Climbing to reach females: Romeo should be small |
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Authors: | Moya-Laraño Jordi Halaj Juraj Wise David H |
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Institution: | Department of Entomology, University of Kentucky, S-225 Agricultural Sciences Building North, Lexington Kentucky 40546-0091 |
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Abstract: | The race for reaching mates by the time they are receptive, or sexual selection by scramble competition, has received little attention. We argue that smaller males are favored in species in which the male must climb to reach females located in high habitat patches. This new explanation we term the "gravity hypothesis" of sexual size dimorphism (SSD). We show that a simple biomechanical model of animal movement predicts that: (1) selection should favor a comparatively smaller size in the searching sex when searching involves climbing; and (2) this effect should be stronger in larger species than in smaller species. In reaching high habitats, smaller, faster searchers will be favored either through sexual selection by scramble competition and/or by escaping predation easier by running faster on vertical surfaces. Different spider species are found at a wide range of heights. We compiled a dataset of spider taxa and arranged their habitats according to four height categories, ranked from soil surface to trees. We show that, after controlling for phylogeny, both predictions of the gravity hypothesis of SSD are met. Thus, it appears that the constraint imposed by gravity on climbing males is a selective factor in determining male dwarfism. |
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Keywords: | Araneomorph spiders biomechanics climbing gravity male dwarfism natural selection scramble competition sexual selection sexual size dimorphism |
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