Traditional interpretations of mating behaviors assume cooperation between the sexes. The field of sexual conflict provides a contrasting view: the sexes are commonly in conflict because they diverge in benefits/costs of reproduction. Precopulatory mate guarding, wherein males attempt to monopolize mates by physically pairing before fertilization, is one expected case of conflict. Initial guarding models focused mainly on male reproductive success, considering mate guarding as a pure male mating strategy. Intersexual conflict models instead consider costs and benefits for both sexes. We provide new insights exploiting a promising study system for intersexual conflict: the androdioecious crustacean Eulimnadia texana. Androdioecy (coexistence of males and hermaphrodites) provides unique opportunities to test sexual conflict hypotheses because hermaphrodites can facultatively self-fertilize. The ability to self-fertilize is expected to decrease the benefits of mating with males, while it is less likely to affect guarding costs.
This asymmetry can thus intensify conflicts, which can be easily detected, if present. We show that hermaphrodites suffer higher costs than males during a guarding event and thus prefer shorter guarding times, while males, seeking to avoid loss of mating opportunities, prefer longer guarding times. Moreover, larger male size (relative to hermaphrodite size) confers greater control over hermaphrodites, leading to increased guarding durations. As sexual conflict theory predicts, guarded mates are not “passive” interactors but struggle to maximize their own fitness. The result of the conflict is a compromised guarding time that differs from the optimal strategy of either single sex.