Evolutionary Pathways Maintaining Extreme Female-Biased Sexual Size Dimorphism: Convergent Spider Cases Defy Common Patterns

Several animal and plant lineages exhibit pronounced sexual size dimorphism (SSD). Here, we review the evolution of female-biased, extreme SSD (hereafter eSSD; females at least twice male size) in two model spider clades, Nephilidae and Argiopinae. Although these two clades exhibit comparable levels of eSSD, we show that the phenomenon takes different evolutionary pathways. In nephilids, no correlation between male and female size changes is detected while this correlation is maintained in argiopines. In nephilids, sizes in both sexes increase through evolutionary time, but female sizes rise faster, which maintains eSSD. In contrast, argiopines exhibit no directional size change in either sex, and eSSD slowly declines. Model fitting analyses reveal that in nephilids, female size and eSSD adhere to Brownian motion, but male body size evolves toward an optimum between 3.5 and 5.7 mm. In contrast, no directional trends can be detected in argiopines with Brownian motion as the best-fit model. Finally, phylogenetic allometric analyses reveal no relationships between male and female sizes in nephilids, while argiopine size evolution is isometric. The sole agreement between the clades seems to be falsification of both Rensch’s rule and its converse. However, to establish pervasive patterns in spider size evolution, studies on other comparable lineages are essential. We point toward candidate clades and pose open questions in eSSD research.