Adult-obtained pyrrolizidine alkaloids defend ithomiine butterflies against a spider predator

Abstract

Ithomiine butterflies, the original distasteful models in neotropical mimicry complexes, show warning coloration and are usually rejected by both vertebrate and invertebrate predators^1–4. Based on co-evolutionary theory^1,5–7 and the Danainae–cardenolide example⁷, this protection has been ascribed to toxic compounds sequestered by larval Ithomiinae from their host-plants, usually poisonous Solanaceae^8–10. However, although the giant tropical orb spider Nephila clavipes cuts out field-caught Ithomiinae from its web^4,11 it readily eats freshly emerged adults. Solanaceae toxins are not found stored in adult Ithomiinae^6,11 and when applied to the palatable¹ butterfly Biblis hyperia do not induce rejection behaviour in Nephila (Table 1). I show here that the protection of ithomiines against this abundant predator is due to dehydropyrrolizidine alkaloid monoesters and their N-oxides, absent from Solanaceae but sequestered by these butterflies as adults from flowers (mostly Eupatorieae) and decomposing foliage (mostly Boraginaceae)^11–13. These compounds are also important in the reproduction of the Ithomiinae^11,14.