Orange Tip tipoff
The Orange Tip butterfly, Anthocharis cardamines, is a familiar spring species across Europe. The eponymous orange wingtips are present only in the males, but both male and female have a green mottled pattern on the underside of the wings, which helps with camouflage when they’re at rest.
When they’re not at rest, there’s some a surprisingly sophisticated bit of chemistry going on during mating. The male passes not only sperm to the female for the fertilisation of her eggs but also a chemical cocktail that acts as an anti-aphrodisiac to other males that might try to mate with her subsequently.
When another male approaches the mated female, he detects the pheromone cocktail from the first male and abandons his pursuit, avoiding wasted time and effort. From the first male’s perspective, this increases the likelihood that his genes are passed on without competition.
The exact chemical composition of this cocktail has not been fully characterised for A. cardamines, but in closely related butterflies in the same family, such as the Green-veined White, Pieris napi and the Large White, Pieris brassicae, the equivalent cocktail contains substances such as methyl salicylate and benzyl cyanide, sometimes accompanied by trace phenolic molecules, such as guaiacol.
Chemically, these are small volatile, aromatic molecules derived with relative ease metabolically speaking from the ubiquitous amino acid phenylalanine. That shared biosynthetic origin suggests that the Orange Tip probably uses very similar compounds as an economical and effective signalling system to warn away males once a female has mated.
The unrelated Holly Blue, Celastrina argiolus, one of the Lycaenidae, also uses anti-aphrodisiacs, but rather than small volatile molecules it uses oily or fat-like lipids.
The males produce long-chain hydrocarbons and fatty-acid derivatives, which form part of his cuticular chemistry. During mating these compounds are passed to the female and create a close-range chemical signature that tells other males that the female has been mated. The result is the same, but the molecular toolkit is different and shows a different evolutionary solution to the problem.
Brush-footed butterflies, the Nymphalidae, such as the European Peacock, Aglais io, also use fatty acids like the Lycaenidae, but add terpenoids to their anti-aphrodisiac chemical cocktail.