A chemical produced by the male sex organs of this tropical butterfly is so repellent that scientists call it an “anti-aphrodisiac”.
New research published today in PLOS Biology describes the genetic underpinnings of a chemical compound produced by men Heliconius melpomene butterflies. It is a striking example of chemical signaling, where scent is used for communication. In this case, the chemical called ocimene “acts as an anti-aphrodisiac pheromone, which is transferred from males to females during mating to ward off further courtship of later males,” the authors write in their paper.
Fascinatingly, some plants also produce ocimene, but they use it for a different purpose. The new research highlights a unique case of convergent evolution. In addition to identifying the genes in Heliconius melpomene responsible for producing the chemical, it is the first time that scientists have documented the production of this chemical, a terpene, in an animal.
Accordingly, the new research dispels the previous belief that Heliconius melpomene ocimene extracted from plants (experiments conducted in 2007 showed that females covered with ocimene were less likely to be approached by males than females covered with a control substance). However, as the new research shows, these butterflies can make this chemical all by themselves.
Members of Heliconius melpomene have an exceptionally long service lifens for butterflies, which live for about six months instead of the usual month. They live in Central and South America and exhibit a wide variety of wing color patterns, depending on their geographic location. These butterflies are poisonous, so the patterns act as a warning to potential predators. These patterns also play a role in sexual selection, as butterflies of this species choose partners that resemble themselves.
G / O Media can receive a commission
“While the patterns have attracted the most attention, it has also long been noted that these butterflies smell,” explains Kathy Darragh, lead author of the novel paper, in an email. “We now know that this scent plays a role in mating guarding, acting as an anti-aphrodisiac to ward off future mating attempts by other males.“ Darragh conducted this research during her PhD at the University of Cambridge.
Humans are highly visual creatures, but many animals depend on chemical signalsing as their main form of communication. Orchids, for example mimic the scent of female insects to attract males for pollination. Anti-aphrodisiacs have also been documented in other insects, including burying beetles and plant bugs.
Butterflies, with their striking colors and patterns, use clear visual cues to communicate, but that doesn’t mean they don’t use chemicals either. Darragh, now at UC Davis, wanted to investigate how chemical signals may play a role in butterfly communication.
“An interesting aspect of this, which has been the focus of the current study, is how butterflies can make these chemicals that they use in signage.ling, ”she said. In other words, what genes do butterflies have that allow them to make these connections? And are these the same genes used to make these compounds in plants? “
Indeed, ocimene is also produced by tropical plants to attract butterflies for pollination (more on this apparent contradiction in a bit). This is an example of convergent evolution, in which two independent species develop a similar trait. However, what’s unique about this finding is that plants and insects use this chemical for different purposes (although still for chemical signaling), and plants and insects use different genetic mechanisms to produce it. In the new article, the researchers identified a new gene in Heliconius melpomene responsible for ocimene, which is unrelated to previously described genes in plants with the same function.
“Ocimene production has evolved independently through different genes in plants and butterflies, showing how different molecular mechanisms may underlie the production of a specific chemical compound,” said Darragh. “The independent evolution of the same trait, in this case pheromone production, multiple times, provides a great system to help us understand evolution.”
That plants use ocimene to attract insects, but this butterfly uses it as a repellent, is a strange observation. The authors don’t fully understand why, but they think it has something to do with the way visual and olfactory information interact. In other words, context is important; a butterfly interprets the chemical as an attractant when it comes from a plant but as an anti-aphrodisiac when it comes from another butterfly. The “smell itself doesn’t vary,” said Darragh, “but the context, and thus how the signal is interpreted, does.”
This is all well and good, but if a man successfully mates with a woman, why should he care about other suitors? The answer is that female Heliconius melpomene store semen for months, during which time they patiently fertilize their eggs. Later mating episodes would introduce new sperm. This has resulted in the perceived hand-to-hand competition and this bizarre arms race with smelly genitals.
This gets even weirder, as it creates scenarios where eager females are very eager to hook up, but the males don’t want anything to do with it. Think of it as an insect-like version of sexual frustration.
At the same time, Darragh believes it can work in favor of women. The “anti-aphrodisiacs can also serve as honest signals of receptivity, reducing male harassment while keeping females unresponsive to further mating,” she said. This is beneficial for both men and women, as “men do not waste time courting non-receptive women, and women can reduce harassment by men.”
I feel there is an important lesson here for us humans, but I can’t quite put my finger on it.