Introducing the refined gravity hypothesis of extreme sexual size dimorphism

Abstract

BackgroundExplanations for the evolution of female-biased, extreme Sexual Size Dimorphism (SSD), which has puzzled researchers since Darwin, are still controversial. Here we propose an extension of the Gravity Hypothesis (i.e., the GH, which postulates a climbing advantage for small males) that in conjunction with the fecundity hypothesis appears to have the most general power to explain the evolution of SSD in spiders so far. In this "Bridging GH" we propose that bridging locomotion (i.e., walking upside-down under own-made silk bridges) may be behind the evolution of extreme SSD. A biomechanical model shows that there is a physical constraint for large spiders to bridge. This should lead to a trade-off between other traits and dispersal in which bridging would favor smaller sizes and other selective forces (e.g. fecundity selection in females) would favor larger sizes. If bridging allows faster dispersal, small males would have a selective advantage by enjoying more mating opportunities. We predicted that both large males and females would show a lower propensity to bridge, and that SSD would be negatively correlated with sexual dimorphism in bridging propensity. To test these hypotheses we experimentally induced bridging in males and females of 13 species of spiders belonging to the two clades in which bridging locomotion has evolved independently and in which most of the cases of extreme SSD in spiders are found.ResultsWe found that 1) as the degree of SSD increased and females became larger, females tended to bridge less relative to males, and that 2) smaller males and females show a higher propensity to bridge.ConclusionsPhysical constraints make bridging inefficient for large spiders. Thus, in species where bridging is a very common mode of locomotion, small males, by being more efficient at bridging, will be competitively superior and enjoy more mating opportunities. This "Bridging GH" helps to solve the controversial question of what keeps males small and also contributes to explain the wide range of SSD in spiders, as those spider species in which extreme SSD has not evolved but still live in tall vegetation, do not use bridging locomotion to disperse.

Sociality level correlates with dispersal ability in spiders

“…Trials were performed in local makeshift laboratories and always at room temperature (range = 13·7–24·1 °C; Mean = 20·07 °C, SD = 2·46 °C). We followed the methods described in Corcobado et al. (2010) to induce bridging in the laboratory.…”

“…Comparing dispersal tendency and efficiency, as well as morphological adaptations to dispersal, can help us understand how the transition from one social system to another took place. Dispersal tendency and efficiency can be determined in the laboratory with experimental bridging trials (Corcobado et al. 2010).…”

“…Bridging is a common mode of spider locomotion that involves the production of a thread of silk that is pulled by the wind until it attaches to a nearby plant; after tensing, the spider crosses the line bridge hanging upside down from the silk (Moya‐Laraño et al. 2008; Corcobado et al. 2010).…”

Evolution and maintenance of sexual size dimorphism: aligning phylogenetic and experimental evidence

“…Thus, future experimental studies might profitably focus on selection pressures that could disfavor large male body size, including those that may arise through conflict with female interests, dispersal between webs and long-term survival. Several authors have suggested that small male size is favored in response to sexual cannibalism (Elgar, 1991), protandry (Danielson-Francois et al, 2012;Elwood and Prenter, 2013), or gravity (Moya-Larano et al, 2002Corcobado et al, 2010), but these ideas have attracted either little attention or no empirical support (Table 1; see also Prenter et al, 2010). Perhaps the low variance in male mating success that frequently characterizes the mating system of these spiders limits the opportunity for selection on male size.…”

Biology, ecology and demography of the tropical treehopper Ennya maculicornis (Hemiptera: Membracidae): relationships between female fitness, maternal care and oviposition sites

“…In the case of body size difference, intersexual selection and specifically male preference can explain the larger body of females, particularly as larger females are generally associated with greater fecundity (Bonduriansky, 2001;Clutton-Brock, 2008;Stillwell et al, 2010). On the other hand, the smaller body size of E. maculicornis males could be explained by the gravity hypothesis, which proposes a negative correlation between moving ability on vertical surfaces and body size (Blanckenhorn, 2005;Corcobado et al, 2010). On the other hand, the smaller body size of E. maculicornis males could be explained by the gravity hypothesis, which proposes a negative correlation between moving ability on vertical surfaces and body size (Blanckenhorn, 2005;Corcobado et al, 2010).…”