Four-thousand seven hundred and five infested hay 
ricks examined between January and June gave a 
total of 46,126 rats, weanlings or older, or an 
average of 9.8 rats per rick. Unfortunately, they 
do not provide statistics on the variability of number 
of rats per rick. However, in 32 subsamples, each 
of which contained 50 or more ricks, the average 
varied between 6.7 and 15.2 rats per rick. The 15 
densest populations averaged about 100 rats per 
rick. This indicates a skewness of the distribution 
such that the more typical density was probably 
nine or less. Since these unthreshed ricks repre- 
sented a concentrated source of both food and 
harborage, it is apparent that rats do exercise 
control of group size toward an optimum level 
which approximates that of the preweaning litter 
size. For these rick-inhabiting rats Leslie, et al 
(60) list the mean number of embryos found in 225 
pregnant females as 8.65 and the mean number 
per litter in the nest as 7.31 for 85 nests. A 
similar number of young per litter was found by 
Emlen and Davis (22) from a rowhouse residential 
area in Baltimore. They found an average of 9.0 
embryos during the latter one-fourth of pregnancy. 
D. Reinterpretation of the Wistar Institute Studies on 
Captive Gray Norway Rats. Beginning in 1919 the 
Wistar Institute initiated a colony of gray Norway 
rats stemming from a small group of wild caught 
individuals. Certain of the papers (61, 62, 63, 64) 
concerning the first 25 generations are of particular 
import to the present discussion. Knowledge of 
the biology of free ranging rats revealed by my 
present study, and the increase in knowledge of 
the interrelationships between stress, endocrine 
function, growth, and reproduction arising since 
King’s publications provide the basis for gaining 
further insight into the biology of the Norway rat 
through examining the data presented by King. 
a. Methods of rearing and selection: Only 6 of the 20 
wild caught females bore litters in captivity. Their 
young were reared by domesticated albino mothers, 
unce the change to captivity so disturbed the wild 
females that they destroyed their young when left 
with them. During the 2d to 25th generation the 
grays reared their own young even though con- 
siderable mortality took place during the suckling 
period through the first few generations. Those 
rats selected for breeding and further stud es on 
growth, etc. had to meet the criteria of member- 
ship in medium sized litters (4 to 7 per litter) in 
which no members died from birth to 60 days of 
age. Males from one litter were placed in a large 
cage (10 by 10 by 57 inches) with females from a 
nonclosely related litter. Sometimes males from at 
least two litters were placed with females from at 
least two other litters. Nesting material was placed 
in the cage, which the rats used to build nests. 
These groups were left intact throughout their life 
span. Litters were conceived, born, and reared in 
the presence of these several adults of both sexes. 
This pattern of breeding was maintained through 
generations 2 to 20. Brother-sister inbreeding was 
practiced through generations 21 to 25. We shall 
primarily be concerned with what hereditary 
selection was taking place during generations 2 to 
20. It is quite apparent that the program of 
breeding minimized random gene drift. Selection, 
though this was not apparent to King and Donald- 
son, was directed toward greater tolerance to the 
disturbances accompanying confinement. 
For initial generations the behavior upon 
approach of a worker was for the rats to become 
highly excited, dash about the cage, dig down and 
hide in the nesting material, and even to jump out 
at the observer when the cage door was opened. 
If the rat escaped, it ran violently about the cage 
upon capture and return, and sometimes died 
very shortly after return to the cage (see p. 259 
for another type of traumatic death). Within the 
cage considerable fighting occurred. The largest 
males frequently bit and sometimes killed his 
smaller brothers and other younger males ap- 
proaching adulthood. Some fighting also occurred 
among females. The suboridnates were frequently 
forced to cower together at one end of the cage. 
Because of the inability to escape, I believe that 
gray Norway rats housed under these conditions 
are under much greater stress from social inter- 
action, than those living under the feral state. 
One would therefore anticipate much more vari- 
ability due to environmental causes among those 
caged gray rats. The young during foetal, or 
neonatal life, as well as during adolescence, were 
subjected to the strife engendered among their 
elders. Death prior to weaning, according to 
King, resulted from (a) the disturbed mother eating 
the young or (b) other adults crowding in the nest 
and smothering the young when the approach of 
workers disturbed these adults. 
Members of such litters, a part of whom died 
from this and other causes prior to 60 days of age, 
were never used for further breeding. Therefore, 
one would anticipate that selection toward greater 
tolerance to crowded group life and to the dis- 
262 
