Sexual dimorphism, trait variability, and allometry in two camel spiders (Arachnida: Solifugae) species with different mating tactics

Abstract

Sexual dimorphism and trait allometry provide a framework to explore the potential selective pressures shaping morphological evolution. We analyzed two camel spider species with contrasting mating tactics: Titanopuga salinarum (persuasive mating) and Oltacola chacoensis (coercive mating). Specifically, we assessed sexually dimorphic traits, sex-specific allometric patterns, and coefficients of variation to examine how morphological variation aligns with these divergent reproductive strategies. Both species showed female-biased dimorphism in absolute body size and cheliceral structures, whereas males had relatively larger legs, pedipalps, and malleoli. These patterns suggest that natural selection favors larger females via fecundity benefits, while sexual selection enhances male traits related to mate searching, stimulation, or grasping. Pedipalpal spiniform setae used in grasping during mating displayed opposite dimorphic directions between species, male-biased in O. chacoensis and female-biased in T. salinarum, linking morphology to mating strategy. Locomotor and sensory traits often presented negative allometry in males, indicating functional consistency across sizes. Likewise, sperm-transfer structures such as cheliceral fingers and flagellum showed negative allometry and low variability in males. Overall, sexual and natural selection appear to act on different components of the solifuge phenotype, producing complex patterns of morphological diversification. This integrative use of allometry, dimorphism, and variability advances our understanding of sex-specific trait evolution in understudied arachnids.