October 2015 Featured Paper

"Flash Signals, Nuptial Gifts and Female Preference in Photinus Fireflies," by Christopher K. Cratsley.
Integrative and Comparative Biology, Vol. 44 (2004), pp. 238-241.

In the typical courtship rituals of the Photinus firefly, the male typically flashes while in flight, and if interested, the perched female responds with a return flash. The male lands to approach; they couple and mate. When Photinus fireflies mate, the male delivers a “nuptial gift” to the female along with his sperm. This nuptial gift is a protein mass that will nourish her eggs. Since adult Photinus fireflies do not eat, this protein provides important nutrients for the eggs’ development.

Early in the season, when males outnumber the females, females can be selective in choosing their mates. Choosing the male that offers the largest nuptial gift would be to the female’s advantage, and to the species as well.

This study seeks to discover what factors influence a female’s choice of a mate. To find a male that provides a reproductive advantage, is the Photinus female influenced by a particular male flash characteristic? Does the male’s flash pattern indicate the size of his nuptial gift? If so, the female preference for a specific flash pattern may be an evolutionary driver of the male mating signal.

Flash pattern as indicators of male quality

The male Photinus consimilis flash pattern consists of four to ten individual flash pulses repeated every ten seconds. In one study it was shown that P. consimilis females preferred males with a shorter time interval between individual pulses. Most Photinus fireflies, however, display a single-pulse flash, so this characteristic alone is not a good indicator of nuptial gift size.

In two species of Photinus fireflies, P. pyralis and P. ignitus, females were shown to prefer males with a brighter and longer flash, as long as the flash is within a certain range of flash pattern recognized by that species of firefly.

While P. ignitus females preferred males with a brighter flash, the females could not distinguish between bright flashes at a distance and dimmer flashes nearby. This suggests that flash brightness may not be a reliable indicator of nuptial gift size.

Studies demonstrate that females prefer males whose flash is of longer duration. When measuring flash duration against body mass and nuptial gift size, males with the longer flash times were shown to have both a larger body mass and a larger nuptial gift. So in choosing a male whose flash is longer in duration, the female receives a larger nuptial gift, providing greater nourishment for her eggs.

This was true only at the beginning of the season, however. Adult males do not feed, and with no way to replenish the protein in their nuptial gift after mating, the nuptial gift reduces in volume with each successive mating. As noted above, females early in the season are selective in their choice of mate, responding to males with a longer flash indicative of a larger nuptial gift. As the season progresses, the number of males decline and the duration of their flash is no longer a reliable indicator of nuptial gift size. At that point females will respond more readily to any male flash.

Conclusion

The preference of female Photinus fireflies for males with particular flash characteristics in pulse rate, light intensity, or flash length may have shaped the evolution of Photinus courtship signals. Males with such traits would be first to mate, and would sire the next generation of fireflies, genetically passing along their longer, brighter flash signals.


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