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Female Glowworms With the Brightest Gleam Make the Best Mates

A new study examines what’s behind a glowworm’s light.

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An adult female glowworm lights up the night looking for a mate.

Researchers have found that the brightness of a female glowworm is a signal to males about her qualities as a mate.

A study published October 20 in Biology Letters says the brightness corresponds to the number of eggs she will produce. It’s a relatively rare example of females having ornamentation to attract males, says Bruce Lyon, an ecologist at the University of California, Santa Cruz.

In many other species, males have gaudy ornaments, from brightly colored feathers to large antlers that help females decide who to mate with, as they indicate good health and, consequently, good genes to pass on to offspring.

Female ornamentation is rarer, probably because reproduction takes more time and energy from females and they are pickier about their mates. Scientists have documented several exceptions, especially those where males do a disproportionate amount of child-rearing, such as two-spotted gobies (Gobiusculus flavescens).

In their larval stage, glowworms use light as a warning to predators that the bugs weren’t good to eat. The larvae don’t have distinct sexes, so everyone appears to glow. As the larvae mature into adults, however, the males lose their ability to shine. Adult female glowworms produce light via a chemical reaction that turns energy and oxygen into a yellowish-green glow that emanates from a special body part called the lantern, located on the lower abdomen. (Despite their name, glowworms aren’t actually worms—they’re a variety of flightless beetle.)

To test the idea that a female’s glow serves as a symbol of her fecundity, the study’s senior author Arja Kaitala, an ecologist at the University of Oulu in Finland, and her colleagues captured glowing females from the Tvärminne Zoological Station on the Gulf of Finland and the town of Nurmijärvi, just north of Helsinki, and brought them 435 miles (700 kilometers) north to her lab in Oulu. They found that larger females had larger lanterns, and the brightest females produced about four times as many eggs as the dullest ones.

“We knew there would be differences in size and egg production, but we had no idea they would be so large. I was really surprised by this,” Kaitala said.

Back in the field, they tested whether males were more attracted to brighter females using tiny LED lights, half of which they rigged to glow extra bright and half of which they altered to be more dim. Placing the lights in traps, which they built out of a plastic 1.6 quart (1.5 liter) bottle, they counted how many males were trapped in each. Kaitala and colleagues found significantly more males in the bright light traps.

But Bob Montgomerie, an ecologist at Queen’s University in Canada, says he isn’t convinced that they have discovered true sexual selection. Larger females are known to produce many more eggs than smaller ones, and since larger females also have larger lanterns, it could be that the males are using glow as a proxy for size.

“To know for sure, you’d have to measure glow per unit area. Then you could say whether it was glow or size,” Montgomerie said.

To Kaitala, many mysteries of the glowworm remain. The adults are unable to eat, which means that females can’t eat extra to fuel their nighttime radiance. The energy has to come from somewhere, and the researchers believe it could impact the number of eggs she produces. They also don’t know how the female benefits from glowing.

Kaitala and other researchers hope that they won’t be in the dark much longer.

Follow Carrie Arnold on Twitter.