Defensive alkaloid variation and palatability in sympatric poison frogs

Chemical defense in poison frogs derives from lipophilic alkaloids sequestered from dietary arthropods. Alkaloid composition varies extensively among individuals, populations, and species. Numerous causes of intraspecific variation have been identified, but the causes of interspecific variation are less clear, with both intrinsic (e.g., mechanism of sequestration) and extrinsic (e.g., arthropod availability) explanations being possible. Sympatric species afford a unique opportunity to investigate the causes and consequences of interspecific variation in natural populations, since they are potentially exposed to the same arthropod prey and predators. We used gas chromatography–mass spectrometry to identify alkaloids from 36 individuals of six species and three genera of dendrobatid poison frogs (Adelphobates, Ameerega, and Ranitomeya) collected in three Amazonian localities. We then compared alkaloid composition, richness, and quantity among sympatric species and analyzed the variation in alkaloid composition among con- and heterospecific populations at the two nearest localities. We also performed arthropod palatability experiments to investigate the biological significance of differences in alkaloids among sympatric species. Sympatric species differed in alkaloid composition, richness, and quantity, and conspecific individuals from different localities shared more alkaloids than heterospecific individuals from the same locality, strongly suggesting that variation is due to intrinsic causes. All analyzed alkaloid secretions were unpalatable, but palatability scores did not differ for most sympatric species, despite significant differences in alkaloid composition, richness, and quantity. Our results provide insights into the causes and consequences of interspecific variation in alkaloid profiles, but additional data are required to identify specific intrinsic causes and predator responses.