Rhinella spinulosa is a toad common in rivers, streams, and steppe wetlands where they face a variety of insect predators. Rhinella spinulosa tadpoles are known to be unpalatable to dragonfly naiads at some developmental stages. In this paper, we studied susceptibility of R. spinulosa tadpoles to predation by two species of predators, water bugs (Belostoma bifoveolatum, Belostomidae) with piercing-sucking mouth parts and dragonfly larvae (Rhionaeschna variegata, Aeshnidae) with chewing mouth parts. We conducted feeding trials to test the effectiveness of tadpole chemical defenses against water bugs. We also performed behavioral and choice experiments to study whether tadpole behavior influences their vulnerability to predators, and whether R. variegata has the capacity of discern between palatable (stages 38–45) and unpalatable tadpoles (stages 24–26). Rhinella spinulosa tadpoles, when fully palatable, responded by reducing their activity levels in the presence of dragonfly larvae but did not alter their behavior at unpalatable-early stages. Chemical defenses were not effective against the sucking predator (B. bifoveolatum) and no changes in tadpole activity were observed. Selectivity experiments revealed the ability of dragonfly larvae to distinguish between unpalatable and palatable tadpoles. Inducible (behavior) and constitutive (toxins) defenses observed in R. spinulosa tadpoles and the transition between them could result in an important survival mechanism in response to variable predator risk in the habitats in which they co-occur.
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