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The tailed frogs get their name from the copulatory organ of the male, which resembles a short tail, but is really an expanded portion of the cloaca. These species are among the very few frogs with internal fertilization; the copulatory organ is used to transfer sperm to the female. They inhabit cold streams in humid forests and the areas around them. The two species of Ascaphus range from British Columbia south to Mendocino County, California, the Rocky Mountains of Idaho and Montana, and adjacent Washington and Oregon. Ascaphus is not known to have an advertisement call, and thus many aspects of their reproductive behavior are poorly known because they cannot be easily observed. The frogs are semi-aquatic and most active in the vicinity of streams at night.
The tadpole has a large suckerlike mouth that occupies about one-half of the ventral surface of the body. It also has a large number of denticles (2-3 upper and 7-12 lower rows). Tadpoles of most species of frogs have only two upper and three lower rows of denticles. Ascaphus uses its sucker to adhere to and move among rocks in cold, swift streams. The function of the suction mechanism was studied by Gradwell (1971).