CAMBRIDGE, United Kingdom — Plants and animals alike seem to have a perfume for any occasion. While most creatures, including humans, may want to smell nice to attract that special someone, a new study finds one popular insect has a different idea. During sex, researchers say male Heliconius melpomene butterflies mark their mates with a revolting smell to “turn off” other suitors.
Curiously, the odor is also produced by a flower species that is highly attractive to butterflies. A from St. John’s College, University of Cambridge and UC Davis explored this dual-purpose perfume as a new example of convergent evolution.
The scent produced by the butterfly, H. melpomene, is a pheromone called ocimene. Pheromones are natural chemicals that organisms secrete for social communication. Scientists are just beginning to identify and understand insects that produce ocimene.
“For a long time, it was thought insects took the chemical compounds from plants and then used them, but we have shown butterflies can make the chemicals themselves – but with very different intentions,” says lead author Dr. Kathy Darragh in a university release. “Male butterflies use it to repulse competitors and flowers use the same smell to entice butterflies for pollination.”
For the H. melpomene butterfly, ocimene is synthesized in the genitals from precursor molecules. A key step in the synthesis relies on an enzyme called terpene synthase, or TSP. In the paper, the researchers set out to determine if the TSPs in the butterfly are genetically distinct from other known natural sources.
Flowers also make ocimene
Another natural source of ocimene is the butterfly’s favorite snack, a flower found in Panama. The flowers also use TSP to make the ocimene. However, the TSPs in flowers are not evolutionarily related to those in the butterfly.
“The evolution of ocimene production in male butterflies is independent of the evolution of ocimene production in plants,” Darragh explains.
The ability to produce ocimene is relatively rare. Researchers estimate that there are around 20,000 species of butterflies worldwide and it is not known to be produced by other species. Indeed, the team notes that, “H.melpomene’s closely related sister species, H. cydno, does not produce B-ocimene.”
Study authors compared the genomes of both species of butterflies, flowers, and other insects to determine that H.melpomene had independently evolved the ability to make ocimene.
‘The birds and the bees’ according to butterflies
The study finds H. melponene butterflies likely evolved to produce ocimene to increase reproductive success. Usually, females have very few sexual partners but can store sperm for months after mating to fertilize their eggs.
Male butterflies are more promiscuous. They mate frequently and transfer the ocimene to females after each romp. The males do this to ensure that only their sperm will be used to fertilize the female’s eggs. The strategy gives them an evolutionary advantage by increasing reproductive success.
“The butterflies presumably adapted to detect it and find flowers and they have then evolved to use it in this very different way,” says co-author Dr. Chris Jiggins, professor at St. John’s College. “The males want to pass their genes onto the next generation and they don’t want the females to have babies with other fathers so they use this scent to make them unsexy.”
‘Context is key’ when it comes to stinky perfume
Ocimene is an effective deterrent for competing males, and also an attractive cue when produced by a flower. According to Dr. Darragh, the discrepancy has to do with context.
“The visual cues the butterflies get will be important – when the scent is detected in the presence of flowers it will be attractive but when it is found on another butterfly it is repulsive to the males – context is key,” the lead author concludes
With Valentine’s Day coming up, perhaps we should take notes from the butterfly: a sexy perfume is best received when paired with flowers.
The study is published in PLOS Biology.