Don’t let the makeup companies find out. Lady glow-worms are setting an unattainable beauty standard by using bright light to show males how fertile they are. It’s a rare (in the animal world) example of females decorating themselves while their mates choose between them.
The European glow-worm, or Lampyris noctiluca, is a member of the firefly family in which the females do most of the glowing. Males are ordinary-looking beetles with brown wings. Females are much larger and don’t have wings at all—they look more like overgrown larvae. In their adult stage, glow-worms usually live for less than two weeks. They don’t even eat, focusing all their energy on finding a mate.
Females, stuck without wings, emerge at night and glow in place. Greenish light shines from the end of a female’s abdomen—an organ called the lantern—for up to several hours each night. Usually, it takes just one or two evenings before a male shows up in response to the beacon. But some females glow for weeks without ever attracting a mate. One they’ve mated and laid their eggs, the females die.
It’s clear that male L. noctiluca beetles are drawn to a female’s glow. But Juhani Hopkins, a graduate student at the University of Oulu in Finland, wanted to know whether males cruising for a mate are especially attracted to females who glow more brightly. And does this glow reveal anything about the female who’s making it?
Hopkins and others captured glow-worms from the wild to bring back to the lab. Females could simply be plucked from their perches. The researchers gathered male beetles using traps with green LEDs at the top. They hung the traps in pairs, a dimmer light near a brighter light, to see which of these artificial females drew more males. Back in the lab, they let the males mate with the (real) females by putting them in jars together.
The trapping experiment showed that male beetles greatly preferred a brighter light. Whether there was a large or a small difference between the intensity of the lights in a pair, male glow-worms sought the fake female making the brighter glow.
After the real females laid their fertilized eggs and expired, the researchers counted the eggs and measured the area of each female’s lantern. They had also scored each female’s brightness on a five-point scale while she was alive. The researchers found that larger lanterns made a brighter glow, and the best-endowed females laid the most eggs. (The findings were published in Biology Letters under the title “I’m sexy and I glow it.”)
There was wide variation in the size of females’ lanterns, as well as their fertility. The 26 female beetles in the lab laid anywhere from 25 to almost 200 eggs. This might explain why males have evolved to be choosy. They want to invest their sperm in the most fertile mate they can find, and the glow of the females’ lanterns lets them judge.
Adorning themselves with light is risky for females, since it makes them easier for predators to find. But evolution has favored this strategy, maybe because of how it lets the females spend their limited energy budget. Instead of flying, they invest resources in making eggs—and advertising them with a glowing sign. “Most likely the female ornament evolved due to the females being flightless,” Hopkins says. “By losing the ability to fly, females can produce more eggs, but reduce their chance of finding males. The glow then acts to compensate for this loss.”
In other words: Maybe she’s born with it. Maybe it’s ma…le sexual preferences driving evolution? The cosmetics industry can work out the slogan.
Image: “Glimworm Lampyris noctiluca crop” by derivative work: Dysmorodrepanis (talk)Glimworm_Lampyris_noctiluca.jpg: Jasja Dekker – Glimworm_Lampyris_noctiluca.jpg. Licensed under CC BY-SA 2.0 via Wikimedia Commons.
Hopkins, J., Baudry, G., Candolin, U., & Kaitala, A. (2015). I’m sexy and I glow it: female ornamentation in a nocturnal capital breeder Biology Letters, 11 (10) DOI: 10.1098/rsbl.2015.0599
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