Female mate choice and male-male competition for females and/or the resources which females need to breed have been studied intensely since the birth of behavioral ecology. As the field has grown, many ideas about these components of sexual selection have changed drastically. Early research was often conducted under natural conditions or by simple experiments with stable conditions. However, drastic spatio-temporal changes often occur in the actual environment, and consequently, evidence that the decisions of individuals depend on the costs and benefits of fights, courtships and mate choice under changing conditions has been increasingly reported. Here, we will present studies of this field using fiddler crabs, a taxon where there has been much recent research on sexual selection. In addition, two related topics in other crab species will be presented. To conclude, we hope to discuss the present situations and perspectives of this field.