Female Preference for Male Courtship Flashes in Photinus ignitus Fireflies

During mating, male fireflies deliver a nuptial gift to the female.  This nuptial gift contains the sperm that will fertilize her eggs and a protein mass that will nourish them.  Since adult Photinus fireflies do not feed in the wild, this nuptial gift may be the female’s only source of nutrition.  For this reason, females would be expected to show preference for males with larger nuptial gifts.  This study will explore how the females choose their mates, what male traits elicit greater female response, and if there is a correlation between these traits and the size of the male nuptial gift.

This study includes three experiments related to the following questions:

1. What is the best predictor of the size of the male nuptial gift in P. ignitus: flash duration, body size, or lantern size?
   
   Males were collected and weighed. The size of their lantern was measured and the duration of their flash was timed.  The males were then mated with females; these females were later dissected to remove and weigh the nuptial gift they received.  Comparison of the weight of the nuptial gift to the male’s flash duration, body weight, and lantern size was noted.


2. Which factors influence female choice of mate: flash duration, lantern size, or light intensity?
   
   Females were exposed to artificial flashes of differing lengths of time (flash duration) that replicated the flashes from male fireflies in search of a mate. The flash duration that elicited the greatest number of responses from the females was observed. LED lights in tubes covered by tape with differently-sized cutouts, simulating lanterns of three different sizes, were used to determine if lantern size affected female response. The lanterns were also displayed at various distances from the female. These experiments tested female response to light intensity both for size of lantern as well as distance.


3. Are females more responsive to males before receiving nourishment, whether from a nuptial gift received during prior mating or from having been fed in captivity?
   
   Three sets of females were tested for their response to artificial flashes: wild-caught females, females mated in the laboratory, and females fed in the laboratory.

Results

A larger nuptial gift corresponded to a longer flash duration.  Female response to the artificial flashes increased as flash duration increased.  However, once the duration of the artificial flash surpassed that of the true upper range of males in nature, female responsiveness declined significantly.
Lantern size was not a good predictor of nuptial gift size. Females responded to large lanterns more often than to small lanterns, but there was no appreciable difference between their responsiveness to large and medium sized lanterns. However, responsiveness decreased as the lantern’s distance increased.
Wild-caught females responded to 50% of the flashes presented to them, while mated and lab-fed females responded to significantly fewer flashes.

Discussion

This study demonstrates that female P. ignitus prefer males with longer flash durations, which corresponds to choosing males with larger nuptial gifts.  The females also prefer males with brighter flashes, but cannot discriminate between flash intensity due to lantern size versus distance from the flashing male. The intensity of the flash is therefore a poor predictor of nuptial gift size, as the flash of a nearby male with a small lantern and small nuptial gift will appear brighter than that of a male with larger lantern and larger nuptial gift that is much farther away.
Females were also less likely to respond to males after having received a source of nutrition, whether from prior mating or laboratory feeding, suggesting that the female’s nutritional status may influence her mating behavior.   Females may prefer one male with a large nuptial gift over multiple males with smaller ones. 
Early in the season, females are not as responsive to males as they are later in the season.  Males outnumber the females early in the season, and hold larger nuptial gifts, since males produce smaller nuptial gifts with each successive mating.  Females can thus take their time and be more selective; they respond to males with a longer flash duration and larger nuptial gift.  As the season progresses, there are fewer males to choose from, and females cannot afford to be as particular. Females will then respond more often and with less selectivity to male flashes.
Although females were more responsive to longer flash durations, the fact that their response declined dramatically when flash duration exceeded that found among male P. ignitus in nature indicates that flash duration as well as timing between flashes is significant in identifying mates of their own species.