(A) The electric fields produced by two electric fish (red and blue lines; top) interfere to produce a beat pattern (brown and black lines; middle); the horizontal red and blue lines indicate that the frequency of each field does not change with time. If the red fish wants to communicate with the blue fish, it increases the frequency of the electric field it produces for a short time (red-green-red line; bottom). This "chirp" changes both the frequency and phase of the beat pattern (brown and black lines; bottom). Image from Metzen et al. (B) Computer simulation (computed using the dipole approximation) showing the electric fields produced by two electric fish. The fish on the left is able to detect the presence of the fish on the right because the latter changes the electric field in the vicinity of the former, and vice versa. The different colors represent different strengths and directions of the electric field.