A tale of two camel spiders: divergence in sperm parameters in two Neotropical solifuge species with contrasting mating systems

Abstract Understanding how divergent mating strategies influence reproductive investment is crucial for elucidating the evolution of sexual traits. Here, we compare postcopulatory traits in two sympatric solifuge species with contrasting mating tactics: the coercive Oltacola chacoensis and the persuasive Titanopuga salinarum. We examined deferent duct morphology, sperm quantity, sperm quality, and their relationships with male body size. Oltacola chacoensis exhibited larger deferent ducts than T. salinarum. In both species, larger males presented more voluminous deferent ducts. Remarkably, both species stored comparable numbers of sperm and maintained consistently high sperm viability. Notably, T. salinarum produced seminal secretions positively associated with sperm number, whereas O. chacoensis lacked these secretions. These findings indicate alternative ejaculate allocation patterns aligned with distinct mating tactics: O. chacoensis might prioritize ejaculate volume and rapid transfer in coercive conditions, whereas T. salinarum invests in secretion-rich ejaculates, possibly associated with sperm performance during prolonged and persuasive courtship. The absence of differences in the number of sperm despite variation in deferent duct size suggests that ejaculate investment might vary in composition rather than quantity alone. This study provides new insights into the interplay between mating strategies and postcopulatory traits, reinforcing the relevance of solifuges as promising models for investigating reproductive diversification.