Same-sex sexual behavior in a male-dimorphic arachnid with a resource-defense polygyny: evidence for female mimicry?

Female mimicry is a reproductive tactic typically employed by small males (minors) in species with female- or resource-defense polygyny. By mimicking females, minors may secure copulations without being detected by territorial males (majors). We used three complementary approaches to investigate the possibility of female mimicry in the male-dimorphic harvestman Serracutisoma proximum, a chemically-oriented arachnid with a resource-defense mating system. Minors rely primarily on sneak copulations, and if they exhibit female mimicry, we expect them to elicit behavioral responses from majors similar to those directed toward females. Indeed, a field experiment showed that majors displayed reduced aggression toward minors and even attempted copulation with some of them. To examine the mechanisms underlying of mistaken sex identification, we compared the cuticular chemical profiles of majors, minors, and females, and quantified the density of chemoreceptors on the sensory legs of majors and minors. Contrary to female mimicry predictions, the overall chemical profile of minors did not resemble that of females. However, some compounds that signal female reproductive status in spiders showed similar abundances in minors and females. Additionally, chemoreceptor density did not differ between majors and minors, suggesting that mistaken sex identification is also not due to sensory limitations in majors. In conclusion, while behavioral evidence is consistent with female mimicry, the underlying mechanisms remain unresolved. Regardless of the underlying mechanism, the ability of some minors to access and mate with females even in the presence of majors likely confers reproductive advantages, which could maintain alternative reproductive tactics within the population.