1. Moya-Larano et al. (2002 ; Evolution 56, 420-425) showed that female-biased sexual size dimorphism is more pronounced in tree-dwelling spider species than in species inhabiting lower-lying substrates. They proposed the Gravity Hypothesis for sexual size dimorphism: small size should be advantageous for males in habitats where successful mate searching requires climbing, because mass-specific power and hence the speed of climbing against gravity, must decrease with increasing size. 2. Their biomechanical model is based on a false premise, that the cross-sectional area of a muscle determines its power output. In fact, muscle power is proportional to muscle volume, and hence mass-specific power should be independent of size. We therefore predict, contrary to the Gravity Hypothesis, that climbing speed should be independent of body size. 3. We tested these contrasting predictions using adult male Western black widow spiders Latrodectus hesperus, a species in which males span a broad range of sizes. 4. In accordance with our prediction, and contrary to the Gravity Hypothesis, we found no relationship between vertical climbing speed and any of a series of size measures, whereas on a horizontal surface, speed was positively related to size. 5. Our results, as well as the error we exposed in the biomechanical model underlying the Gravity Hypothesis, indicate that selection for vertical climbing speed in males cannot account for patterns of sexual size dimorphism in spiders.
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