DEVELOPMENT OF ARRHYTHMICITY BY THE COLONY 
Figure 97. — Development of arrhythmicity of activity by the 
colony. As the number of rats increased, the variability 
in the amount of activity among comparable intervals of 
time decreased. 
greater for comparable seasons of the two years as 
the progression is made from (1) midwinter reduced 
sexual activity through (2) inception of sexual 
activity, to (3) addition of juveniles to the popula- 
tion. This is in line with the evidence that the 
more factors there are which exert stress upon the 
social relationships between members of the popu- 
lation, the greater will be the arrhythmicity of 
nocturnal activity. 
C. The Relationship of Temperature and Barometri c 
Pressure to Intensity of Activity. From the previous 
analyses it is apparent that there are many factors 
which influence the intensity of activity. These 
make it difficult to utilize absolute indices of 
activity for the investigation of the role of meteoro- 
logical conditions upon activity. The most valid 
comparison is that between two adjoining days 
since conditions other than the one being investi- 
gated are most likely to be similar. Utilizing this 
concept of paired days, an index of the effect of 
temperature upon activity was developed as 
follows: 
UD/HD-r-TU Relative amount of change in 
activity per 1° change in temperature. 
Where: 
UD= Difference in activity units between the pair 
of days. 
//Z)=Activity units on day of higher activity. 
TD— Difference in temperature between the pair 
of days. 
Sign of index: 
+ = Increase in activity with decrease in tempera- 
ture. 
— ^Decrease in activity with decrease in tem- 
perature. 
The results for 96 pair of days are shown in figure 
98. It is apparent from the wide spread of points 
that temperature can be only one of several factors 
affecting intensity of activity. Nevertheless, the 
trend exhibited by the median activity changes 
certainly suggests that temperature does exert an 
important effect. Above 61° F. an increase in 
temperature exerts an inhibitory effect, and below 
61° F. a decrease in temperature exerts an inhibi- 
tory effect. Below 48° F. there is apparently a 
rather constant effect of temperature in exerting a 
2.25-percent reduction in activity with every de- 
gree drop in temperature. Since changes of 
5° in the mean nightly temperature between 
adjoining nights were common below 48° F., 
temperature exerted an influence, which if operat- 
ing alone would have caused approximately a 10- 
percent change in intensity of activity. Other 
factors influencing intensity of activity were 
sufficient either to accentuate greatly or to override 
completely the effect of temperature. 
The influence of temperature upon absolute 
activity was also investigated, although it might be 
expected that there would be considerable more 
variability in the results. The total number of 
passages per rat per night into and out of the Food 
Pen was taken as this index of activity, and the 
influence of temperature upon it is shown in table 
23. Although the variability among the means of 
activity within the seven temperature class intervals 
is significantly well below the 1 percent level of 
confidence, the actual trend of the means only 
partially validates the conclusions arrived at in the 
previous analyses. About all that may definitely 
be said is that with reference to the temperature 
range, 30° to 60° F., a decrease in temperature 
generally produces a decrease in activity, while an 
increase in temperature usually brings about an 
increase in activity. 
If the mean number of activity units (25.13) per 
rat shown between 55.0 and 74.9° F., is an accurate 
126 
