In general, amphibian larvae dispose of nitrogenous wastes as ammonia and change to urea excretion during metamorphosis (Balinsky 1970). Ammonia excretion, however, poses a problem for tadpoles and embryos that live in a limited volume of fluid because it is toxic in higher concentrations. One solution is urea excretion or ureotelism. In amphibians it occurs in species with terrestrial modes of reproduction when larvae develop within the uterus as in the ovoviviparous Salamandra salamandra (Schindelmeiser & Greven 1981), when embryos develop in foam nests as in Leptodactylus bufonius (Shoemaker & McClanahan 1973), or when offspring develop in brood pouches as in Gastrotheca riobambae (Alcocer et al. 1992).