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’
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.
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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
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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.
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