broad range of species sizes; however, patterns are similar in other monogamous subfamilies. In (a–c) and (g–i), all relevant subfamilies are plotted. Criteria used to categorize subfamilies: (a) ≤−0.5 and obligate male-only parental care, (b) polygamy >0.10 and some species exhibit obligate male-only parental care, (c) obligate interspecific brood parasitism, (d) polygamy between −0.06 and 0.06 and size dimorphism <0, (e) polygamy between −0.06 and 0.06, and dichromatism between 0 and 0.20, (f) polygamy between −0.06 and 0.06, and dichromatism >1.0, (g) polygamy between 0.125 and 0.25, (h) polygamy between 0.25 and 0.75 and (i) polygamy >0.75.
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fig4: Standardized size dimorphism (log male wing length−log female wing length) versus male size in subfamilies with different mating systems. Polygyny is associated with positive allometry (i.e. slopes >0 in these kinds of plots), and polyandry is associated with negative allometry (i.e. slopes <0). Subfamily names are provided in the order of increasing body size. Grey points comprise the dimorphism versus size relationship for all birds, while coloured points comprise target subfamilies. Representative monogamous subfamilies (d–f) were selected to cover a broad range of species sizes; however, patterns are similar in other monogamous subfamilies. In (a–c) and (g–i), all relevant subfamilies are plotted. Criteria used to categorize subfamilies: (a) ≤−0.5 and obligate male-only parental care, (b) polygamy >0.10 and some species exhibit obligate male-only parental care, (c) obligate interspecific brood parasitism, (d) polygamy between −0.06 and 0.06 and size dimorphism <0, (e) polygamy between −0.06 and 0.06, and dichromatism between 0 and 0.20, (f) polygamy between −0.06 and 0.06, and dichromatism >1.0, (g) polygamy between 0.125 and 0.25, (h) polygamy between 0.25 and 0.75 and (i) polygamy >0.75.

Mentions: The relative differences in the nature of allometry between groups with different mating strategies are shown in figure 4. The strongest difference occurs between the two groups with the highest expected degrees of differential sexual selection between the sexes. In strongly polygynous subfamilies, characterized by high proportions of species with lekking or defence polygyny mating systems, Rensch's rule is demonstrated clearly and in remarkably similar fashion across a broad range of taxa. In monogamous groups, there are no apparent allometric relationships, despite high variance in sexual dimorphism in size and coloration. In groups with clear gender-role reversal, allometry goes in the opposite direction. These strongly contrasting patterns provide conclusive support for the hypothesis that Rensch's rule is driven by a correlated evolutionary response in one sex to stronger size selection in the other sex.


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)

Standardized size dimorphism (log male wing length−log female wing length) versus male size in subfamilies with different mating systems. Polygyny is associated with positive allometry (i.e. slopes >0 in these kinds of plots), and polyandry is associated with negative allometry (i.e. slopes <0). Subfamily names are provided in the order of increasing body size. Grey points comprise the dimorphism versus size relationship for all birds, while coloured points comprise target subfamilies. Representative monogamous subfamilies (d–f) were selected to cover a broad range of species sizes; however, patterns are similar in other monogamous subfamilies. In (a–c) and (g–i), all relevant subfamilies are plotted. Criteria used to categorize subfamilies: (a) ≤−0.5 and obligate male-only parental care, (b) polygamy >0.10 and some species exhibit obligate male-only parental care, (c) obligate interspecific brood parasitism, (d) polygamy between −0.06 and 0.06 and size dimorphism <0, (e) polygamy between −0.06 and 0.06, and dichromatism between 0 and 0.20, (f) polygamy between −0.06 and 0.06, and dichromatism >1.0, (g) polygamy between 0.125 and 0.25, (h) polygamy between 0.25 and 0.75 and (i) polygamy >0.75.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig4: Standardized size dimorphism (log male wing length−log female wing length) versus male size in subfamilies with different mating systems. Polygyny is associated with positive allometry (i.e. slopes >0 in these kinds of plots), and polyandry is associated with negative allometry (i.e. slopes <0). Subfamily names are provided in the order of increasing body size. Grey points comprise the dimorphism versus size relationship for all birds, while coloured points comprise target subfamilies. Representative monogamous subfamilies (d–f) were selected to cover a broad range of species sizes; however, patterns are similar in other monogamous subfamilies. In (a–c) and (g–i), all relevant subfamilies are plotted. Criteria used to categorize subfamilies: (a) ≤−0.5 and obligate male-only parental care, (b) polygamy >0.10 and some species exhibit obligate male-only parental care, (c) obligate interspecific brood parasitism, (d) polygamy between −0.06 and 0.06 and size dimorphism <0, (e) polygamy between −0.06 and 0.06, and dichromatism between 0 and 0.20, (f) polygamy between −0.06 and 0.06, and dichromatism >1.0, (g) polygamy between 0.125 and 0.25, (h) polygamy between 0.25 and 0.75 and (i) polygamy >0.75.
Mentions: The relative differences in the nature of allometry between groups with different mating strategies are shown in figure 4. The strongest difference occurs between the two groups with the highest expected degrees of differential sexual selection between the sexes. In strongly polygynous subfamilies, characterized by high proportions of species with lekking or defence polygyny mating systems, Rensch's rule is demonstrated clearly and in remarkably similar fashion across a broad range of taxa. In monogamous groups, there are no apparent allometric relationships, despite high variance in sexual dimorphism in size and coloration. In groups with clear gender-role reversal, allometry goes in the opposite direction. These strongly contrasting patterns provide conclusive support for the hypothesis that Rensch's rule is driven by a correlated evolutionary response in one sex to stronger size selection in the other sex.

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