There have been many theories proposed to try to explain the synchronous flash behavior of fireflies. Yet none of these theories have been satisfactory, given that some element of logic or reason has always been lacking, an empirical proof made impossible or improbably, that one might as well say synchronicity is "magic." Indeed, this may have been what Howard Ensign Evans was implying when he wrote, "In Defense of Magic: The Story of Fireflies." However, having read and reviewed the various theories with regard to synchronous flash behavior in fireflies and not being of mind to place much faith in a magical explanation, I have come to the conclusion that synchronous flash behavior is simply and elegantly the result of an evolutionary process, that fireflies have evolved to flash in synchrony because those individuals which did not tend toward a synchronous flash behavior were not able to successfully compete for a mate, such that they died out, unable to pass along their genes to future generations.
The synchronous flash behavior of fireflies is thus the result of hundreds of thousands, perhaps millions of years of evolution. It is not difficult to understand how this process would evolve. In fact there are examples of fireflies which appear to be evolving toward synchronous flash behavior. A species I have observed and studied since 1968, Photinus pyralis clearly exhibits the tendency to flash in synchrony when flying about seeking females. Males of P. pyralis fly in arching, dipping flights a few feet above the ground or vegetation, flashing every 4 - 6 seconds, pausing in their flight to observe a female response which occurs some 2 seconds after perception of the male flash. Time intervals may vary with temperature and/or humidity such that on hotter evenings males may flash more rapidly than on cooler evenings.
Early or late in the season when there are fewer P. pyralis males searching for females, they may fly about in an endless search for females, never finding a mate, their flash patterns being unaffected by other male fireflies. Yet at times when there are large numbers of male P. pyralis active as when emergence has reached a peak level, then there are many males in close proximity to each other, and they tend to flash in synchrony. If fact, on rare occasions, when population densities are very high, males will shift their flashing, each blinking when it sees another male flash, the result being that many males come to flash in synchrony, often forming small aggregates of males all competing to copulate with a single female.
P. pyralis males seem to interrupt their normal flash pattern when they see other males flash, such that they exhibit a flash in response to a flash behavior, similar to that which induces them to flash and fly when they see other males flashing. So P. pyralis males have at least two distinct flash behaviors, their normal rhythmic flying and flashing, and their flash in response to seeing other flashes, which enables them to adjust their flashing and flash in synchrony when males see one another.
My first observation of this behavior was in 1968 when I was living in Jacksonville, Alabama upon the edge of Talladega National Forest. I recorded the individual flashes of numerous P. pyralis males, following them in their mating flights, using a recorder to note when they flashed. It was not long before I observed that males would tend to flash when they observed the flash of other males. Then later I observed that males exhibit a preflight behavior, where they are resting or alert, and that they can be stimulated to begin their mating flights by seeing the flash of another male already in flight, a response which can be induced by blinking a flash light.
Thus males of P. pyralis, I observed, tend to flash when they see the flash of other males. This also stimulates males to flash and fly. Thus when males and females are emerging in greatest numbers, males tend to flash in synchrony, forming small aggregates of males competing for a female. The males which take flight first and which locate a female first and copulate with her, are the males which pass upon their genes to the next generation.
One may easily conclude from this observation that at least in the case of Photinus pyralis
, males which tend to flash in synchrony have an advantage over males which do not exhibit this behavior. The same is probably true for other species of fireflies which aggregate in large numbers upon "firefly trees" in tropical Asia and Indonesia. In fact, it is my understanding from a review of John Bonner Buck's studies in this regard that this is exactly what is happening. Many males and females aggregate upon trees in densely forested areas of mangroves along river banks. Thousands of fireflies congregate together such that you have many small aggregates of males competing for females. In such high population densities the tendency would be for males to flash in synchrony and females to respond by copulating with males which flash in synchrony, that over hundreds of thousands of years, perhaps millions of years, this behavior would have evolved, to result in exact synchronous flashing of males competing for females.
The manner in which synchronous flash behavior evolved is easier to understand if one looks at this behavior in a species like P. pyralis
and charts their flash behavior over time.
Male flight
x----x----x----x----x----x----x-->
0 10 20 30
Time (sec.)
Fig. 1. Male flashes at a rate of once each five seconds.
Male flight
--x----x----x----x----x----x----x-->
0 10 20 30
(----x----x----x----x--)
Female response - stationary
Time (sec.)
Fig. 2. Male flashes at 5 second intervals. Female sees male and flashes approximately 2.5 seconds later. Male orients toward female and lands close to her.
Male flight
x----x----x-------------------------->
0 10 20 30
Female response
(--------x----x----x--------------------------)
^
Copulation
Fig. 3. Male locates female, female accepts male and copulation occurs. Flashing of male stops after copulation and female stops responding to flash of any other males in her vicinity.
Male A
--x----x----x----x----x----x----x-->
0 10 20 30
--x----x----x----x----x----x----x-->
Male B
Time (sec.)
Fig. 4. Two males flying do not see each other, flashing at same rate but not in synchrony.
Male A
--x----x----x--//---x----x----x----x-->
0 10 20 30
----x----x----//----x----x----x----x-->
Male B
Time (sec.)
Fig. 5. Firefly males A and B flashing out of synchrony see each other and flash immediately in response which brings them to flash in synchrony. Two or more males flashing in synchrony may compete for copulation with the same female.
Male A
--x----x----x----x----x----x----x-->
0 10 20 30
-----------------x----x----x----x-->
Male B
Time (sec.)
Fig. 6. Male B in alert position upon leaf or other vegetation sees Male A flash and is stimulated to flash in response and fly. Male A and B thus are flashing in synchrony.
Figures 1 through 6 show the various flash patterns exhibited by P. pyralis.
In Figure 1 a solitary male flies and flashes in search of a female. The interval or period, P, between flashes is T1 and T2 may vary with the temperature and humidity.
In this illustration P = 5 seconds which is a good value to use for demonstration and line graph modeling.
In Fig. 2 Male A is shown flashing at a period of one flash each five seconds. A female is shown to respond after an interval of approximately 2.5 seconds. This represents the basic male-female flash response in courtship of P. pyralis.
In Fig. 3 the male locates and copulates with the female and flashing of the copulated pair stops. However, other males in an aggregate may continue to flash and try to copulate with the female.
Figure 4 shows two males A and B flying and flashing at the same rate but not in synchrony.
Figure 5 shows two males A and B which are not flashing in synchrony. The males, one or the other or both see each other and respond by flashing each in response to the other. This tends to bring them to flash in synchrony.
Figure 6 shows how a male A in flight and flashing stimulates a male B which is alert to flash and fly blinking in synchrony with males already in flight and flashing. It is this male-male flash response which enable males to establish and flash in synchrony.
Through male-male flash response, Photinus pyralis males are able to adjust their flash that they flash in synchrony. I have observed this behavior numerous times when there would be large numbers of P. pyralis active. On more rare occasions I have observed large numbers of fireflies flashing in synchrony. Conditions must be just right for such displays, there being large numbers of fireflies active on dark or cloudy evenings in areas away from bright city lights. This is behavior, I believe, that is not limited to P. pyralis but which is also exhibited by other Photinids.
What other species of fireflies exhibit the aggregate synchronous flash behavior; i.e., where the formation of small aggregates of males occur which tend to synchronize their flash periods?
To answer this question one needs to make collection of specimens in the field which are observed to synchronize their flash periods and also endeavor to record this behavior using sensitive electronic equipment. I personally have observed synchronous flashing in Photinids in northern Alabama and eastern Texas. Others have recorded this behavior in the Smokey Mountains. How common is synchronous flash behavior among fireflies in North America? Hopefully this may be determined before urban sprawl destroys the habitat of synchronous flashing fireflies.
An experiment that I suggest is described below which may also be applied to synchronous flashing fireflies in Asia and Indonesia.
The idea is to conduct some basic controlled experiments with regard to synchronous flash behavior. This would involve collecting fireflies and putting them in vials or jars with a small, moist piece of paper towel or filter paper (Small Petri dishes would be ideal) to observe and record their flash behavior.
In the control groups "A" and "B" place four males each. Then record the flash behavior. As "A" and "B" are identical they should exhibit the same synchronous flash patterns. Set the jars close together separate by an opaque barrier (piece of plywood board or other opaque board) and record their flash. When the two groups are flashing out of synchrony with each other remove the barrier and record how the two groups adjust their flash to be flashing in synchrony. Use a line graph to model and diagram the flash pattern and show how synchrony is achieved. (My report shows how these line graphs are made).
Repeat this experiment using a group "A" of four males and a group "C" of four females. Repeat this experiment using a group "A" of four males and a mixed group "D" of two males and two females.
Then conduct a second experiment to determine how individuals form small aggregates to flash in synchrony. This involves isolating males and females into small, clear Petri dishes or other containers. Then place the fireflies in their small Petri dishes upon a table in a darkened room.
1. Begin with two males and see how the two males come to flash in synchrony when a barrier between the Petri dishes is removed.
2. Then have a pair of male fireflies (either two in one Petri dish or two in separate Petri dishes set one atop the other) in one group and a single male in another group, such as to see how the single firefly comes into synchrony with the pair when a barrier is removed.
3. Repeat this experiment with various combinations of male-female pairs, as a male-female pair and then remove the barrier to see how a single male comes to flash in synchrony with them.
4. Also vary the number of males and/or the distance between Petri dishes to gain some insight into how far small aggregates can be to maintain their synchronous flashing. Make line graphs to model and describe flash behavior for each experiment.
Through these type of experiments one may be able to gain some insight into synchronous flash behavior of various species of fireflies. Such experiments are ideally suited for species which occur in large numbers which are relatively easy to collect, identify and sex. This would especially apply to areas where there are abundant synchronous flashing fireflies occurring upon "firefly trees" in Asia, Indonesia or Malaysia.
I would be curious to know what conclusions others may be able to come to by conducting simple experiments like this. Basically the idea is to gain some insight into how fireflies form small aggregates with groups of males competing for a female. It is my experience that in P. pyralis males tend to form small aggregates which flash in synchrony so that they may compete successfully for a female, that synchronous flash behavior has evolved as this enables males to compete more successfully for a female. Any insight others may bring to the proof of this theory as by studying small aggregates and how they come to flash in synchrony may be significant.
Should others doing original research on fireflies read this article be, please let me know if you have access to enough fireflies to try any of these experiments. I would be curious to know what results you obtain and any conclusions you may reach. Please contact me via writing to Project Firefly.