Nuptial Gifts and Sexual Selection in Photinus Fireflies

During courtship and mating, males of many species of insects provide females with a source of nutrition to help ensure reproductive success. This nutrition might include captured prey, nutritional substances produced by the male, or body parts of the male himself. This paper reviews what is known about these gifts — called "nuptial gifts" in Photinus fireflies.

Most fireflies do not feed as adults. Therefore, the adult female relies on food she acquired in the larval stage for the energy required to reproduce. The nuptial gift that the male supplies during mating may provide an important additional source of energy. This suggests that females should continue to look for mates to supplement their diminishing energy reserves, even after they have gained sufficient sperm to fertilize their eggs. Like the females, males do not feed as adults, and with each successive mating produce a smaller nuptial gift to offer the female. These two facts suggest that as the season progresses, the courtship roles may reverse.

Male Nuptial Gift Production and Transfer

During mating, the male firefly provides the female with a nuptial gift called a spermatophore, which includes the sperm that will fertilize the eggs. During mating, the spermatophore is transferred to the female, who stores the sperm in a storage organ known as the spermatheca. The remainder of the spermatophore enters a spermatophore-digesting gland (SDG) where it disintegrates over the next few days.

Fate of Spermatophores within the Female

Using radioactive tracers, the researchers examined the fate of the proteins from the digested spermatophore. Three hours after the beginning of copulation, the majority of the labeled protein appeared in the SDG. However, over the next two hours, the amount of protein increased in the eggs as it diminished in the SDG. This finding suggests that the protein from the spermatophore is used to help nourish the developing eggs.

Effects on Female Fecundicity

In this study, females that mated three times — and thus were provided with three spermatophores — produced significantly more eggs than females who only mated once. The extra matings, however, did not affect the percentage of eggs that hatched or the lifespan of the female.

Costs and Benefits to Males

Spermatophore production appears to be costly for the males. There was an average decline of 75% in mass between the males first and fourth spermatophore. Studies also show that smaller spermatophores result in lower paternity success.

Conclusion

It is clear that nuptial gifts have the potential to play an important role in mate selection in fireflies. For example, the Photinus female's ability to produce a larger number of eggs with multiple matings suggests that she should continue to look for mates to supplement her own diminishing nutritional reserves, even after she has gained sufficient sperm to fertilize their eggs. Similarly, decreases in spermatophore size as the season progresses may cause seasonal changes in nuptial gift availability.
The decrease in nuptial gift size, coupled with the seasonal changes in male to female ratio, has the potential to alter courtship behavior. Early in the season, when there are many more females than males and when male nuptial gifts are larger, males must compete for females, while females can choose prospective mates. However, later in the season when females outnumber males, and when nuptial gifts are smaller and harder to come by, females may have to compete for males, and the males can be selective. This change in mating behavior is suggested by this study, but it has yet to be tested by research.