of vibration, even a spot of blue sky can disclose the sun’s position by its
polarization pattern. Are bees endowed with this capacity?Von Frisch went on to do experiments with polarizing filters to
conclusively demonstrate that honeybees use sunlight polarization
patterns as a navigational aid, another extraordinary discovery.
(Perhaps von Frisch’s most extraordinary discovery was the dance lan-
guage of honeybees, alluded to in the paragraph above, through which
bees communicate the location of food sources to their hivemates.)
Many insects and other animals have since been found to be sensitive
to skylight polarization and to use it as a source of navigational infor-
mation. This could be very helpful, for example, to a beetle crawling
in a crevice between large boulders and only able to see a tiny piece of
blue sky above.
Ultraviolet, infrared, and polarization are aspects of electromagnetic
radiation to which humans lack sensitivity. Other animals have
evolved capacities to detect these electromagnetic phenomena and
use them in ways that are useful for them. Similar differences occur
for sound. The human ear and auditory system (see Chapter 15) are
exquisitely sensitive and sophisticated and, as with vision, are also
limited in their sensitivity. In terms of frequency of mechanical vi-
bration, we humans are capable of hearing within the approximate
range of 20 to 20,000 cycles of vibration per second. The unit of cycles
per second is called a hertz (Hz), after Heinrich Hertz (1857-1894),a
physicist who made contributions to the study of vibrating electro-
magnetic fields.
Very low-frequency sound, with frequencies less than 20 Hz, is
referred to as infrasound. Though inaudible to us, elephants can both
generate and hear infrasonic frequencies and use infrasonic calls in