Hunting Guild Mediates the Effects of Fecundity and Mortality on Sexual Size Dimorphism in Spiders.

Sexual size dimorphism (SSD) arises from sex-specific selection on body size, primarily through sexual selection, mortality-driven selection on males, and fecundity selection on females. A key ecological factor that may shape both processes is foraging behaviour, which determines movement, predation exposure and resource acquisition. Spiders provide a useful model system to test this idea because they exhibit extensive ecological diversity in prey-capture strategies, and previous work has linked hunting strategies to reproductive traits, body size and SSD. However, these relationships remain poorly resolved and rarely tested within a comparative mechanistic framework. Here, using a phylogenetic comparative analysis of 264 species across eight hunting guilds, we tested whether hunting guild predicts variation in SSD and modulates the relationship between SSD, sex-biased mortality and fecundity. We used sex differences in pitfall trap catches, taken from the literature, as a proxy for sex differences in mobility and mortality. SSD differed markedly among guilds, with the strongest female bias occurring in orb-web weavers. The relationship between SSD and sex-biased mobility/mortality was context-dependent, emerging in sheet weavers but not in ground hunters. In contrast, we consistently predicted female and male body size, but not SSD, with effect sizes varying across guilds. Together, these results suggest that SSD does not arise from universal mechanisms, but from guild-specific sexual asymmetries associated with mobility, predation exposure, and resource access, linking foraging strategy to the macroevolution of body size differences between the sexes.