Sensing Electricity

Superman’s fictional heat vision notwithstanding, humans sense their world for the most part passively. We don’t beam anything out of our eyes, we just respond when light hits our retinas. We do send out sound, however. Our ears, sensing this very different kind of energy—sometimes bounced back as echoes—still respond passively, as minute changes in air pressure move our eardrums. But when we converse, we both send sound and receive it. We can hear ourselves and at least use this feedback to modulate our own volume and pitch. Bats emit sound and listen for variations in echoes to help them sense their surroundings. Using a sense very different from either hearing or vision, certain fish communicate and sense their surroundings with electricity.

“Just as the eyes are organs that evolution has fine-tuned and engineered to optimally detect light, ears are optimal organs for detecting sound, and taste buds are chemical receptors, these fish have very sophisticated receptor organs for detecting electric fields,” says physiologist Dr. Brian Rasnow, now at Amgen.

A wide variety of fish can detect electric current. Some, like the shark, use the sense to “see” prey. Some sharks can detect a field as weak as “five nanovolts per centimeter, which is equivalent to stretching out a one-and-a-half-volt battery over 30,000 kilometers,” Rasnow says.

But some fish have taken their electric sense a step farther; they produce electricity. Electric eels have been known since before humans understood the nature of electricity. Some smaller, gentler cousins also produce electric fields, albeit weak ones. Two families of freshwater fish have evolved the ability to create electric fields around their bodies. Their ability to sense minute changes in these electric fields, the fields of other electric fish, the weak fields produced by all living things and disturbances in all these fields produced by inanimate objects in their environment, makes up a sense and a system of communication as different from vision as hearing is from vision and as different from hearing as vision is, Rasnow says.

Weakly electric fish live in muddy water and only become active at night. In this lightless environment, weakly electric fish use their electric sense like many other animals use sight or hearing, to “see” where they are going, to find prey, and communicate with each other. They can distinguish the electrical discharge of their own species, and can further determine the size, sex, maturity and even possibly the individual identity of any fish of their own species that passes by. Each fish, in other words, broadcasts many aspects of its identity in fluctuations and characteristics of its electric field. This sense, however, has its limits.

To picture these limits, take off your glasses or remove your contacts, Rasnow suggests. Then fill the room with fog. The power of an electric field fades rapidly—at twice the distance, the power falls to 1/8—so the electric sense of these fish is limited to about half a body length. It’s as if you could see only a little beyond arm’s length.

This short-distance communication system, however, serves the social electric fish well. In some species, social groups organize into a dominance hierarchy. The most dominant male emits the most extreme frequency, highest in some species and lowest in other species. The dominant female will settle on the opposite extreme. In one species, for example, males have a frequency of about 60 Hz (the frequency of household electric current in the US that causes the buzzing you hear in poorly grounded stereo systems turned to high volume), whereas females buzz at 120 Hz. Immature animals fall between these extremes.

While many animals who live in societies organized on dominance can’t do anything about their status, electric fish can change their frequency. If one animal chooses to challenge the dominant male, he may begin to match the dominant male’s frequency. Responding to this insult, the dominant male may emit an aggressive electrical “chirp,” a sudden, brief increase in frequency—the electric-fish equivalent of a glove across the face. Once engaged in battle, two males may spend an entire night locked jaw to jaw. Finally one male wins out, earning the exclusive right to spawn. The loser signals submission with a sudden cessation of electric discharge. In the laboratory, scientists can model an aggressive electric fish. Energizing the model provokes an attack, always to the “head” end, electrically speaking. With the electrical poles switched, the insulted male attacks the opposite end of the model, which now emits the electrical signal typical of the head end.

In one species, females defend spawning territories and the dominant male will mate only with the dominant female, the one with the best spawning territory. She hangs vertically among the plants while the male emits electrical chirps 60-80 times per minute, courting all night. This electrical display excites the female to lay eggs, whereupon she chirps quietly, an apparent signal for the male to fertilize the eggs.

Some females try to sneak in to lay eggs among those of the dominant female. These females remain electrically quiet. If the royal couple drives them off, sneaker females will lay eggs elsewhere, in which case, her eggs remain unfertilized. Nonmating females thus appear so excited by the male’s electrical love song that they lay eggs even in the absence of a male. In laboratory playback experiments, scientists can duplicate this effect, playing a love song that causes lone females to lay eggs in an aquarium.

Another species, this one a bottom dweller, shows different mating behavior. A male will mate with any female who responds to his call. Successful fertilization in this species is complicated compared to most fish. She lays only one egg at a time, and the male must fertilize it just as she lays it, or it spurts to the river (or aquarium) bottom to remain unfertilized. To signal the male, the female chirps just as the egg is about to emerge.

Swimming in a muddy river and relying on your electric field can pose a problem, however. What happens if another electric fish swims by? Your frequencies may overlap. “It’s like CB radio when the channel is too noisy,” Rasnow says. “The only sensible thing to do is switch to another frequency.” Scientists studying electric fish call this the Jamming Avoidance Response (JAR). Two passing electric fish change frequencies, each moving its frequency slightly away from the other’s. This response functions as a reflex, like the leg-jerk reflex when the doctor taps you on the knee. On the other hand, electric fish sometimes synchronize their frequencies, for reasons scientists have yet to fathom.

Next: Fish and Sound