Sexual size dimorphism in spiders: patterns and processes

Abstract

This chapter uses data for 489 spider species from fifteen families to describe patterns of variation in sexual size dimorphism (SSD), and to evaluate hypotheses explaining these patterns. The direction and magnitude of SSD is found to depend strongly on the size measure chosen, and the use of carapace width is recommended because it is less affected by condition than body mass or length. Comparative analyses reveal that spiders do not exhibit allometry consistent with Rensch's rule. Instead, females appear to have diverged more than males over evolutionary time, and male and female body size show uncorrelated co-evolution, which is unusual for animals. Only two adaptive hypotheses — fecundity selection favouring large size in females and gravity selection favouring small size in males — have general explanatory power for patterns of SSD in spiders. However, processes may differ among species and comprehensive studies of selection within given species are needed.

Fecundity and sexual size dimorphism of wolf spiders (Araneae: Lycosidae) along an elevational gradient in the Arctic

“…We used carapace width as a measure of body size (Foellmer and Moya-Laraño 2007). All adult males and females were measured, and the females' egg sacs dissected and contents counted using a Nikon Ò SMZ1000 stereoscopic microscope fitted with an ocular micrometer.…”

“…Differences in SSD have been detected among populations within species, and the direction and strength of SSD can be influenced by geographic variation (Blanckenhorn and Demont 2004;Schäuble 2004; or vary in response to changes in biotic or abiotic factors (Foellmer and Moya-Laraño 2007;. Sex-specific responses to various environmental drivers can arise through sex-differences in phenotypic plasticity or genetic makeup Stillwell and Fox 2009).…”

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

“…Our understanding of sexual size dimorphism has converged on the equilibrium model (Blanckenhorn, 2000), suggesting that a combination of selection pressures is expected to operate on spider males (Foellmer and Moya-Larano, 2007). Combined, they must push male size down, hence the detected genetic decoupling of male and female size evolution in nephilid spiders.…”

“…Such extreme female biased sexual size dimorphism has attracted numerous experimental and correlational studies to investigate the role of natural and sexual selection on size evolution in this clade (Kuntner et al, 2013). These studies have identified a number of potential fitness components influencing male and female body size, including fecundity, developmental rates, mate search, male-male competition, sperm competition, and sexual cannibalism (reviews in Elgar, 1992;Elgar and Schneider, 2004;Foellmer and Moya-Larano, 2007;Schneider and Fromhage, 2010;Schneider and Andrade, 2011). An earlier comparative study revealed no phylogenetic correlation between male and female size in nephilid spiders, with a steady evolution toward female gigantism but no clear trend for male size ).…”

Dangerous mating systems: Signal complexity, signal content and neural capacity in spiders

“…Chen and Wise, 1999) with the ability to withstand long periods of hunger (Nakamura, 1986). Foraging success has direct fitness consequences, as fitness is size and condition dependent in both sexes (Foellmer and Moya-Larano, 2007). For example, in females, fecundity is directly correlated to adult body size and to how many nutrients are stored (e.g.…”