in a southerly direction, even though they are inside cages and don’t
get very far. Five months later they again exhibit restlessness, hop-
ping and attempting to fly in a northerly direction. Thus, these migra-
tory animals must have an endogenous circannual biological clock—
a physiological mechanism that tells them what time of the year it is
—as well as an inborn sense of knowing which direction to migrate at
which time of the year. In addition, they must have some mechanism
of obtaining information as to which direction is which, even inside a
closed room. This they accomplish by detecting the geomagnetic field,
using it as a source of directional compass information (see Chapter
11). Researchers have artificially manipulated the magnetic fields in
these experimental rooms and have shown that this messes with the
birds’ ability to exhibit migratory restlessness in the seasonally appro-
priate direction.
There are endogenous rhythms of other lengths, too. Reproductive
cycles in mammals are an example. Female hamsters have a cycle of
ovulation that is about four days long. In human females, the cycle of
ovulation is typically about twenty-eight days, although this varies
between women and even within the same woman.
Much of the study of biological rhythms has occurred using animals
in laboratories where the conditions of light and dark can be precisely
controlled. For example, one can work with rats or mice and monitor
their activity—things like how much they move around in their cages
or how much they run around in a wheel. A rat living in a cage under
conditions of a constant light-dark cycle—twelve hours of light and
twelve hours of dark—will be relatively active during the hours of
darkness and relatively inactive (mostly sleeping) during the hours of
light. Rats are nocturnal creatures, and in their natural habitat they
are most active during the night. Okay, nothing surprising so far.