Exercise wheels and closed-circuit television were used to record the activity rhythms of 34 1. macrourus
and 32 P, nasuta (1) outdoors; (2) indoors with natural light; (3) indoors with artificial light in which
the phases of night and day were normal; (4) same as (3) but with the phases of night and day reversed;
(5) continuous dark; (6) continuous light. In tests (3)-(6) a faint red light was on continuously. These
tests were made on animals caged singly or in pairs and lasted for periods of from 8 to 357 days.
When a male and a female of different body weights were tested together, the animal using the wheel
was identified by its weight.
Five I. macrourus and 23 P. nasuta exposed to normal phases of night and day were active mainly
during the dark periods. The daily activity pattern varied considerably but was fairly constant for
individuals. When the phases of night and day were reversed, the animals continued to be active during
the dark periods and, in many of them, the pattern of activity was similar to that under normal phases
of night and day. They were, therefore, entrained to the artificial lighting system. However, in 17
out of 31 P. nasuta and 5 out of 31 1. macrourus exposed to reversed lighting conditions, activity started
later each day. When the trigger for exercise began to fall close to the beginning of a light period,
no activity was taken; the animals did not run in the wheel or take other exercise for 2-5 days, feeding
and drinking were greatly reduced and some animals did not leave the nest box for 1-2 days. After
this pause, exercise began again at the beginning of a dark period.
In 10 I. macrourus exposed to continuous dark the period of the endogenous rhythm was either
more than or less than 24 h, and in continuous light (four animals), the wheel-running activity was
greatly reduced.
Wheel-running activity rhythms in two inbred strains of laboratory rats under different photoperiods
