Scientific American 2019-04

April 2019, ScientificAmerican.com 67

work showed that the eels were, in fact, attacking in response to

doublet-generated fish movements.

Thus, the electric eel has two modes of remote control, which to-

gether make for one of the most insidious hunting tactics in the an-

imal kingdom: it can unmask hidden prey by making them move,

and it can freeze moving prey once they have been discovered.

DOUBLING DOWN

remotely controlling another animal is pretty cool, but it is not

the eel’s only trick. The creature also has an ingenious solution to

a fundamental problem with its electric output. Unlike superhe-

roes or wizards who can aim lightning bolts, every time the eel

gives off a high-voltage pulse, the electricity is distributed

throughout the surrounding water. As a result, only a tiny fraction

of the eel’s prodigious power is transmitted to prey. English phys-

icist and chemist Michael Faraday, who coincidently worked with

electric eels in 1838, gave us a convenient way to visualize the

problem: The eel’s electric field is a so-called dipole, with lines

representing forces on a positive charge emanating from the pos-

itive head of the eel and ending on the negative tail. The density of

lines reflects the strength of the electric field at any given point; it

is strongest at the poles and falls off in strength rapidly with dis-

tance. In introductory physics, you learn that bringing a negative

pole close to the positive pole greatly increases the field strength

in between. Eels have apparently taken physics because they use

this move on difficult, struggling prey. The eel holds the victim

firmly in its jaws and curls its tail (the negative pole) around the

animal before delivering a series of high-voltage volleys.

To measure the effect of the eel’s maneuver, I designed an eel

“chew toy”—a pair of recording electrodes on a plastic holder in-

side a dead fish. The eels grabbed the apparatus, and I shook the

attaching wires to simulate struggling. The eels obliged, curling

around and shocking the electrodes. As expected, the field

strength more than doubled. It is a great strategy, allowing the

eel to concentrate its otherwise fixed power output on a target,

like focusing the fixed power of a flashlight to a single bright spot.

What happened to prey was predictable yet awe-inspiring.

Subsequent experiments showed that the eel’s amped-up at tack

causes muscle contractions at abnormally high rates, totally and

utterly exhausting prey in just a few seconds. It is the electric an-

alogue of a neurotoxin, allowing the eel to capture and subdue

otherwise dangerous animals, such as large, clawed crayfish.

MORE THAN A WEAPON

during my studies of the eel’s hunting behavior, I noticed some-

thing that made me wonder whether the shocks might function

as more than just a weapon. Typically three things happen when

electric eels go in for the kill. First, they give off a full volley of

high-voltage pulses, then they rapidly strike at the prey and suck

it into their mouth. But in my experiments, when the dead fish

was made to twitch in the insulated plastic bag, the eel’s attack

was always cut short. The eel gave off the high-voltage volley and

struck toward the fish but missed and aborted the attack with-

out the final suction-feeding bite. Why?

I had assumed the eels’ strike was ballistic—a preplanned

event that takes place without sensory feedback. But now it oc-

curred to me that the animals might use high-voltage pulses as

a tracking system. This would explain why they overlooked prey

insulated in plastic. Electric eels evolved from weakly electric

(^1234)

The closer the positive

and the negative, the

stronger the field

Voltage

experienced

by prey

High-voltage

doublet

Involuntary

prey movement

High-voltage

attack volley

Low-voltage

electroreception

Eel’s Voltage Output^500
volts

1 2 3 4

Stages of a Kill

Eel uses both low- and high-voltage electric output to sense its

surroundings ● 1. While searching for prey hidden among plants,

the eel gives off pairs of high-voltage pulses called doublets that

cause a powerful twitch in nearby prey ● 2. The twitch displaces the

surrounding water, revealing the prey to the eel ●^3. On finding a target,

the eel launches a high-voltage attack volley that paralyzes the prey,

which the eel then strikes at with a suction-feeding bite ● 4.