Hyla versicolor and h. chrysoscelis are frogs which are morphologically indistinguishable, non-hybridizing and broadly sympatric in central texas. there is no evidence that these species utilize different natural resources and both reproduce throughout the spring and summer. mating involves males clutching the females but fertilization is external. these species are easily distinguished by their mating calls; males and females of h. versicolor are "slow trilling" whereas males and females of h. chrysoscelis are "fast-trilling." the factor that most likely maintains these forms as species is