Testing the gravity hypothesis of sexual size dimorphism: are small males faster climbers?

Abstract

Summary1. Moya-Laraño 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.

Optimal climbing speed explains the evolution of extreme sexual size dimorphism in spiders

“…As our climbing trials revealed an optimal size for climbing speed at approximately 43 mg body mass (≈7.4 mm body length; according to Edwards, 1996 equation for a wide range of spider taxa; see the section), we predicted that in species that live in high‐habitats males should not evolve beyond this optimal size and hence pronounced SSD should become apparent with increasing size of the females – in contrast to species that live on the ground where male size is predicted to co‐evolve tightly with female size, as running speed on horizontal surfaces typically increases with size (Bauwens et al. , 1995; Brandt & Andrade, 2007a; Moya‐Laraño et al. , 2008).…”

“…However, studies testing directly whether small males are faster climbers than large ones or have an advantage in scramble competition over access to females (i.e. find females first) have yielded mixed results: small males were favoured (Linn, 2001), no size effects were detected (Andrade, 2003; Foellmer & Fairbairn, 2005; Brandt & Andrade, 2007a; Kasumovic et al. , 2007), intermediate‐sized males were favoured (Vollrath, 1980) and even large males had an advantage (Foellmer & Fairbairn, 2005).…”

“…, 2007), intermediate‐sized males were favoured (Vollrath, 1980) and even large males had an advantage (Foellmer & Fairbairn, 2005). Brandt & Andrade (2007a) questioned the validity of the model underlying the GH; however, the alternative model they presented has since been shown to be flawed and unable to describe biological processes (Brandt & Andrade, 2007b; Moya‐Laraño et al. , 2007a).…”

Functional relations between locomotor performance traits in spiders and implications for evolutionary hypotheses

“…We calculated the intraclass correlation coefficient between the fastest and the next fastest trails for each assay of locomotor performance to determine repeatability of running and climbing performance [5,6] using SPSS 16.…”

Resurrecting the differential mortality model of sexual size dimorphism

“…, 2002), which states that extreme SSD with dwarf males could evolve and be maintained because smaller males climb faster and thus are better able to reach females that live high up in the vegetation. Although this hypothesis has been recently criticized (Brandt & Andrade, 2007a,b), additional evidence strongly suggests that although the relationship between climbing speed and body size is complex (i.e. positive below certain body size threshold and negative beyond the threshold), the gravity hypothesis can still accurately explain the evolution of extreme SSD (Foellmer & Moya‐Laraño, 2007; Moya‐Laraño et al.…”