Vertebrate colonization of land occurred multiple times, including over
50 origins of terrestrial eggs in frogs. Some environmental factors and
phenotypic responses that facilitated these transitions are known, but
responses to water constraints and risk of ammonia toxicity during early
development are poorly understood. We tested if ammonia accumulation and
dehydration risk induce a shift from ammonia to urea excretion during in
early stages of four anurans, from three origins of terrestrial
development. We quantified ammonia and urea concentrations during early
development on land, under well-hydrated and dry conditions. Where we
found urea excretion, we tested for a plastic increase under dry
conditions and with ammonia accumulation in developmental environments.
We assessed the potential adaptive role of urea excretion by comparing
ammonia tolerance measured in 96h-LC50 tests with ammonia levels in
developmental environments. Ammonia accumulated in foam nests and
perivitelline fluid, increasing over development and reaching higher
concentrations under dry conditions. All four species showed high
ammonia tolerance, compared to fishes and aquatic-breeding frogs. Both
nest-dwelling larvae of Leptodactylus fragilis and late embryos of
Hyalinobatrachium fleischmanni excreted urea, showing a plastic increase
under dry conditions. These two species can develop the longest on land
and urea excretion appears adaptive, preventing their exposure to
potentially lethal levels of ammonia. Neither late embryos of Agalychnis
callidryas nor nest-dwelling larvae of Engystomops pustulosus risked
toxic ammonia levels under dry conditions, and neither excreted urea.
Our results suggests that an early onset of urea excretion, its increase
under dry conditions, and elevated ammonia tolerance, can all help
prevent ammonia toxicity during terrestrial development. High ammonia
represents a general risk for development that can be exacerbated as
climate change increases dehydration risk for terrestrial-breeding
frogs. It may also be a cue that elicits adaptive physiological
responses during early development.