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Sexual Selection in Fireflies

Fireflies are justifiably famous for their spectacular bioluminescent courtship displays. In North American Photinus fireflies, species differ both in male flash signals and females’ flash responses (see chart). Firefly females most often mate with several different males during their short adult lives, so sequential episodes of sexual selection happen before, during and after mating. Our firefly research, which is funded by the National Science Foundation, is helping to elucidate several key features of this evolutionary process. 

What makes a firefly attractive?
Using photic playback experiments, we’ve shown that females of Photinus fireflies differentially respond to male courtship signals produced by males of their own species. Females show directional preferences for longer pulse duration in single pulse species such as Photinus ignitus, while females in double-pulse P. greeni prefer faster pulse rates.  We also documented that male mate choice occurs later in the mating season, as firefly males will preferentially mate with those females that carry more eggs.  View graph >

How costly is bioluminescent flashing?
We used a combination of open-flow respirometry and field experiments to measure two potentially important costs associated with producing bioluminescent signals: elevated energy costs and elevated predation risk.

We discovered that from an energetic perspective firefly flashing (without flight) is surprisingly inexpensive; metabolic rate measured during flashing was comparable to that during walking. However, field experiments revealed a striking cost of bioluminescent signals from specialist predators: simulated courtship signals attracted many more predators (Photuris female fireflies) compared to non-flashing controls.

This risk of predation may balance the force of sexual selection in which males with more conspicuous flash signals gain higher mating success.
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Firefly Nuptial Gifts
During each mating, a firefly male transfers a nuptial gift to females. This coiled, sperm-containing package (spermatophore) is manufactured by several male reproductive glands, and represents a substantial investment by the male.

Once inside the female, the male spermatophore enters a specialized structure where it disintegrates over the next few days. Radiolabelling studies show the majority of spermatophore-derived amino acids are found within the female’s developing eggs.

Female fireflies can gain a direct benefit from male nuptial gifts. In these two species, females that mate sequentially with three different males produce significantly more offspring over their lifetime compared to singly-mated females.  View graph >