Meeting Abstract
Phenotypic plasticity is hypothesized to play a role in evolution and facilitate colonization of new environments. In frogs, this may have been an important step in the evolution of terrestrial egg-laying. A major physiological change in terrestrial tetrapods was the shift to excreting more costly, less toxic urea, as an adaptation to avoid ammonia toxicity under water constraints. Urea is the main form of nitrogen waste excretion in adult frogs, but is known from embryos or tadpoles in few species. We hypothesize that these stages have evolved adaptive plastic responses to transient terrestrial conditions, including N-excretion plasticity. We studied early life stages of two closely related, sympatric species in the family Leptodactylidae. Engystomops pustulosus lays eggs in aquatic foam nests, in pools that sometimes dry, and Leptodactylus fragilis makes foam nests in terrestrial burrows that usually flood later. We manipulated water availability and quantified ammonia and urea excretion by embryos and hatchling tadpoles in nests of both species, to test if facultative shifts from ammonia to urea excretion occur in response to desiccation risk. We kept aquatic foam nests of E. pustulosus either above water or on a dry substrate, We kept foam nests of L. fragilis in soil with water content matching or 50% less than field conditions. E. pustulosus foam nests over water accumulated only ammonia, but some nests on dry substrate also contained urea. In contrast, L. fragilis nests accumulated both ammonia and urea at both hydration levels tested. Our results indicate that urea excretion can occur in the aquatic foam nests (the ancestral condition) as a plastic response to drying, but it may be fixed in the derived terrestrial foam nests.
