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You can determine the sex of your pet bullfrogs (Rana [Lithobates] catesbeiana) by examining several secondary sexual characteristics. However, you also can observe your frog’s behavior, which may provide further clues about your pet’s sex. Unfortunately, it is not possible to determine the sex of your pet until sexual maturity, which does not occur until at least one year after the conclusion of metamorphosis.
Throat color is one of the easiest ways to identify male bullfrogs. It was the primary method of determining the sex of bullfrogs employed by Richard D. Howard of Purdue University, Lafayette, Indiana, when they conducted one of the first explorations of sexual dimorphism in the species, in the late 1970s. Upon reaching maturity, male bullfrogs develop yellow and gray markings on their throats.
Another anatomical difference between the sexes relates to the frogs’ tympanums, which are akin to human eardrums. Both male and female bullfrogs have large tympanums relative to other species, but those of males are much larger than those of females. Mature males possess tympanums that are larger than their eyes, whereas the tympanums of females are slightly smaller than their eyes.
Small males exhibit parasitic breeding strategies, meaning they inhabit a large male’s territory, refrain from calling females, and then attempt to intercept females as they approach the large males. By contrast, large males usually claim and defend a territory. Resident males attempt to mate with any females entering their territory, while they greet trespassing males with intimidation or violence. Therefore, if you keep more than one bullfrog in the same habitat, you may be able to observe one of the animals defending a territory, which indicates that the animal is male.
Male bullfrogs emit loud calls, which consist of one or more 0.8-second, 1-kilohertz notes. The calls often occur in threes, leading observers to describe the sequence as sounding like the phrase “jug-o-rum.” They produce these calls by inflating their vocal sacs and forcing the air out under high pressure via the corners of their mouth. The males produce these calls to attract females and defend territories from conspecific males, while females rarely vocalize. However, large females occasionally may call as males do, which prompts greater competition among the males. Both sexes may emit release calls when threatened or grasped by a predator.
It is difficult to determine the sex of bullfrogs before they are sexually mature. In the wild, males mature about one year after metamorphosis, while many females take two years to reach maturation. Males mature at lengths of about 3.75 to 4 inches, while females mature about 4.25 inches in length.