their social communication.
Very high-frequency sound, called ultrasound, is inaudible to us
but audible to many other animals. Dogs and cats can hear sounds
having frequencies of 40,000 Hz and higher. These animals are not
known to use ultrasound in their communication; however, ultra-
sound perception is useful to them in their hunting, as many rodents
communicate using high-frequency sounds.
Bats, dolphins, and whales emit high-frequency sounds and then
hear the sound’s reflection (echo) from nearby objects, including, in
the case of bats, insects. Using this echolocation, or biological sonar
(sound navigation and ranging), these animals can navigate and hunt
in cases of poor visibility, even total darkness. Ultrasonic frequencies
higher than 100,000 Hz are often used by bats because the very short
wavelengths associated with very high frequencies permit discrim-
ination of a very fine degree of detail (spatial acuity), useful when
trying to locate a tiny insect when both the bat and the bug are flying
through the air—really quite astounding! Some insects, such as some
moths, also can hear ultrasound, allowing them to sense the approach
of a bat about to eat them and take evasive action in their flight.
Akind of sensory perception that is unlike anything currently known
to exist in humans is electroreception—the detection of electric fields
generated by living organisms. Every living creature is surrounded
by electric fields, produced as a result of the movement of charged
ions within the organism. A classic experiment demonstrating the
existence of electroreception was conducted with sharks. Some sharks
are capable of locating fish that are well camouflaged on the ocean
bottom, and it was generally believed that the shark must be locating
the hidden fish by smell. While odor may indeed be helpful, it is now
known that sharks use the electric field generated by the fish as the