ACTIVITY ACTIVITY 
PERIOD 9 
PERIOD 10 
Figure 89. — Activity period 9,- February-March 1949; 127 
adult rats present in the pen,- this is the time of the initial 
sexual activity of the breeding season. 
Figure 90. — Activity period 10; March-April 1949; 126 
adult rats in pen,- juveniles were not yet exploring as far as 
the Food Pen. 
and attempts to eat or drink. In addition, there 
is a heightened aggressiveness among males be- 
ginning with the descent of their testes and the 
onset of the breeding season. This aggressiveness 
is accentuated at the time a female is in estrous. 
Whenever aggressiveness is accentuated, there is a 
greater likelihood of a rat being repelled by a more 
dominant individual when it attempts to enter the 
Food Pen and thus it must attempt reentry at a 
later period during the night. These relationships 
are discussed in more detail later in the paper 
(pp. 183 to 184), but mention of them here is 
requisite to an understanding of the shift in the 
activity periodism toward increased arrhythmicity. 
Despite this trend toward nocturnal arrhythmicity 
the premidnight activity is still accentuated. 
Periods 5, March to April 1948 (fig. 85), 6 
May 1948 (fig. 86), and 7, June to July 1948 
(fig. 87) may each be considered as representing 
different examples of the same set of conditions 
which modify periodicity of activity. In addition 
to continued sexual activity there are three addi- 
tional factors which contribute to the trend toward 
increased arrhythmicity of the population. The 
storage of food by lactating females becomes 
markedly increased. This affects the ease with 
which other rats gain access to the Food Pen. 
And finally, during most of these three periods 
over half the population consisted of recently 
weaned rats, whom presumably all had the ar- 
rhythmicity of activity demonstrated for juvenile 
laboratory rats. 
Unfortunately the activity recording machine 
was not in operating condition from August 1948 
through January 1949, when the large group of 
rats born during 1948 were attaining sexual 
maturity. Fortunately the machine was back in 
operation during February 1949, the month 
before the beginning of the 1949 breeding season. 
Period 8 (fig. 88) thus represents a time with a 
high population density, 135 rats, of mature but 
sexually inactive rats. Because of the later in- 
ception of sexual activity during 1949 (see fig. 
94), Period 8 in 1949 is most nearly comparable 
to Period 3 during 1948, insofar as the physio- 
logical condition and the age of the majority of 
the rats is concerned. In comparing these two 
periods it will be noted that there is a greater 
degree of arrhythmicity of nocturnal activity 
during 1949. Even though both groups of rats 
were essentially sexually inactive, the presence of 
135 rats in 1949 in contrast with only 25 in 1948 
produced more competitive action between in- 
dividuals for entry into the Food Pen and thus 
forced a greater proportion of individuals to 
enter the Food Pen later during the night. 
The inception of heightened sexual activity 
during 1949 is represented by Period 9 (fig. 89), 
116 
