Experts find some male butterflies force a ‘chastity belt’ on females to stop advances from rivals

Reproduction is a driving force across the animal kingdom, with creatures developing diverse strategies to ensure their genes are passed on.

Some male butterflies employ a unique technique – they force the female into a ‘chastity belt’ that prevents her from reproducing with other suitors.

In response, some females seemed to have evolved larger, more complex genitalia that are harder to block.

The result has been an all-out battle of the sexes, with males devising increasingly ornate mating plugs  -some with winglike projections, slippery scales or pointy hooks.

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A female butterfly fertilizes most of her eggs with sperm from her last partner, so its to the male’s benefit to block access to rivals.

But external mating plugs, also known as sphragis, are found in just one percent of all butterflies. 

Other species employ sphragis, including kangaroos, bees, rats, spiders and several kinds of primates.

Typically they ensures paternity, but they can have other benefits for butterflies, too: The Rocky Mountain parnassian’s mating plug delivers protein to the female.

The zebra longwing butterfly’s sphragis includes predatory defense chemicals and an anaphrodisiac that turns off other males.

While the female will eventually expel the plug, it may be in place long enough for a male to ensure his sperm have reached the egg and achieved fertilization.

Rather than use a mating dance or song, a plug-producing male butterfly will grab a female in midair and drag her down to the ground.

After depositing his sperm, he excretes a pre-molded plug from intricate abdominal ducts that give it its shape. 

It then hardens on the female, blocking her genitalia but leaving the orifice she uses to lay her eggs unobstructed. 

This natural contraceptive allows him to fertilize more females rather than spend time fending off advances from other males.

For the female, though, it’s a bummer.

Multiple mates mean higher quality sperm that can result in greater genetic diversity of her offspring and more nutritious ‘gifts’ for herself.

Ana Paula dos Santos de Carvalho, a doctoral candidate at the Florida Museum of Natural History’s Kawahara Lab, was curious if this sexual tug-of-war was behind the large diversity of butterfly species.

She studied brush-footed butterflies in the museum’s collection, analyzing the rate at which new species appeared across the Acraeini tribe.

In a study published this month in the journal Systematic Biology, Carvalho found that lineages with and without sphragis evolved at essentially the same rate.

That points the finger at some other factor fueling butterfly diversity, which Carvalho said ‘came as a surprise.’

‘I was expecting to see an association between plugs and new species appearing faster, but my work suggested there was no link at all.’

While a mating plug frees a male from guarding his mate, it’s a major investment in resources and time.

And simpler plugs may be torn or ripped off by a particularly determined female.

In species with large, complex plugs, Carvalho noted she rarely encountered a female without one.

When tracing an evolutionary family tree for Acraeini butterflies, Carvalho found some species employed sphragis at one time but then stopped.

This likely means the females in that line figured out how to circumvent the plug to the point that it was no longer an effective evolutionary strategy.

The variety in shape and size of female butterfly organs also suggests they were evolving to defeat the sphragis.

‘Butterflies and moths continue to surprise us,’ said co-author Akito Kawahara, curator at the Florida Museum’s McGuire Center for Lepidoptera and Biodiversity.

‘This study suggests we still have a lot to learn about what drives insect diversity and the role sexual conflict plays in evolution.’