Fire-bellied toads are small frogs that have brightly colored bellies and warty skin. It is difficult to determine male and female frogs apart because they are similar in appearance. According to “Popular Amphibians” by Philippe de Vosjoli, the only time you can determine their sex is during breeding season. To tell the sex of a frog, you need to own more than one so that you can watch their interactions and behaviors.
Place your fire-bellied toads in the same tank to breed. Fire-bellied toads breed in the spring, so if you have a frog tank, you will need to create artificial winter and spring environments for the frogs to cue their bodies to mate. The book, "Quick and Easy Fire-Bellied Toad Care" by Tom Mazorlig, has information on creating artificial breeding environments.
Listen to see if any of your toads begins to perform a mating call. The call may sound like a soft “tink” noise or an “oo-oo-oo” sound. The male gender of the species is the vocal sex during breeding season.
Identify which male toad is making the noise. If you cannot tell, look at the throats of the toads. Males who have spent time “calling” tend to have baggy-looking throats or darkened-colored throats.
Look to see if a toad is grasping another toad from behind. Males will hold females around the waist area and go into the mating position to mate with them. This is another way to tell males and females apart.
Examine your fire-bellied toads for nuptial pads. Only breeding males develop nuptial pads, or rough dark areas, on the inside of their thumb, second finger, forearms and feet when they are breeding. Nuptial pads help a male frog hold onto a female while mating.
Compare the sizes of your toads. If you notice that certain toads are plumper and rounder than others, these may be the female toads. Males tend to have a more slender appearance.
Compare the forearms of your toads. Males tend to have more muscular forearms than females.