These butterflies use their putrid genitals to ward off their rivals from their mates

A chemical produced by the male genitals of this tropical butterfly is so repellent that scientists call it “anti-aphrodisiac.”

New research published today in PLOS Biology describes the genetic basis of a man-made chemical compound Heliconius melpomene butterflies. This is a vivid example of chemical signaling, in which scent is used for communication. In this case, the chemical, called ocimene, “acts as an anti-aphrodisiac pheromone, transferred from males to females during mating to repel courtship of subsequent males,” the authors write in their article.

Oddly enough, some plants also produce ocimene, but they use it for different purposes. The new research highlights a unique case of convergent evolution. In addition to identifying genes in Heliconius melpomene responsible for the production of the chemical, this is the first time scientists have documented the production of this chemical, a terpene, in an animal.

As a result, the new research dispels the earlier belief that Heliconius melpomene ocimene obtained from plants (experiments carried out in 2007 shown that females covered with ocimene were approached less frequently by males than those covered by a control substance). As the new research shows, however, these butterflies can make this chemical on their own.

Members of Heliconius melpomene have an extraordinarily long lifespanns for butterflies, living about six months instead of the usual month. They live in Central and South America and feature a wide range of wing color patterns, depending on their geographic location. These butterflies are poisonous, so the the models act as a warning to potential predators. These patterns also play a role in sexual selection, as butterflies of this species choose mates that resemble them.

“While the patterns have attracted the most attention, it has also long been noted that these butterflies smell,” explained Kathy Darragh, main author of the new paper, in an email. “We now know that this scent is involved in protection against the mate, acting as an anti-aphrodisiac to repel future mating attempts from other males.“ Darragh carried out this research while doing his PhD at the University of Cambridge.

Humans are very visual creatures, but many animals depend on the chemical signalas their primary form of communication. Orchids, for example, imitate the smell 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, clearly use visual cues to communicate, but that doesn’t mean they don’t use chemicals. Darragh, now with UC Davis, wanted to study how chemical signals can play a role in butterfly communication.

“An interesting aspect of this, which was the focus of this study, is how butterflies can make these chemicals which they use in signaling, ”she said. “In other words, what genes do butterflies have that allow them to make these compounds? 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 just a bit). This is an example of convergent evolution, in which two independent species develop a similar trait. What is unique about this discovery, however, 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 not linked to the genes previously described in plants with the same function.

“Ocimene production has evolved independently via different genes in plants and butterflies, demonstrating how different molecular mechanisms may underlie the production of a specific chemical compound,” said Darragh. “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 repellant, is a strange observation. The authors don’t fully understand why, but they think it might have something to do with how visual and olfactory information work together. In other words, context matters; 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 scent itself doesn’t vary,” Darragh said, “but the context, and therefore the way the signal is interpreted, does.

This is all fine and good, but if a man successfully mates with a woman, why should he care about other suitors? The answer is that the woman Heliconius melpomene store semen for months, a period during which they patiently fertilize their eggs. Subsequent mating episodes would introduce new sperm. This resulted in the observed man-to-man competition and this bizarre arms race involving smelly genitals.

It gets even stranger, as it creates scenarios where enthusiastic women would love to hook up, but men don’t want anything to do with them. Think of it as an insectoid version of sexual frustration.

At the same time, however, Darragh believes it may actually work to the benefit of women. “Anti-aphrodisiacs can also act as honest signals of receptivity, reducing harassment by men while women are not receptive to further matings,” she said. This is beneficial for both men and women, because “men don’t waste time courting unresponsive women, and women can reduce harassment from men.”

I think there is an important lesson here for us humans, but I can’t really put my finger on it.