Firefly sex success: the secret is in the gift-giving

Fireflies illuminate the woodland The scientists still do not understand how female fireflies sense the size of a male's nuptial gift

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Male fireflies, known for attracting mates with a flash of light, also seduce with a gift, say scientists.

This gifts comes in the form of a spermatophore: a package containing sperm and nourishment for the female.

Researchers from Tufts University in Boston, US, found that females preferred males that had the largest, most nourishing gift.

The team presented their findings at the First Joint Congress on Evolutionary Biology in Ottawa, Canada.

With supervision from his colleague Sara Lewis, who has been studying fireflies for 20 years, Dr Adam South used LED lights to mimic the flashes of amorous male fireflies.

Invertebrate love gifts

Photinus firefly after failing to transfer a gift
  • Female Paratrechalea ornata spiders receive food parcels wrapped in perfumed silk from suitors
  • The male fruit-fly (Drosophila subobscura) regurgitates a nutritional soup for its belle
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They showed one group of females artificial male flashes in patterns and durations that had been proven attractive in previous studies. Another group of females saw "unattractive" flashes.

In the wild, females are very picky about what males they reveal themselves to during this part of the courtship routine. Females will only "flash back" to males they are attracted to.

But in this experimental set-up, after several minutes of the courtship flashing, males and females were paired together in miniature chambers. The Tufts biologists filmed the encounters under infrared illumination to see what was happening when the lights went out.

Fireflies mating under infrared light The fireflies' nocturnal activities were revealed under infrared light

Their footage revealed that females were much more likely to mate with males that had larger nuptial gifts to offer. Once the males and females were together, the quality of the flashes did not seem to affect the outcome of their meeting.

The results have presented the scientists with a further mystery; since the spermatophore is transferred internally, it is not clear how the female uses the size of this gift to decide whether to mate with a male.

Dr South, who presented the findings, said he was surprised to discover that "attractive flashes only seem to benefit males during the early stages of firefly courtship".

"Initially, flashes are important," he explained. "[But] once males make physical contact, females switch to [this] alternative cue."

The team study fireflies in order to fully understand the remarkable displays and sometimes bizarre behaviour that has evolved in the pursuit of sex.

Dr South told BBC Nature that it was "critical to study all possible episodes" in the insects' sexual behaviour, "to truly understand the reproductive ecology of [the] species".

"If we had stopped studying the mating habits of fireflies after the flashing stopped," he said, "we would have missed this amazingly complex and incredible story."

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