Chemical defense is one of the most widespread secondary defenses against predators among animals. In chemically defended species that depend on dietary source for defensive chemicals, the necessity for the chemicals would influence their foraging ecology. The Japanese grass snake, Rhabdophis tigrinus, possesses unique organs of chemical defense on its neck and sequesters chemical defense from highly toxic toads consumed as prey. The defensive toxins are derived solely from toads among various prey animals. In addition, females provide their offspring with toxins while they are gravid. Therefore, gravid females are expected to require additional toxins to produce chemically defended offspring. We hypothesized that gravid females selectively forage for toads because of the greater need for toxins. First, we compared habitat use patterns between sexes to test the hypothesis. We predicted that gravid females use forests more frequently than males because toads generally occur in forests. Our radiotelemetric research clarified that gravid females use forests more frequently than males. Second, we conducted Y-maze experiments to clarify relative preference for toads to non-toxic frogs. Gravid females tended to choose toads compared to males. These results supported our hypothesis that females selectively forage for toads to acquire additional toxins for provisioning.