If you have ever been outdoors in the suburban or rural USA, you have probably seen fireflies cheerfully lighting the evening. Fireflies, or lightning bugs (which are neither true flies nor bugs), are actually beetles – and though as a 5 year old I thought they glowed for my own amusement, the fireflies’ light organs serve an important purpose. Adult fireflies use their light to find and identify mates, and larval ‘glow worms’ may keep predators away by using light organs to signal their unpalatability.
Even though firefly light signals serve common purposes, they come in a variety of colors, ranging from green to yellow to light red. Interestingly, variation in light color seems to have little to do with mate attraction and this led David Hall, Sarah Sander and colleagues at the University of Georgia, USA, to ask the next obvious question: why did variation in light color evolve in the first place? Earlier research on signal reception provided one possible explanation: sensory drive.
The theory of sensory drive suggests that evolution should favor conspicuous signals – signals that maximize signal-to-noise ratio and minimize degradation. So, Hall, Sander and colleagues translated the principles of sensory drive into some firefly-specific hypotheses. First, firefly species that are active earlier in the evening should have yellower lights to maximize contrast with the green ambient light reflected from vegetation (increasing signal to noise). Second, fireflies that are active later in the evening – especially ones with sedentary females – should have greener light, to increase the amount of light reflected off plants (minimizing signal degradation).
To test these hypotheses in an evolutionary framework, the researchers sampled the emission spectra (light color) of a massive number of fireflies (including males and females from 25 species and totaling over 750 individuals) across the eastern USA over 3 years. In addition, they collected data on the density of vegetation at each site and the time of day when the tantalizing creatures were active.
The results of all of this data collection proved tremendously rewarding. While the color variation across populations of single species was larger than the authors had anticipated, the species and populations that were active earlier in the day produced yellower light. Further still, populations that live in areas of abundant vegetation (with more green ambient light) emitted yellower signals, enhancing the contrast with their environment. In addition, the fireflies that were active later in the evening produced greener light and the team suspects that in the males, this may be due to ancestral conditions, the sensitivity of photoreceptors, or a number of other hypotheses. However, the researchers wanted to test the specific assumption that sedentary females would produce greener light than males to reflect off their leafy perches. Fortunately, the team had collected the light spectra of enough males and females from two species to test this hypothesis and they found that the females from both species were greener than the males, presumably to minimize signal degradation.
Having demonstrated these relationships both without and with phylogenetic context, Hall, Sanders and their collaborators suggest that light color within and across firefly species has evolved multiple times as predicted by sensory drive. The beautiful colors of these natural lights serve a functional purpose: and it has nothing to do with my childhood lantern preferences.