A chemical produced by the male genitals of this tropical butterfly is so repulsive, 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. This is a striking example of chemical signal, in which fragrance is used for communication. In this case, the chemical, called ocimene, “serves as an anti-aphrodisiac pheromone, which is passed on during mating from males to females to ward off further courtship from subsequent 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 converging evolution. In addition to identifying the genes in Heliconius melpomene responsible for the production of the chemical, this is the first time that scientists document the production of this chemical, a terpene, in an animal.
Consequently, the new research makes the belief that Heliconius melpomene obtains ocimene from plants (experiments done in 2007 shown that females covered with ocimene are less frequently approached by males than those covered with a control agent). However, as the new research shows, these butterflies can make this chemical on their own.
Members of Heliconius melpomene have an exceptionally long lifespanns for butterflies, which live about six months as opposed to the usual one month. They live in Central and South America and shows a wide range of wing color patterns, depending on their geographical location. These butterflies are poisonous, so the patterns serve as a warning to prospective predators. These patterns also play a role in sexual selection, as butterflies of this species choose mates that look like themselves.
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“Although the patterns attracted the most attention, it was also long noticed that these butterflies smell,” Kathy Darragh explained. lead author of the new paper, in an email. ‘We now know that this scent is involved in protecting a partner, and serves as an anti-aphrodisiac to ward off future mating efforts from other men.” Darragh did this research while studying at the University of Cambridge.
Humans are very visual creatures, but many animals are dependent on chemical signalinging as their most important form of communication. Orchids, for example, imitate the scent of female insects to attract males for pollination. Anti-aphrodisiacs have also been documented in other insects, including burrows of beetles and plant beetles.
Butterflies, with their striking colors and patterns, clearly use visual cues to communicate, but that does not mean they do not use chemicals as well. Darragh, now with UC Davis, wanted to investigate how chemical signals may play a role in butterfly communication.
An interesting aspect of this, which was the focus of the current study, is how butterflies can make these chemicals that they use in signa.ling, ”she said. ‘In other words, what genes do butterflies have that enable 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). This is an example of converging evolution, in which two independent species develop a similar trait. What is unique about this finding, however, is that plants and insects use this chemical for different purposes (although it is 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 not related to genes previously described in plants with the same function.
“The production of ocimene has evolved independently through different genes in plants and butterflies, showing how different molecular mechanisms may underlie the production of a specific chemical compound,” he said. Darragh. “The independent evolution of the same trait, in this case, pheromone production, often provides an excellent 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 do not fully understand why, but they think it has something to do with the way visual and olfactory information work together. In other words, context matters; a butterfly interprets the chemical as a lure when it comes from a plant but as an anti-aphrodisiac when it comes from another butterfly. The “smell itself does not differ,” Darragh said, “but the context, and therefore how the signal is interpreted, does.”
This is all well and good, but if a male successfully mates with a female, why would he care about other suitors? The answer is that the woman Heliconius melpomene store sperm for months, during which they patiently fertilize their eggs. Next mating episodes will introduce new sperm. This led to the perceived competition between men and men and this bizarre arms race with smelly genitals.
It gets even weirder, because it creates scenarios in which eager females would very much like to join, but the males want nothing to do with it. Consider it an insectoid version of sexual frustration.
At the same time, however, Darragh believes that it can actually succeed to the female advantage. The ‘anti-aphrodisiacs can also serve as honest signals of receptivity, reducing the harassment by men while women are receptive to further matings,’ she said. This is beneficial for men as well as women, as ‘men do not waste time receiving female women, and women can reduce the harassment by men.’
I feel here is an important lesson for our people, but I can not put my finger on it.