VARIATION IN ALKALOID DEFENSES OF THE DENDROBATID POISON FROG OOPHAGA PUMILIO LEAD TO DIFFERENCES IN AVAOIDANCE BY ARHTROPODS

Conspicuously colored dendrobatid frogs sequester alkaloid-based defenses from dietary arthropods, resulting in considerable alkaloid variation among populations. Although alkaloids act as a defense against predation, relatively little is known about how alkaloid variation is perceived and functions as a defense against predators. Throughout its geographic range, previous studies have found the dendrobatid frog Oophaga pumilio to have particularly variable alkaloids, and that differences in these alkaloids are associated with differences in toxicity to laboratory mice. Although toxicity is one measure of alkaloid variation, predator avoidance of dendrobatids might simply be due to the bitter or unpalatable nature of alkaloid defenses. Arthropods are natural predators that use chemoreception to detect prey, including frogs, and may therefore perceive variation in alkaloid profiles as differences in palatability. The goal of the present study is to gain an understanding of how arthropods respond to variable alkaloid defenses in O. pumilio. Frog alkaloids were sampled from individual O. pumilio from ten geographic locations throughout the Bocas del Toro region of Panama and the Caribbean coast of Costa Rica. Alkaloids were used in feeding bioassays with the fruit fly Drosophila melanogaster and the ant Ectatomma ruidum to investigate how arthropods respond to different suites of dendrobatid alkaloids. Drosophila melanogaster and E. ruidum feed less on frog alkaloid solutions when compared to controls, and variation in alkaloid profiles among O. pumilio populations result in differences in palatability. Differences in palatability were observed among populations, as well as between sexes and life stages of a single population. In particular, alkaloid quantity, diversity, and type appear to play an important and complex role in arthropod avoidance of alkaloid profiles. The findings of the present study represent the first direct evidence of a palatability spectrum in a vertebrate that sequesters its chemical defenses from dietary sources. The presence of a palatability spectrum suggests that variable alkaloid defenses in O. pumilio (and likely other dendrobatids) are ecologically relevant and play an important role in natural predator-prey interactions, in particular with respect to arthropod predators