Sexual Size Dimorphism: Evolution and Perils of Extreme Phenotypes in Spiders

Sexual size dimorphism is one of the most striking animal traits, and among terrestrial animals, it is most extreme in certain spider lineages. The most extreme sexual size dimorphism (eSSD) is female biased. eSSD itself is probably an epiphenomenon of gendered evolutionary drivers whose strengths and directions are diverse. We demonstrate that eSSD spider clades are aberrant by sampling randomly across all spiders to establish overall averages for female (6.9 mm) and male (5.6 mm) size. At least 16 spider eSSD clades exist. We explore why the literature does not converge on an overall explanation for eSSD and propose an equilibrium model featuring clade- and context-specific drivers of gender size variation. eSSD affects other traits such as sexual cannibalism, genital damage, emasculation, and monogyny with terminal investment. Coevolution with these extreme sexual phenotypes is termed eSSD mating syndrome. Finally, as costs of female gigantism increase with size, eSSD may represent an evolutionary dead end.

On the male of the orb weaving spider Micrathena cyanospina (Lucas, 1835) (Araneidae, Araneae)

Most species of spiders exhibit some degree sexual size dimorphism (SSD). Females are usually larger than males, about 20% on average (Vollrath 1998), although this difference can be much larger. By convention, species in which the adult males are half or less of the adult female size are considered sexually dimorphic, some representing cases of extreme SSD (Hormiga et al. 2000). It is worth mentioning that although males are called dwarfs, large SSD is often a consequence of female gigantism rather than male dwarfism (Hormiga et al. 2000).