= DAYS INTERVENING BETWEEN CONCEPTIONS 
LIFE CYCLE OF REPRODUCTION IN CAPTIVE GRAYS 
(KING, 1939) 
AGE IN DAYS 
Figure 152 — Life cycle of reproduction. Maturation and decline of reproductive performance result in an ever increasing 
average span between conceptions in so long as the proportion of visibly pregnant females increases to not more than 
0 333 and then declines S = 5 1 days at maximum performance. When the proportion of rats visibly pregnant exceeds 
0.333, an intervening decline in the span between conceptions will result. 
optimum environment than that experienced by 
urban out-door rat', in Baltimore. This conclusion 
corroborates a similar one (pp. 262 to 264) derived 
from the slower growth of these caged rats than 
those in the Towson study. 
Size of litters of King’s captive grays followed a 
similar pattern of change with age as did pregnancy 
rates Tig. 151). Litter size increased from 3 to 8 
months, with the maximum at 8 months; a gradual 
decline characterized the 8- to 17-month age 
range, which was followed by a rapid decline in 
size of litters between 17 and 20 months of age. 
There exists one additional change in the pattern 
of the life cycle of reproduction as revealed by data 
presented by Davis and Hall ( 68 ) for rats living in 
warehouses in Baltimore. Judging by the fact 
that the median size of the 409 females was 20 mm. 
less than those living out of doors ( 66 ). 1 suspect 
that their 155 mm. rats were actually about 64 
days old; the 165 mm. rats were 74 days old, etc. 
272 
