g. Social rank and growth as factors influencing the 
prevalence of infections with microorganisms. It has 
already been shown that the probability of ill 
health and mortality, as given in table 64, is greater 
for males, who are also more likely to be exposed to 
fighting and other stressful conditions. The sample 
of autopsied females was examined to determine 
if prevalence of infection was associated with such 
indicators of stress as social rank and growth. 
Capillaria and Streptobacillus were grouped to- 
gether as organisms causing “social contact” dis- 
eases since their transmission is facilitated by one 
rat biting or eating another. Heterakis, Hymeno- 
lepsis and the condition of bronchiectasis were 
grouped together as “random contact” diseases, 
since all rats came more nearly having the same 
probability of exposure through water, food, or 
soil contaminated with the eggs or the bacteria. 
Fifty-three females were included in the total 
sample. An individual rat might have none or all 
of the infections. All of the rats were not examined 
for all five conditions. The total examinations 
were 197. Most of the records supplied by the 
Microbiological Institute were in terms of intensity 
of infection as well as presence and absence. For 
the purposes of this examination mild infections 
were listed in the “absent” category along with 
those individuals for whom no infections was 
detected. 
Several comparisons showed no statistical signifi- 
cance between the “present” and “absent” cate- 
gories. These included: (1) Random contact dis- 
eases with social rank; (2) social or random 
contact diseases with the Maturity Index classifi- 
cations of growth; (3) social and random contact 
diseases among those rats who were in the lower one 
third of the rank ordering of the 53 females in both 
social rank and growth. These latter females were 
presumably the ones which had experienced the 
most stress. 
Two comparisons resulted in significant differ- 
ences: (1) Social contact diseases: (a) present 
according to social rank: high — 21, medium — 9, 
low — 7; (b) absent according to social rank: high — 
6, medium — 7, low — 10. Chi square, 6.17; p of 
Chi square less than 0.05. See table 56 for related 
data. (2) Disease prevalence among the more 
favored rats: The 53 females were rank ordered 
according to both social rank and growth (Ma- 
turity Indices). The more favored rats were 
designated as those who fell in the upper third of 
the rank ordering in both categories. There were 
90 examinations which gave none, one, or more 
of the diseases for each of these rats. Social contact 
diseases: present — 19, absent — -9; random contact 
diseases: present — 22, absent — 40. Chi square. 
6.90; p of Chi square was less than 0.01. 
From the significant and nonsignificant com- 
parisons certain tentative inferences are suggested. 
Rats who have high social status, and who are more 
likely to bite (and probably also eat) other rats 
are more likely to contract infections of Capillaria 
and Streptobacillus , whose transmission is facilitated 
by such action. Such rats appear to be more 
resistant to the condition of bronchiectasis and to 
infections of Hymenolepsis and Heterakis. However, 
increased size alone does not necessarily contribute 
to resistence. Many of the larger rats must ex- 
perience more stress than some of their smaller 
associates as a result of their unsuccessful compe- 
tition for favorable status. Among the smaller 
rats, or those of lower social rank, there is an equal 
probability of getting or avoiding the diseases. 
No conclusions concerning this are warranted. 
These results point to the desirability of planning 
studies on population ecology to accurately deter- 
mine the effect of social organization and growth 
upon the prevalence and incidence of disease. 
D. Population Growth. During the 27 months of 
observation, the colony of Norway rats increased 
from 10 individuals weighing 3.5 kilo to 171 
weighing 60 kilo (fig. 146). It is quite unlikely 
that the number of adult rats, alive at the begin- 
ning of later spring breeding seasons, would have 
ever exceeded 200. See the previous discussion 
(pp. 54 to 63) on the growth of trails as an indicator 
of population level. 
The 10 adult females alive from March through 
May 17, 1948, weaned on the averge 9.2 rats. 
During the same period in 1949, there were 59 
adult females surviving from 1948 which produced 
only an average of 2.2 young which were still alive 
on May 17. Many of these latter were not yet 
weaned, so that the average number surviving 
through weaning would probably have been 2.0 or 
less. All the evidence is that this trend in decrease 
of number of weaned young per female would have 
continued at least into the next year. Although 
projection of trends is a poor tool, the best estimate 
that might be made is that by 1950 there would 
have been 77 breeding females during the spring 
breeding season. These would have produced on 
the average only 0.55 weaned young from March 
through May. Probably not more than 72 rats 
244 
