In passive electrolocation, the animal senses the weak bioelectric fields generated by other animals and uses it to locate them. These electric fields are generated by all animals due to the activity of their nerves and muscles. A second source of electric fields in fish is the ion pumps associated with osmoregulation at the gill membrane. This field is modulated by the opening and closing of the mouth and gill slits. Many fish that prey on electrogenic fish use the discharges of their prey to detect them. This behavior of a species collecting information from the communicative signals of a different species is referred to as "Eavesdropping" and has been observed in the electroreceptive African sharptooth catfish ("Clarias gariepinus") whilst hunting weakly electric Marcusenius macrolepidotus.This has driven the prey to evolve more complex or higher frequency signals that are harder to detect.