Bidder's organs in both male and female bufonids are hypothesized to represent a transitory state in an evolutionary transformation from extensive, hyperfecund ovaries to smaller, less fecund ones-a transformation that other families of anurans experienced in forms now long extinct. Some members of the genus Bufo retain considerable hyperfecundity, but in derived species of Bufo and genera of Bufonidae a spectrum of ovarian reduction exists. In those groups, Bidder's organs normally persist, always in males and often in females, as an undifferentiated ovaroid or ovary in a non-functional transition between the ancestral state and modification as part of the fat body. The organs are gynomorphs in males and highly variable as vestigial structures, but may well have endocrinogenic functions in both sexes although gametogenic functions in nature have been lost in males. Presence of Bidder's organs is a derived condition in the context of anurans as a whole, but primitive within the family Bufonidae.