Covert Electronic Surveillance
How some animals use electricity to spy on their neighbors
Posted Mar 09, 2016
We learned from Edward Snowden that the U.S. National Security Agency spies on a vast number of electronic communications. But we humans did not invent covert surveillance of electrical signals.
All animals need to keep tabs on their neighbors, which may be friend or foe (or on the dinner menu). Some animals rely on overt signals that are available to all species, such as sights and sounds, while others use covert signals that are invisible to most species.
Weakly electric fish (also called knifefish), which live in muddy waters and often hunt at night, use covert electrical surveillance. (These fish are different from strongly electric fish, like electric eels, that stun their prey using extremely high voltages.) They generate an electric field around their body. This field is too weak to harm anything or even to be detected by most species. Other animals in the vicinity unknowingly alter the fish’s electric field just by being good electrical conductors, because they are largely filled with salt water. The weakly electric fish detect these electrical disturbances using an array of electrical sensors that cover their skin (video).The sensor signals feed into a map of the neighborhood that is created within their brain, in the same midbrain structure that other vertebrates use to create visual, auditory, and somatosensory maps of the neighborhood. Suitable activation of neurons within this map triggers appropriate behaviors, such as attacking prey, fleeing a predator, or chirping a greeting to a potential mate.
Weakly electric fish run into a potential problem, though, if others of the same species are nearby: they can jam each other’s signals. To avoid this problem, each fish raises or lowers its own frequency to make it differ more from the neighbor’s frequency (see photo). This requires that they know whether the neighbor is using a higher or lower frequency. How the brain does this was largely worked out in the lab of Walter Heiligenberg and is now perhaps the best understood vertebrate brain circuit for any behavior.
The individual brain cells that detect the electrical interference by another fish are imprecise and unreliable. You can actually fool a fish by exposing only a small part of their skin to the electrical signal. But when these imprecise neurons act together as a large population—what Heiligenberg called a democracy of neurons—they reliably vote for the right answer, with a precision that is among the best of any animal and any kind of sensory perception.
It used to be thought that strongly electric fish, like electric eels, use high-voltage electricity only to stun prey, not for surveillance. But Kenneth Catania recently discovered that electric eels can also use high-voltage signals to find electrical conductors (like prey) in the surrounding water (video). They can even be fooled into striking a conductive carbon rod instead of a fish (video). In addition, the eels probe potential prey hiding places by shooting a quick pair of strong electrical pulses at the hideout. These pulses, like a remote control, directly activate all the prey’s muscles to contract maximally, triggering a strong twitch of the prey’s whole body and revealing the prey’s presence to the eel. This is followed by a sustained stream of strong electrical pulses that stun the prey, and dinner is served.