Mass-mediated sex differences in climbing patterns support the gravity hypothesis of sexual size dimorphism

J Moya-Laraño,M W Foellmer,D Vinkovi&Amp;Cacute;,C M Allard

doi:10.5194/we-7-106-2007

J Moya-Laraño, M W Foellmer + Show 2 more

Open Access

https://doi.org/10.5194/we-7-106-2007

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Abstract

Abstract. The gravity hypothesis of sexual size dimorphism can explain the patterns of extreme sexual size dimorphism in spiders (males smaller than females) because small males climb faster and therefore may be better at reaching females that live in high habitats. Recently, the main prediction of a negative relationship between climbing speed and body size in spiders has been called into question. Here we induced males and females of the spider Leucauge venusta (Tetragnathidae) to run on vertical surfaces and found partial support for the gravity hypothesis. As predicted, males climb faster than females and we demonstrated that this effect is an indirect effect mediated by the negative relationship between body mass and climbing speed. We validate our results using simulated data showing that there is enough statistical efficiency in our data set to support our conclusions. We distinguished between direct and indirect effects (through mass) on sex differences in climbing speed by means of path analysis. Thus, we provide empirical evidence that by being smaller, males are able to climb faster than females. However, we found only a barely significant negative relationship between climbing speed and body size when only males were considered. Reasons for such results are discussed within the text.

Highlights

The gravity hypothesis of sexual size dimorphism can explain the patterns of extreme sexual size dimorphism in spiders because small males climb faster and may be better at reaching females that live in high habitats
When considering males and females separately we found a negative relationship between climbing speed and body mass in females (r = –0.39, p = 0.023, Fig. 1) but not in males (r = –0.09, p = 0.398)
We had not very high power to detect the effect of mass (41%), we can be confident that a significant effect is real, and most importantly, that if sex has no direct effect in the model, it will be very unlikely that it shows significant

Summary

Introduction

The gravity hypothesis of sexual size dimorphism can explain the patterns of extreme sexual size dimorphism in spiders (males smaller than females) because small males climb faster and may be better at reaching females that live in high habitats. Comparative analyses have lent good support for the fecundity advantage hypothesis, which states that large size in females is favored because of the associated higher reproductive potential (Head 1995, Prenter et al 1998, 1999) It is far from clear what factors have been and/or are responsible for the evolution and maintenance of small male size (relative to female size) (reviewed by Foellmer and Moya-Laraño 2007). The gravity hypothesis is based on a simple biomechanical model and states that size (mass) is inversely proportional to achievable running speed on vertical structures (Moya-Laraño et al 2002) It predicts that small size should be favored in males during mate search in species where males have to move through a three-dimensional habitat and have to climb to reach females, which is the case in many spider species (MoyaLaraño et al 2002). A second prediction of the gravity hypothesis is that female-biased SSD should be more pronounced in species where females build their webs high up in the vegetation than in species where females place their webs close to the ground (MoyaLaraño et al 2002)

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