Female preference for male courtship flashes in Photinus ignitus fireflies

Christopher K. Cratsley 0 Sara M. Lewis 0 0 Department of Biology, Tufts University , Medford, MA 02155 , USA In Photinus fireflies, males produce spontaneous bioluminescent courtship flashes. Females preferentially respond to particular male flashes with flashes of their own. This study explored variation in female flash responsiveness as a function of male flash duration, female condition, lantern size, and lantern distance, as well as the relationship between male characteristics and spermatophore mass in Photinus ignitus fireflies. We determined female preference by scoring female flash response to simulated male flashes and determined variation in overall female flash responsiveness for laboratory-mated, laboratory-fed, and control P. ignitus females. Flash duration, lantern size, and body mass were recorded for field-collected males. Males were then mated to determine spermatophore mass. Females exhibited greater preference for artificial flashes representing the upper range of conspecific male flash duration and lantern size as well as flashes produced at a closer distance. Both laboratory-mated and laboratory-fed P. ignitus females showed lower overall responsiveness across all flash durations relative to control females that did not mate or feed in the laboratory. Male flash duration predicted a significant proportion of the variation in spermatophore mass for early-season males. These results suggest that female Photinus ignitus may prefer long flashes in order to obtain the direct benefit of larger spermatophores and may adjust their overall flash responsiveness as the relative importance of this benefit varies with changing female condition. Key words: fireflies, lampyrids, mate choice, nuptial gifts, Photinus, sexual selection, spermatophore. [Behav Ecol 14:135-140 (2003)] - F been demonstrated in many species (Andersson, 1994; emale choice for particular male courtship traits has Johnstone, 1995) and has been proposed to play a role in the evolution of elaborate male traits (Darwin, 1871) and speciation (Endler, 1989; Lande, 1981; Ryan, 1990). Mate choice is the pattern of mating that arises at least in part through the mating preferences of one sex (Heisler et al., 1987). As potentially adaptive behaviors, female mating preferences are predicted to vary in response to many factors, including the females ability to discriminate between different signals and the relative costs of sampling multiple males ( Jennions and Petrie, 1997). Until recently, variation in female preference and the factors underlying this variation have received little attention ( Jennions and Petrie, 1997; Widemo and Saether, 1999). To determine the potential for sexual selection through female choice, we need to explore the factors that influence female mating preference. Nuptial gifts provided by males during courtship or mating can influence female mating preference. Under certain conditions females may benefit from choosing carefully among phenotypes of prospective mates, while under other conditions females may benefit more by obtaining a males contribution regardless of male phenotype. In many insects females can increase their reproductive output by obtaining spermatophores through additional matings (reviewed by Boggs, 1995; Gwynne, 1997; Vahed, 1998). However, in some species, females use male traits to predict spermatophore quality (e.g., Dussourd et al., 1991). Therefore, female preferences may vary with the reliability of male traits as predictors of spermatophore quality as well as with the effects of these nuptial gifts on female condition. We studied the relationship among female preference for male traits, male spermatophore quality, and female condition in Photinus fireflies. Courtship and flash behavior have been described for several Photinus firefly species (Branham and Greenfield, 1996; Buck, 1937; Buck and Buck, 1972; Carlson and Copeland, 1988; Cicero, 1983; Lewis and Wang, 1991; Lloyd, 1966; Wing, 1984). Male Photinus fly while spontaneously flashing, and females perched in the vegetation may or may not respond to the flashes of particular males with bioluminescent flashes of their own. Males that elicit a higher female response rate are more likely than competing males to successfully mate with the responding female (Lewis and Wang, 1991; Vencl and Carlson, 1998). Females differentially respond to conspecific male flashes as a function of flash rate in a species that produces trains of multiple flash pulses (Photinus consimilis; Branham and Greenfield, 1996) and intensity in a single-pulse species (Photinus pyralis; Vencl and Carlson, 1998). In single-pulse fireflies the duration of male flashes may be an important timing component assessed by females, but this has yet to be investigated. Furthermore, the adaptive significance of female preferences remains unknown for Photinus fireflies. Male fireflies of the species Photinus ignitus transfer a protein-rich spermatophore to females during mating (van der Reijden et al., 1997). Amino acids obtained from male spermatophores are distributed to the females eggs within 2 days after mating (Rooney and Lewis, 1999). Because adult Photinus fireflies do not feed in the field (Lloyd, 1997; Williams, 1917), male spermatophores may represent the only nutritional supplementation that females receive as adults. Therefore, females are expected to show preference for any male traits correlated with large spermatophores. In contrast, under some conditions females are not expected to show mating preferences because the costs of rejecting any spermatophore contribution may outweigh the benefits of choosing a particular male. In this study we examined the hypothesis that female P. ignitus fireflies respond preferentially to males based on male flash duration. We also investigated whether male flash duration, body size, or lantern size might be a useful predictor of spermatophore size. Finally, we tested the prediction that P. ignitus female responsiveness would decrease after mating, as the costs of sampling additional males decline relative to the benefit of receiving additional spermatophores. MATERIALS AND METHODS We collected P. ignitus fireflies throughout their mating season from late June to early August 19972000 at the Smith Andover field in Lincoln, Massachusetts, USA. Nightly flight periods of male P. ignitus last from approximately 2100 to 2200 h. Fireflies were maintained in the laboratory on a natural light cycle in clear 250-ml plastic containers with moistened filter paper. Intraspecific variation in male flash duration To examine intraspecific variation in flash duration, we observed male P. ignitus both in the field and in the laboratory. We transferred spontaneously flashing males to mesh containers and recorded flashes using a photomultiplier tube connected to a portable data acquisition system (DASport, Intelligent Instrumentation, Tucson, Arizona). Flash data were acquired at 1000 Hz and streamed t (...truncated)