Poison frogs, defensive alkaloids, and sleepless mice: critique of a toxicity bioassay

Abstract

In studies of defensive allomones, appropriate methods of presenting chemicals and measuring their deterrent effects on consumers are essential for understanding the contributions that chemicals make to the survivorship of potential prey. However, unnatural chemical presentations and/or ambiguous bioassay responses occasionally have left open questions on some allelochemical effects. This discussion critiques a toxicity bioassay of Neotropical poison frogs (Dendrobatidae), a group whose skins are known to possess a diverse array of bioactive alkaloids. The problematic bioassay entails injecting laboratory mice with the skin extracts of frogs and monitoring the time taken for mice to fall back to sleep to estimate extract toxicity, where longer latencies of sleep onset were claimed to reflect greater toxicity. Dendrobatids do not invasively deliver skin alkaloids to offenders, hence the method of injecting mice with skin extracts does not correspond to the frogs' natural defense mechanisms. Neither does the injection of frog extracts permit gastrointestinal deactivation or clearance mechanisms that may reduce the bioavailability of toxins. Whether or not increased sleep latencies induced by injected skin extracts reflect toxicity, irritability or other effects, the ultimate protective value for frogs of prolonging the wakefulness of mice (or relevant predators) is unclear. Defensible bioassays entail both modes of chemical delivery consistent with those by which would-be predators normally encounter chemicals and quantified measures of responses by predators that detract from their success.