Such adjustments were made in assigning an age 
to the rats in each of their 10 mm. class intervals 
The proportion of females visibly pregnant rose 
from zero at 64 days of age to about 0.434 at 245 
days of age then dropped to about 0.33 at 438 days 
of age. If we assume that r bears the same pro- 
portion to p in this Davis-Hall data as is shown for 
r and p in columns 4 and 5 of table 68, then S' 
attained the optimum value of 51 days between 
conceptions at approximately 1 50 and 450 days of 
age. Intervening between these two ages there is 
an increase in p beyond 0.33 and a corresponding 
decrease ef S below 51. Since a second maximum 
reproductive performance occurred at about 450 
days of age, the inception of the climacteric must 
have begun here, or possibly somewhat earlier. 
Between 1 50 and 450 days of age one must conclude 
that these females were physiologically and be- 
haviorally quite capable of achieving conception, 
but that some stressful conditions in the environ- 
ment upset the normal course of maternal behav- 
iors so that litters were lost shortly after parturition. 
During the climacteric when r=1.0, or at least 
is no longer increasing and p is below 0.33 and 
decreasing, it is only possible to derive maximum 
estimates of S if some of the litters are not reared 
to weaning. Where loss of litters, arising from 
incomplete expression of maternal behavior, occurs 
throughout the lifespan, one cannot calculate 
accurately the age of maximum reproductive 
performance or the inception of the climacteric 
when these are derived soley from estimates of r 
and p. Since S beyond this age will always be great- 
er than 51 days, P, the success of rearing litters, will 
have an unreal value of greater than 1.0 and one 
cannot gain any insight, based only upon r and p, 
as to whether the females are partially or entirely 
successful at rearing litters. 
In the above discussion I have taken considerable 
liberty with published data in estimating age from 
measurements and in arriving at estimates of 
pregnancy and parity rates Irom smoothed curves 
through observed data. Such approximations 
have enabled development of formulations of re- 
productive performance, as well as possible con- 
ditions influencing it. Though far from exact, I 
believe that these formulations approximate reality. 
If we are to approximate the reality of repro- 
ductive performance more closely, S and / 2 as well 
as r and p must be determined directly for the popu- 
lation studied. This can only be done where the 
individuals composing the population are followed 
even in more detail than I did for my Towson 
colony. Patently this is impossible in extensive 
studies of populations in their native habitat. 
However, the minimum effort demands simulta- 
neous determinations of p and r. Age can be approx- 
imated by assigning 74 days of age to the midpoint 
of the 10 mm. body length class interval when p 
first exceeds 0.01, and than assigning the ages 
shown in column 2 of table 68 to the other class 
intervals. 
The above discussions in conjuction with results 
from my Towson colony lead to the following 
generalizations. 
1. Norway rats attain anatomical sexual matu- 
rity by 85 days of age. 
2. The first conception may be delayed as much 
as 8 months. Why this is so is not entirely 
clear. The only rats in my Towson colony 
which did not conceive by nearly a year of 
age (tables 51 and 53) were members of 
socially low-ranking colonies. This suggests 
that amount of delay in conception is posi- 
tively correlated with amount of social 
disturbance or stress experienced by the rats. 
3. Once the ability to conceive has been attained, 
recurrent conceptions take place at a high 
rate. A threshold phenomenon is indicated. 
The extent to which hormones and matura- 
tion of behavior contribute to the threshold 
is not known. 
4. The effectiveness of maternal behaviors as 
judged by successful rearing of litters gradu- 
ally increases. This increase is of the order 
of tenfold between 3 and 12 months of age. 
5. Even during the mature adult span of 8 to 12 
months of age disturbing experiences may 
greatly decrease the effectiveness of maternal 
behavior. 
6. Somewhere between 8 and 14 months of age 
an increasing delay arises between termination 
of lactation and conceiving again. This is 
the climacteric which terminates at meno- 
pause somewhere around 2 years of age. 
7. The rate of maturation of reproductive per- 
formance as well as the duration of main- 
tenance of successful reproduction in rats may 
be taken as indices of the extent to which the 
environment provides stressful conditions. 
F. Manipulation of the Bond between Mother and 
Young. A general outline of maturational changes 
in relationships of young rats to their mother and 
adults in the naturalistic setting has been given on 
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