single major activity period lasting from 7 p.m. 
to 3 a.m. The existence of a single noctur- 
nal peak of activity has been since confirmed by 
many workers (Richter, 1922, 1927; Shirley, 
1928; Holmgren, 1936; Browman, 1937, 1942; 
Hemmingsen and Krarup, 1937; Herring and 
Brody, 1938; Levinson et al., 1938). The devel- 
opment and modification of this nocturnal pattern 
seems to be an attribute of age (Slonaker, 1907, 
1912; Richter, 1922, 1927). At the time of wean- 
ing short activity periods are equally distributed 
over the entire 24-hour period but is concentrated 
in the hours of 7 p.m. to 3 a.m. The accentua- 
tion of nocturnalism is continued through the 
seventh month so that by the eighth month prac- 
tically all activity is confined to the night hours.” 
“This marked nocturnalism continues through 
the 11th month, but after this time there is a 
general leveling of the activity curve and a shift- 
ing of the major period of activity so that by the 
21st month it occurs between 3 p.m. and 3 a.m. 
The loss of rhythmicity is accentuated from the 
24th month until shortly before death, when acti- 
vity is nearly completely arrhythmic. During 
old age, the periods of activity are of short dura- 
tion and are separated by long periods of rest. 
As the white rat assumes its adult nocturnal acti- 
vity pattern, it also reaches its peak in the total 
amount of activity expressed per day. The age 
of assumption of maximum activity has been vari- 
ously calculated at 75 to 105 days (Browman, 
1942); 87 to 120 days (Slonaker, 1907); 175 days 
(Richter, 1922); 210 days (Shirley, 1928) 240 
to 270 days (Slonaker, 1912); 270 to 390 days 
(Slonaker, 1926). Despite the variations in results, 
they mostly fall within the period of maximum 
nocturnalism and have been preceded by the 
assumption of the ability of the muscle to do 
maximum amounts of work at the age of 90 days 
(Steinhaus, 1941). According to Anderson and 
Smith (1927), rats stunted by insufficient diet do 
not show the change in intensity of activity with 
age shown by normal control rats.” 
During Period 1, November 1947, when the 
population was low, 26 rats, there was little evidence 
of nocturnal variability of activity between 6 p.m. 
and midnight (fig. 81). The absence of the marked 
bimodal nocturnality observed during periods 2 
and 3 may be attributable to the fact that the 
majority of the population consisted of rats no 
more than 3 months of age. According to the 
experimental studies cited above rats of this age 
have not yet developed the complete adult rhythm 
of nocturnal activity. 
Periods 2, December 1947 (fig. 82) and 3, 
January 1948 (fig. 83) may be considered as repre- 
sentative of the rhythm which mature rats (4 and 
5 months and over) exhibit in the absence of the 
increased competition which accompanies sexual 
activity and increased population density. The 
general pattern for each of these periods is that of a 
bimodal nocturnal rhythm with the premidnight 
period being the one of greater intensity of activity. 
Such bimodal activity rhythms follow the typical 
pattern exhibited by most animals which possess 
an endogenous 24-hour rhythm of activity, regard- 
less of whether it is diurnal or nocturnal (13). It 
is suspected that the unimodal nocturnal rhythm 
typical of laboratory rats is in part dependent upon 
their domestication and living in isolation. How- 
ever, the expression of a unimodal peak of activity 
appears to be determined mainly by the periodic 
replenishment of their source of food and water. 
This latter has been shown by Richter (14) and 
Shirley (15) to result in a marked increase in 
activity, during the several hours just preceding 
feeding. 
With regard to the day to day variability in 
rhythms of activity, the rhythms for each of the 
two separate and consecutive days for each of 
these periods reveal little in common as to the 
exact timing of local periods of intensified activity. 
One must conclude that, whereas there is a basic 
underlying endogenous rhythm, local and tempo- 
rary biological or meteorological events may 
modify its exact expression. Similar variability of 
the day to day activity from that of the general 
pattern is revealed by the pairs of days diagramed 
for each of the following seven periods. 
During Period 4, February 20 to March 7, 1948, 
there was an abrupt shift in the character of the 
activity rhythm (fig. 84). Although the population 
was low, 24 rats, this was a time of heightened 
sexual activity. Several females conceived during 
this time. Two factors regarding sexual activity 
contributed to the greater tendency toward noc- 
turnal arrhythmicity during Period 4 in contrast 
to the midwinter striking nocturnal bimodality of 
Period 2 and 3. On those nights when females are 
in estrous several males continuously follow her 
throughout the night. Although some of the males 
may temporarily leave her to go into the Food Pen 
the others give her little opportunity to rest and 
also follow her when she goes into the Food Pen 
113 
