In areas with mild winters, bullfrogs are active all year, but they undergo periods of inactivity or hibernate in cold climates. Their purpose is unknown, but they may be lengthy foraging excursions, or dispersals to new breeding localities. No significant migration by adults from breeding sites has been recorded. They are presumably dispersing from rapidly drying ponds where their development took place. Males had larger areas of activity than females, and large frogs moved more than small ones.
Males defend areas around themselves for sexual display during the breeding season.
Females lay eggs within the territories of chosen males. The bullfrog is the largest frog in California, and it may prey on, or compete for food and space with, native amphibians with which it coexists. Supporting evidence is sparse. Juvenile and adult bullfrogs are taken by a wide variety of vertebrate predators. Size variation and food habits of larval bullfrogs, Rana catesbeiana Shaw, in western Oregon.