Butterfly turn-off, flower turn-on

Seems that some smelly butterflies don’t get a lot of action, thanks to an anti-aphrodisiac.

A new study, led by Kathy Darragh, from the University of California, Davis, has found that male postman butterflies (Heliconius melpomene) make a chemical compound called ocimene in their genitals, which they leave on female butterflies to deter other males.

Fascinatingly, and somewhat ironically, ocimene is also produced by some flowers to attract butterflies, so postman butterflies and flowers evolved the genes that produce the compound independently – for different, context-dependent purposes.

The findings have just been published in PLOS Biology.

“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 Darragh. “Male butterflies use it to repulse competitors and flowers use the same smell to entice butterflies for pollination.”

Postman butterflies can live as long as six months, and the females store sperm from a small number of sexual partners. They can then fertilise their eggs over many months from a single fling.

Males, on the other hand, try to mate whenever they can, so they have much more competition with other males. The ocimene that is transferred to the female during one of these trysts means she will probably keep using the single male’s sperm multiple times, instead of getting other deposits.

“Male butterflies pester the females a lot so it might benefit the females too if the smell left behind means they stop being bothered for sex after they have already mated,” says senior author Chris Jiggins of the University of Cambridge, UK.

This smelly substance is no deterrent when on flowers, though.

“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,” says Darragh.

One potential explanation for this strange convergent evolution is that a single smell requires less scent receptors to recognise, but in combination with how something looks it can communicate a different scenario. This is somewhat like the aroma of garlic cooking in a pan as opposed to its smell on somebody’s breath.

“The butterflies presumably adapted to detect [ocimene] and find flowers and they have then evolved to use it in this very different way,” says Jiggins. “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.”

While not all butterflies produce this anti-aphrodisiac, the male postman’s chemical-making capacity sheds interesting light on smell as a form of communication in insects.