Toxin structure shapes palatability in a chemically defended butterfly

The study investigates the impact of toxin structure on predator deterrence in a chemically defended butterfly species. While toxicity is well understood in relation to chemical defenses, the role of compound structure remains largely unexplored. Researchers examined whether predation acts more strongly on toxicity or unpalatability, a question partially addressed due to the scarcity of systems where toxin structure can vary independently of quantity.

Heliconius sara larvae offer such a system, with those raised on Passiflora auriculata accumulating cyclopentenyl cyanogenic glucosides (CGs) and those raised on Passiflora biflora synthesizing comparable amounts of aliphatic CGs. To test whether this structural variation influences palatability independent of toxicity, the study utilized two invertebrate predators: Camponotus floridanus ants and Hierodula membranacea mantids.

Results showed that mantids rejected larvae containing cyclopentenyl CGs more frequently than those with aliphatic CGs, even when total CG content remained unchanged. This pattern was consistent in extract-based assays with ants, indicating that extracts with cyclopentenyl CGs maintained their deterrent effect, while those with aliphatic CGs showed no significant difference in deterrent capacity compared to water.

Live larvae, conversely, elicited similar reactions from ants irrespective of the CG structure. These findings demonstrate that variations in toxin structure can significantly affect palatability, with certain compounds providing enhanced protection. This underscores the importance of chemical structural diversity in the evolution of chemical defenses within species.