Predation and the evolution of complex oviposition behaviour in Amazon rainforest frogs

Terrestrial oviposition with free-living aquatic larvae is a common reproductive mode used by amphibians within the central Amazonian rainforest. We investigated the factors presently associated with diversity of microhabitats (waterbodies) that may be maintaining the diversity of reproductive modes. In particular, desiccation, predation by fish, competition with other anurans and water quality were examined in 11 waterbodies as possible forces leading to the evolution of terrestrial oviposition. Predation experiments demonstrated that fish generally do not eat anuran eggs, and that predacious tadpoles and dytiscid beetle larvae are voracious predators of anuran eggs. The percentage of species with terrestrial oviposition was only weakly correlated with the occurrence of pond drying, pH and oxygen concentration, suggesting that anurans in this tropical community are able to use the range of water quality available for egg development. There was a tendency for terrestrial oviposition to be associated with the number of species of tadpoles using the waterbody, but we consider this to be spurious as there was no obvious competitive mechanism that could result in this relationship. The percentage of species with terrestrial oviposition was significantly positively related to our index of egg predation pressure, and negatively related to our index of fish biomass. Egg predation pressure was also negatively related to the index of fish biomass. These results allow us to discount as improbable the hypothesis that predation by fish on anuran eggs was an important selective pressure leading to terrestrial oviposition in this community. The strong positive relationship between terrestrial oviposition and our index of egg predation pressure indicates that these predators have exerted, and are exerting, a significant selective pressure for terrestrial oviposition. The strong negative relationship between the occurrence of fish and the egg predators suggests the surprising conclusion that the presence of fish actually protects aquatic anuran eggs from predation in this tropical system, and allows aquatic oviposition to dominate only in those waterbodies with moderate to high densities of fish. Our results suggest that terrestrial oviposition is a “fixed predator avoidance” trait.