Studies in activity. II. Activity rhythms; age and activity; activity after rest.

Abstract

In studying the daily activity cycle in rats, using the revolving drum apparatus, hourly records were taken on 31 animals over a 5-day period. The hr. of greatest activity during the 24 was that just before feeding, 12 m. to 1 p.m. During this hour the rats did 20 % of their total day''s running. Activity subsided to a very low level in the hr. after feeding, increased slightly in the early evening, fell to its lowest point between 5 and 6 a.m., and then gradually rose till the feeding hr. Thus, the daily cycle seems to be conditioned chiefly by hunger, but there is some indication that a nocturnal rhythm also exists. Correlations of +0.91 and +0.80 between the total day''s activity and activity in the hour before feeding and the feeding hour, respectively, indicate that activity during these hours is a very good index of the rat''s total activity. Activity records for 66 rats over varying intervals at ages of 2-14 mos. revealed that activity at 2 mos. averaged about 3000 revolutions a day. There was a gradual increase to 6000 revolutions at 6 mos., and the highest level, 7000 revolutions, was attained at 9 mos., after which activity decreased to the 2-mos. level. Intercorrelations between activity at 2 mos. and succeeding mos. up to 9 mos. vary around +0.30; at 4 mos. the correlations rise to +0.60, and at 6 mos. they vary around +0.80. The rat becomes more consistent in activity as he advances in age. The writer points out the danger of equating 2 groups of animals in activity at an early age and expecting them to maintain this equality as they grow older. The effect of enforced rest by confinement in small stationary cages was that activity after rest periods 1 and 2 days in length was about 25% greater than that when the rats ran continuously; but after rest periods 3 and 5 days in length the animals'' activity decreased. The more active animals seemed to be benefited by rest (i.e., they became more active); whereas the "lazy" animals were even lazier after rest. The cumulative effect of rest was to impair activity rather than to increase it.