Examples of failure of each of these eight behav- 
iors were noted in my Towson colony. Similar 
failures must characterize other populations of wild 
rats as evinced by the gradual increase of success at 
rearing litters revealed in column 9, table 68. 
How much of this failure results from inadequate 
learning of the appropriate behaviors, and how 
much is a consequence of stressful conditions which 
so disturb physiology as to prevent expressing 
"innate” behaviors cannot be stated. Suffice it 
to say, the maturation of maternal behaviors is 
slow. 
Thus, environmental conditions retard the ex- 
pression of both mating and maternal behaviors. 
The gradual attainment of full expression of both 
of these essential categories of behavior is even 
slower than either separately, column 10, table 68. 
Not until the fully mature age of 438 days are all 
rats parous and the frequency of conception has 
been reduced to the point where all litters conceived 
have the opportunity of being reared successfully. 
Remarkably enough, at this age, the span between 
litters comes out to be 51 days, which is exactly the 
sum of the 23 days of pregnancy and 28 minimal 
days of lactation determined independently from 
my Towson population. 
Whenever p exceeds .333, regardless of the value 
of r, one is justified in concluding incomplete ex- 
pression of maternal behavior. A p greater than 
.333 means that litters are lost before weaning and 
conception occurs less than 28 days after par- 
turition. Similarly if p is less than .333 and r is less 
than 1 .0 one is justified in concluding incomplete 
expression of both mating and maternal behavior. 
In either of the above cases S will be less than 51 
days and /, greater than 7.17 per year due to 
recurrent conceptions closely following loss of litters. 
Further insight into this general question may be 
derived from data on captive wild gray Norway 
rats presented by King (63). Nine-hundred and 
seventy-four of the 1,323 females alive at 3 months 
of age survived to 20 months of age. King lists 
the number of litters born to those rats surviving 
during each month of age. 
Let L = proportion of females having litters 
each 30 days. Then p, the proportion of females 
visibly pregnant, can be estimated to be: =17L/30 
or .567/. 
See figure 151 and table 69. Note the decline 
in pregnant females beginning at 9 months of age. 
This Is in marked contrast with the pregnancy rate 
of rats in their native habitat, which continues to 
increase to 450 days of age at least (fig. 151). 
Inferences drawn from this difference should 
provide further insight into the biology of rats. 
As the rate of pregnancy (p) declines, the span (.S') 
between conceptions must increase. King prob- 
ably had exact, but unpublished, data on S, and 
also on r, the proporiton of parous females. How- 
ever, we can make a crude estimate of both r and S 
which shall be designated as r and S. Because 
of reduced fertility in generations 2 to 9 only 
approximately 0.92 of the 1,323 females in the total 
sample reached parity by 20 months of age. This 
fixes the upper limit of f at 0.92 at 600 days. Under 
240 days of age the King rats had a higher p and 
therefore probably had a higher f than the Davis- 
Emlen rats. I therefore assumed r= T 4 * when 
A 
r 4 and p b refer to the r and p values in columns 4 and 
5 of table 68. These r’s when plotted against age 
and connected to the r of 0.92 at 600 days enabled 
an estimate of the intervening f’s. S was then 
calculated (table 69; fig. 152). The age of maxi- 
Table 69. — Estimated reproductive statistics applicable 
to King’s (1939) captive gray Norway rats 
Age in days 
A 
P 
A 
r 
A 
S 
74 
0. 034 
0. 051 
25.6 
88 
. 096 
. 151 
26. 8 
Ill 
. 158 
.268 
28. 6 
144 
. 210 
. 411 
33. 3 
186 
. 249 
. 572 
39. 1 
238 
. 269 
. 690 
43.6 
298 
. 254 
. 720 
48. 1 
366 
. 245 
. 767 
53.2 
450 
. 225 
. 830 
62. 7 
580 
. 140 
. 905 
140. 0 
mum reproductive performance was reached at 
least four months earlier by the King rats than by 
the Davis-Emlen rats. Beyond the age of maxi- 
mum reproductive performance S continued to 
lengthen. Thus, whether or not all litters were 
reared, it is quite apparent that some change in the 
psychology or physiology prolonged the time of 
conception beyond the 28th day of lactation. This 
period of declining reproductive performance must 
represent the climacteric. The life cycle of repro- 
duction from the viewpoint of the average female 
in the population consists of three specific ages and 
two intervening periods of change: 
270 
