To humans, fireflies are rather eye-catching little creatures. This is a consequence of their bioluminescent signals, which are mainly used for signalling mating availability and, thus, attracting mates. Once the interest of another firefly has been sparked, the flash pattern changes and a new pattern is used to convince the potential mate that they have made a wise decision. But to another firefly, not all flashing signals are equally attractive and fireflies that are overly lascivious in their signalling might not be gaining much in the long term. Putting on a great visual performance could, for example, increase the risk of predation or, alternatively, decrease valuable energetic resources with later repercussions for survival. So how important are these various costs to the soap-opera lifestyle of fireflies and what factors keep a check on a firefly's visual display?
William Woods and co-workers at Tufts University set out to explore these questions using two complementary experiments. In the first experiment, using four species of fireflies, they compared the relative energetic costs of flashing by estimating resting metabolic rates under controlled laboratory conditions in several Photinus species differing in their bioluminescence characteristics. The team found no significant differences in metabolic rate between the nocturnal Photinus species that use flashing to communicate and day-active Photinus species, which do not signal by flashing.
In addition, the team monitored flash rate and metabolic rate within one of the nocturnal species that uses bioluminescent signalling, Photinus greeni, as the fireflies signalled, rested or wandered around. Here they found two important and novel discoveries. First, when individual fireflies were flashing, metabolic rates increased by roughly 37%, although this was approximately 20% less than the cost of walking. Second, they saw a positive relationship between signalling rate and metabolic rate: individuals of P. greeni that flashed more quickly incurred a higher energetic cost. Therefore, from an energetics perspective, flashy performances were indeed more costly within a firefly species.
In the second experiment, using field experiments designed to assess the potential predation costs associated with courtship signals, the researchers built tiny light traps that mimicked firefly courtship patterns in the wild and monitored how many predatory Photuris fireflies they caught. Predatory Photuris fireflies were attracted significantly more frequently to the light traps emitting courtship signals that simulated P. greeni signal patterns. Quite literally, hundreds of predators flocked to this bevy of potential meals during these trials. In addition, the more closely the pattern resembled P. greeni's true mating signal, the more predatory Photuris fireflies were trapped at the miniature lights. Eavesdroppers were indeed keeping close track of their food source.
This study by Woods and co-workers represents an exciting glimpse into the world of energetic costs of courtship signalling in fireflies. Furthermore,the team has clearly demonstrated that natural variation in signalling alters the risk of predation and can incur a metabolic cost. In consequence, the flashdance of fireflies probably represents an evolutionary compromise between attracting the right mates without using too much energy, and simultaneously minimizing the risk of becoming another firefly's dinner.