February 2016 Featured Paper

“I’m Sexy and I Glow It: Female Ornamentation in a Nocturnal Capital Breeder,” Juhani Hopkins, Gautier Baudry, Ulrika Candolin, and Arja Kaitala.
Biology Letters, 20150599.

Read the original paper.(Subscription to Fellows of the Royal Society is required. Check your local library for access.)

In nature, males often use some form of ornamentation to attract females. Females rarely use ornamentation to attract a male because ornaments require internal resources that could be better used to produce eggs. Also, ornamentation is often unnecessary for a male to select a fertile female because fecundity, the amount of eggs produced by a female, is often indicated by her abdomen size. When this is not readily apparent, females of some species may have evolved ornamentation to signify their degree of fecundity.

This study seeks to determine if lantern brightness of the female glow-worm is a reliable indicator of fecundity, and whether males prefer females with a brighter glow. If so, this may indicate that the female glow evolved to attract males with the promise of fecundity.

Glow-worm* females are flightless and attract males by emitting a constant glow. Once mating has occurred, usually within one or two nights, the female ceases to glow, lays her eggs, and dies. If unsuccessful in attracting a mate, she may continue to glow for a couple more weeks.

Glow-worms vary greatly in the amount of eggs they produce (25–195 in this study), and the size of their lanterns (7–19 mm2). The area of the lantern directly correlates to brightness: the larger the lantern, the brighter the glow. Examination of the females in this study revealed that females that glowed brightly contained more eggs. The glow reliably indicates fecundity.

In the experiment, several pairs of traps were set up, one of each pair containing an LED imitating a female with a bright lantern and one with a dim lantern. The males were overwhelmingly drawn to the brighter glow (248 to 38). The results suggest that the female glow serves as an ornamentation that allows males to choose females likely to contain the most eggs.

This ability to glow may come with certain costs, however. Fireflies are “capital breeders”—they do not feed as adults, so their energy reserves are fixed at maturity. Of the twenty-six females tested, nineteen still carried unlaid eggs upon death. Possibly some eggs are used as energy for glowing, instead of adding to the number of successful offspring. The bright glow can also be detrimental in attracting predators.

The benefits of glowing, however, may outweigh the costs. Brighter females attract more males than dimmer ones, and so have a greater choice in picking their mate. Brighter females also attract males over a shorter period of time, thus reducing the number of nights a female need glow, thereby reducing her chance of predation.

As capital breeders, female glow-worms cannot increase their fecundity. Perhaps they evolved to be flightless so that the internal resources needed to fly could instead be used to produce a maximal number of eggs. As males preferred females who glowed more brightly, the brighter glow became a sexually selected trait that allows males to see and choose the more fecund females even in the dark.

* Lampyris noctiluca, the glow-worm used in this study, is a European firefly. Glow-worms in the US include fireflies of the genera Pleotomus, Microphotus, Phausis, and Pleotomodes.

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