Developmental age or period 
Age in days 
Remarks 
1 . First ovulation 
Circa 40 
After Davis { 28 ). 
2. Maturation of reproductive behav- 
ior. 
60 to 328 
a. Average interval between conceptions increases from 
23 to 51 days. 
b. Increasing frequency of successfully reared litters. 
3. Maximum reproductive perform- 
ance. 
328 
Most litters reared successfully. 
4. Climacteric 
328 to circa 580 
Average interval between conceptions greater than 51 
days. 
5. Menopause 
Circa 580 
Ages listed in the last four categories are for 
King’s data as shown in figure 152. The develop- 
mental age designated as maximum reproductive 
performance, when 5=51 days, represents the 
most important age for judging the relative inhib- 
itory effect exerted by environmental conditions. 
This follows if the inception of the climacteric 
indicates a suboptimum physiological state. To 
the extent that the logic of my argument to this 
point is correct, the earlier origin of the climacteric 
in King’s rats strongly suggests that the cage 
habitat at the Wistar Institute represented a less 
PREGNANCY IN NORWAY GRAY RATS 
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Ld 
oc 
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CQ 
c n 
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CO 
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a 
AGE IN DAYS 
Figure 1 51 . — Pregnancy in gray Norway rats. An inference from these data is that the stressful conditions of captivity cause 
an earlier origin of the climacteric and menopause. 
271 
