would have been weaned during the entire 1950 
breeding season. Despite the crudeness of this 
estimate, it is certainly unlikely that there would 
ever be over 200 adults at the beginning of any 
future breeding season. 
Furthermore, as the colony aged there was a 
changing sex ratio toward a greater proportion of 
females as a result of the higher mortality of males 
(fig. 147). During 1947 16 males and 12 females 
were weaned. Half of these females but none of the 
males survived to May 1949. During 1948 104 
males and 65 females were weaned. Sex ratio 
prior to weaning was unknown. Why there were 
so many more males than females weaned is also 
unknown. Rats born in 1949 which had been 
weaned or were near weaning age by May 17 were 
sexed; there were 57 males and 63 females. By 
1950 there should have been both an actual and 
relative preponderance of females. 
The number, 200 rats, is particularly instructive 
when we compare it with the number of rats that 
might have been raised in the available space had 
each individual been isolated as a juvenile into 2 
square feet of cage space, as is customarily done in 
the laboratory. Under such laboratory conditions 
5,000 healthy rats might have been reared in 10,000 
square feet of space instead of the 200 which 
utilized such space under free-ranging conditions. 
This figure of 5,000 rats is actually a conservative 
one in regard to representing the biotic potential 
expected from this free-ranging colony. The 
studies of Emlen and Davis (22) and Davis ( 23 ), 
supplemented by my own observations, indicate the 
following conditions for determining the poten- 
tial reproduction, were other limiting factors not 
in operation: (1) eight per weaned litter with 
equal sex ratio; (2) first litter by 5 months of age; 
(3) one litter every 2 months; (4) no beeding 
during the 4 midwinter months; (5) all rats born 
the first breeding season should be dead by the end 
of the third; (6) it is within the potential lifespan 
of a rat for all which were born in the pen to have 
been alive at the end of the experiment; (7) the 
study lasted from March 1947 to June 1949. 
With these conditions as a basis for judgment, 
50,000 descendants from the original 5 females 
might have been alive in June 1949. Nevertheless 
in the comparison above, it is believed that the 
figure 5,000 is a more realistic one in indicating 
the potential density of 10,000 square feet, although 
it is conceivable that 50,000 healthy rats could be 
maintained in a similar space by confining each to 
a cage somewhat less than 8 inches square. What, 
then, was the cause of this 25-fold decrease in 
utilization of space under naturalistic conditions? 
The obvious explanation is that under free-ranging 
conditions the rats expressed genetically determined 
and culturally modified behaviorial potentialities 
which would be impossible under caged conditions. 
This explanation has broad implications. When- 
ever the density of a population becomes increased 
beyond that level to which the heredity-to-enviro- 
ment relationship provides optimum adjustment, 
then the individual and the group must forfeit some 
AGE IN DAYS 
Figure 147. — Survivorship curves. 
245 
