primary means of detecting its location. Electroreceptive structures,
called ampullae of Lorenzini, are densely dispersed over the shark’s
head.
The platypus, an egg-laying monotreme mammal native to
Australia, uses electroreceptors located in its bill to probe the muddy
bottoms of aquatic environments in search of bioelectric fields gen-
erated by prey animals, such as small crustaceans and mollusks. The
platypus, together with its relative the echidna, are the only mammals
known to use this form of sensory perception.
Electroreception is also used by various fish living in the murky
waters of rivers like the Amazon in South America. These fish also
have structures that generate oscillating electric fields (stronger
than the bioelectric fields produced by all living organisms) that then
propagate into the surrounding environment. Some of these oscillat-
ing fields are used to communicate with other members of the same
species. Different species of electric fish are distinguished by fields
having specific frequencies and patterns. The electric fields generated
by the fish are also altered by the surrounding terrain, allowing the
fish to sense the presence of nearby animals and obstacles in a manner
somewhat akin to echolocation—a kind of electrolocation.
Yet another form of sensory perception different from anything cur-
rently known to exist in humans is the sensing of magnetic fields, in
particular the magnetic field produced by Earth. The Earth’s geomag-
netic field is generated by large-scale movement of magnetic atoms
in the molten interior of the planet. The intensity of the geomagnetic
field varies over the surface of the planet—strongest near the north
and south magnetic poles and weakest near the equator (Fig. 11.7).
The direction of the magnetic field vector points toward the poles and
becomes more steeply inclined nearer to the poles.