Precopulatory mate guarding is a complex behavior, influenced by many social and physiological factors, representing a case of intersexual conflict. Mate guarding has often been analyzed with the aid of theoretical models. In these models, it is commonly predicted that mate-guarding time is influenced by encounter rates between males and females, the contenders’ relative sizes, and the possible interaction among males (i.e., “takeovers”: one guarding male displacing another). The factors influencing male and female guarding decisions have been measured in laboratory experiments but never under natural field conditions. In this field-based study, we observed mate-guarding couples of the clam shrimp Limnadia badia in ephemeral pools on granite rock outcrops in Western Australia. We recorded guarding duration, focusing on the factors considered important in mate-guarding models: male and female size, population density, sex ratio, operational sex ratio, and the status of female receptivity. We also estimated time budgets for males, the possibility of male takeovers, and the potential role of female resistance. We found that female receptivity stage (how close the female was to molting), small male size, and low absolute female presence are key factors in decreasing mate-guarding duration. This study adds a field dimension to manipulative laboratory projects and theoretical models of mate guarding. We were able to observe the simultaneous interactions of multiple factors in the field and to make a robust examination of the ideas of intersexual conflict during mate guarding in these crustaceans.