Electroreception is the ability of animals to perceive electrical stimuli. It mainly is seen in sea life, as salt water is a much better conductor of electricity than fresh water or air. It can be used for both electrolocation, and electrocommunication.
One of the key examples of electroreception in the animal kingdom can be seen in sharks. Sharks (and other cartilaginous fish) may have special sensory organs known as Ampullae of Lorenzini. These electroreceptors exist within the lateral lines, in jelly-filled pores. These so-called ampullae pores are visible to the eye as dark spots, with positioning varying species to species. The general way in which the ampullae work is through detecting the potential difference between the voltage of the skin pore, and the voltage at the electroreceptor cells. It is hypothesised that sharks may be more sensitive to electric fields than any other animal, being able to detect changes of as little as 5nV/cm. This is potentially sufficient for sharks to detect the electrical impulses controlling the muscles within prey animals. However, in tests it was discovered that paralysed prey was still able to draw a feeding response from sharks. Sharks are also able to detect prey animals lying sedentary under sand. Another theory is that sharks are able to detect the electrical fields generated by the ion pumps of fish, used for osmoregulation (the ability of the fish to maintain the osmotic of its bodily fluids).
Another use of the Ampullae of Lorenzini within sharks is navigation. The moving conductor surrounding the shark, i.e. the sea, induces an electric field when it moves through a magnetic field, such as the one generated by the Earth. These induced electrical fields are very small, although sufficient for a shark’s ampullae to detect. Tests have shown that sharks do react to the presence of a magnetic field within their tanks. It is hypothesised that sharks may be able to make use of the oceanic electrical currents to orient themselves to the water currents, or for general oceanic navigation. It is also possible that the electrical field generated by the sharks themselves may allow for sharks to sense their magnetic heading, via its interaction with the Earth’s magnetic field. It has been suggested that hammerhead sharks, most notably the winghead shark, may have developed their characteristically wide heads (cephalofoils) to provide a larger surface area for the lateral lines and Ampullae of Lenzini.
The type of electroreception used by the aforementioned sharks can be considered as passive electrolocation, meaning that the ampullae passively detect the bioelectric fields generated by other animals. Another type of electrolocation is active electrolocation. In active electrolocation, an animal generates its own electric fields through a specialised electric organ. It then detects the interference in this field caused by its surroundings via electroreceptors. These fields are modulated so as to have differing frequency and waveform between species, and occasionally even individuals. Some species of weakly electrical fish are even capable of performing a jamming avoidance response. When two weakly electrical fish meet with very similar discharge frequencies, each fish will increase their discharge frequency to avoid accidental jamming. Fish may either discharge electrical fields in small discrete electrical pulses, or via a continuous quasi-sinusoidal wave discharges. The different resistance/capacitances of various objects can allow for electroreceptive fish to identify surrounding objects, although objects with similar levels of electrical impedance to water are undetectable. The range of the active electrolocation systems in fish tends to be about one body length.
Interestingly, electroreception can be seen in some land based mammals, namely the monotremes (meaning “single hole”, referring to the cloaca). This group involves animals such as the platypus, and the echidna. As monotremes do not have lateral lines, their mechanoreceptors consist of free nerve endings located along the mucous glands of the snout. The duck-billed platypus has the most sensitive electrical sense of all the monotremes, with about 40,000 electroreceptors being arranged in stripes along its bill. When the platypus swims, it makes short sweeping head movements much like those used by the hammerhead shark. These movements are known as saccades. These movements constantly expose the most sensitive parts of the bill, allowing for better prey detection. The electrosensory systems of the land based monotremes are much more basic. For example, the Western long-beaked echidna only has around 2,000 electroreceptors. These echidnas feed on earthworms in damp leaf litter, meaning that electroreception is useful in the detection of prey. In these echidnas and others, it seems that moisture in substrate and high humidity in the air is necessary for the transmission of meaningful electrical signals.
It seems that this methodology may not be appropriate for UAVs, as it requires the presence of moisture.