Michaelidis CI, Demary KC, Lewis, SM. 2006. Male courtship signals and female signal assessment in Photinus greeni fireflies. Behavioral Ecology 17: 329-335.

The April 14^th Animal Communication lecture on signals used in mate acquisition enumerated predictions regarding the form and content of signals used for mating. An example was given in the lecture on signaling patterns in the bioluminescent flashing of fireflies. In a recent paper by Michaelidis et al, the firefly Photinus greeni is discussed in terms of the preferences of females for specific male signals. The authors of the study recognized that many fireflies have stereotypical flash patterns that function both as species-specific indicators and as signals displaying readiness to mate, but they also identified the propensity for female choice to be plastic with respect to abiotic factors such as temperature and individual-specific traits, such as personal mating history and nutritional status. The amount of variation in the signals themselves is expected to be low to ensure that female mistakes are avoided since females, as the higher-investing sex, stand to lose more resources in the event of a hybrid mating.

Courtship in Photinus greeni features roving males using bioluminescent signaling to attract females, who also have bioluminescent capability and flash light to respond to males. Michaelidis et al identified the presence of two to three congeneric species, all of which are described primarily in their IPI, or interpulse interval. This time lag between flashes of the bioluminescent organ should have the effect of drawing in only (or primarily) conspecific females with preferences for that quantity of time lag.

The authors were interested in quantifying the amount of variation in P. greeni IPI given shifts in ambient temperature, which they expected would affect the mating signal, as it does in many ectotherms. They used video-taped signals of individual males, allowing them to mate following the recording. To get a baseline understanding of female preference difference, they enacted choice trials where female were able to respond to flash signals from preprogrammed LED lights. The authors were also interested in investigating any relationship of IPI to size of the spermatophore (recoverable in the female body after mating), a nutrient rich sperm packet that is provided to females as a direct benefit of mating. We expect spermatophore size to scale up with male body size. If the bioluminescent signaling is so-called “honest” signaling with respect to body condition, males with particularly suitable flash patterns should be advertising their ability to provide large spermatophores.

The average IPI of P. greeni males at a constant 70 degree Fahrenheit was 1.51 seconds (plus/minus 0.18 seconds), but there was significant between-male variation in IPI, accounting for nearly 70% of the total variation. These IPIs remained roughly constant over a change of ambient temperature, a surprising demonstration of temperature coupling, which should favor female preferences to be as consistent as male signals. There was no effect of spermatophore size being predicted by any certain length IPI, and surprisingly, body size was also not a statistically significant predictor of spermatophore size either. In female preference tests, a strong preference was seen for IPIs of about 1.0 second, but individual females also responded significantly to other IPIs of 50 to 150 milliseconds. I found the experimental design of these trials to be a bit suspect—if, for example, an inexperienced female sees flash patterns of other species first, who’s to say she might gain a newfound preference for that speed? All in all, the authors state that IPI doesn’t appear to provide information to females about male quality, just identity and location. From here I would expect female choice to be exerted at a closer range, possibly with visual inspection or handling of the spermatophore to determine male suitability