Male Reproductive Morphology Across Latitudinal Clines and Under Long-Term Female Sex-Ratio Bias

SynopsisRecent work in evolutionary ecology has capitalized on the species-specific, rapidly evolving nature of male reproductive traits, but it is unclear how the strength of sexual selection mechanisms, such as female choice and intersexual conflict, changes with ecological factors-shifts in latitude, elevation, and sex-ratio. Marginal habitats, such as those found at high elevation or latitude, might be expected to exert energetic hardships on males such that they do not invest heavily in reproductive structures. Conversely, males in these habitats may adapt to limited breeding opportunities, evolving morphologies with efficacy in antagonistic mating. Sex-ratio biases elicited by facultative parthenogenesis may also signal increasing selective pressure for reproductive organs that improve coercion ability, as males must capitalize on mating opportunities when females have alternate reproductive options. We studied the reproductive organs of males of five species of a group of polygynandrous Japanese harvestmen, collected from populations that varied in their elevation, latitude, and observed male: female ratio. Morphological data were analyzed with modern comparative methods, using a phylogenomic framework built with molecular data from double-digest restriction associated DNA sequencing. Although no features with hypothesized utility in coercive mating significantly covaried with elevational change, penis length was found to decrease with increasing latitude, supporting the existence of physiological constraints in marginal habitats. We also found males of parthenogenetic species, with primarily female-biased populations, had potentially greater precopulatory clasping ability, as compared to males of non-parthenogenetic species with more even sex-ratios. Ultimately, our findings suggest long-term sex-ratio biases are one of many factors with the potential to influence evolution of primary and secondary reproductive morphology.