The American bullfrog Rana (Aquarana) catesbeiana has been reported to show significant sexual dimorphism based on the size ratio between the tympanic membrane and the eye. In males the tympanic membrane is much larger than the eye, but not in females. The ratio has been used as a convenient criterion to discriminate sexes (sexing) in the American bullfrog, though its reliability is unknown. In this study, we examined 86 adult American bullfrogs to clarify whether the tympanic membrane long diameter/eye long diameter (Dtm/De) ratio is a reliable index to discriminate sexes in this species. In addition, we examined the growth of this sexually dimorphic trait. Results indicated that there is a significant difference but there is a small overlap in this ratio Dtm/De between sexes. The allometric comparisons showed the sexual dimorphism of the Dtm/De ratio was increased during growth and the dimorphism is attributable to the difference in the growth rate of the tympanic membrane (Dtm). Therefore, sex determination of American bullfrogs cannot be wholly reliably achieved by the Dtm/De ratio alone; other external morphological features are required in addition.
Keywords: Ranidae; eye; sexual dimorphism; tympanum.