Bullfrogs (Rana catesbeiana) have two male genders that both mate with females.
Bullfrogs have two male genders that both mate with females:
(1) large males who call at night, giving bullfrogs their name, and
(2) small males who are silent.
Both are reproductively competent, and females mate with both. Silent males turn into calling males as they grow older. Male frogs in older species and males in many vertebrate groups aslo have to decide when to begin breeding—whether to wait until established enough to flaunt wealth and power, or to begin sooner with fewer resources but lots of charm.
Silent males turn into calling males as they grow older.
Both still mate with females regardless of silence.
Perhaps silent males should not be considered a different gender from calling males, but rather an early developmental stage of the same gender. Compare this case with others, though, and you may agree that it makes more sense to view males who mature from a silent stage into a calling stage as changing genders.
R.D. Howard, 1978,. The evolution of mating strategies in bullfrogs, Rana catesbeiana, Evolution 32: 859-71.
R.D. Howard, 1981, Sexual dimorphism in bullfrogs, Ecology 62:303-10.
R.D. Howard, 1984, Alternative mating behaviors in young male bullfrogs, Amer. Zool. 24:397-406.
Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. p. 76.