U.S. Fireflies Flashing in Unison

Science News,  March 13, 1999  by Susan Milius

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Michael Greenfield of the University of Kansas in Lawrence approaches synchrony from a slightly different viewpoint. He studies acoustic coordination in the great croakfests and chirpfests where male frogs and insects gather to advertise their studly charms, but he also ranks fireflies as special.

Although animals may not call in unison, they often adjust their timing in other ways, he says. Many species of frogs do perceive the beat in a neighbor's calls and respond by avoiding it. One frog who croaks, waits, and croaks again often finds a neighbor croaking during the pauses. This tendency to alternate holds across many species, Greenfield says. Only a very few frogs naturally adjust their timing to call simultaneously.

The evolutionary drive behind this, Greenfield suspects, comes from a neurological tendency to perceive only the first of two closely spaced stimuli. Basically, he who croaks first, even by a few milliseconds, gets the girl. The alternate croaking maximizes the time between the different frogs' signals.

Such simple rules for coordinating with a few buddies, however, can give rise to complicated sounds. For example, Greenfield has found that among the tungara frogs he studies in Panama, two males in an alternating duet will move over to make room, acoustically speaking, when a third joins in. The frogs space their calls so a back-and-forth duet becomes a 1-2-3, 1-2-3 trio. When an abundance of frogs converges, individuals seem to block out the crowd, still taking their timing cues from near neighbors. By overlapping, these local ensembles can create pond music of Wagnerian proportions.

Yet this degree of coordination does not match the feat of certain fireflies, Greenfield says. When frogs or many of the singing insects he's studied join a hootenanny, they do not really change their own basic rhythms, whether they're in or out of synch. Suppose a creature calls every 2 seconds. A neighbor's call may reset the clock, perhaps lengthening the pause between one set of chirps, so that both animals will start the next cycle at the same time. However, both continue to follow their basic 2-second rhythms.

A few firefly species, such as one in Thailand, seem able to do more. A twinkle of light resets the clock to a different starting point but can also change the basic pace. For instance, a firefly that naturally signals every 0.5 second could switch to a once-every-0.4-second pace.

Such adjustability does not occur often in nature, Greenfield says. One of the species of periodic cicadas, Magicada cassini, seems to do it, as can some katydids and perhaps a tree cricket. Even among synchronizing fireflies, this ability to change the natural pace is far from universal.

Fireflies are not flies, taxonomically speaking, but beetles. The whole family, some 2,000 species, covers much of the temperate and tropical world. Most North American species have earned the nickname rovers, because males patrol at night, flashing to advertise their services. Females, often on the ground or a stationary perch, respond with a characteristic answering flash that indicates an interest as well as an address.