Table 67. — Prevalence of mortality or extensive injuries 
among male rats in relationship to the level of their 
maturity index 
Maturity index 
range 
Mortality of rats 
born in 1948 
Extensive injuries 
Died 
before 
May 
1949 
Lived to 
May 
1949 
Injured 
Uninjured 
littermate 
1.00 to 1.39 
0 
7 
9 
3 
1.40 to 1.79 
8 
11 
13 
10 
1.80 to 11.19 
3 
11 
10 
4 
11.20 to 11.59. . . . 
3 
8 
5 
9 
11.60 tolll.0. . . . 
2 
12 
5 
8 
Total .... 
16 
49 
42 
34 
Mortality: It appears from table 66 that rats 
living to 1 50 days of age but dying before they are 
a year old have an average maturational pattern 
which does not differ from the average maturational 
pattern of those rats surviving to approximately a 
year of age. However, the distribution of the 
Maturity Indices which makes up the mean for the 
males is interesting. It is shown in table 67 that 
all males in the top 20 percent of the Maturity 
Index range survived. These were those indivi- 
duals who had made a successful adjustment in the 
contention for social position. On the other hand, 
50 percent of those who died were included in the 
second most favorable range of Maturity Indices, 
1.40 to 1.79. Six of these eight males were etiher 
born at the South Alley Burrow or were members 
of a litter born in a neighboring favorable location 
and whose mother moved to the South Alley Burrow 
at the time they were being weaned. All were born 
during the first half, the more favorable half, of 
the 1948 breeding season. Probably their growth 
had been inhibited just sufficiently to prevent 
their successful competition, yet they had matured 
under conditions which predisposed them to con- 
tend for priority. Five of these rats were noted to 
be social outcasts just prior to their death. They 
had become emaciated, failed to groom their fur, 
and were prevented by other rats from entering 
the burrow systems. Two of the other three died as 
a result of screw fly infestation of extensive wounds. 
The last rat was also extensively wounded and was 
killed and eaten by other rats. Rats of a similar 
or lower Maturity Index, who survive, are generally 
those which adjust by not initiating combats or who 
flee when challenged. However, mortality among 
adolescent rats, 50 to 1 50 days, is certainly associ- 
ated with the degree of inhibition of maturation 
(table 66). 
f. Infestation of the rats by microorganisms. This 
section is concerned with the autopsies performed 
at the termination of the experiment in March 1 949 
by the Johns Hopkins School of Hygiene and Public 
Health, and by the National Microbiological 
Institute of the U.S. Public Health Service. These 
data refer to 31 males examined by Hung-Ying Li 
of Johns Hopkins; and to 77 females and 23 males 
examined by Drs. Chester W. Emmons, Robert T. 
Habbermann, Carl Larson, and W. T. S. Thorpe 
of the National Microbiological Institute. Of the 
adult females in this latter group there were 53 
whose identity was known. 
Negative Results: None of the rats examined at 
Hopkins or the NIH had Salmonella-, nor did any of 
the 31 males examined at Hopkins have Leptospira. 
Emmons (20) found no Histoplasma in any of the 
rats. The absence of these three organisms in the 
experimental population is noteworthy since the 
original members of this population were secured 
from Parsons Island, an isolated 150 acre island, 
which lies across the Chesapeake Bay from Anna- 
polis, Md. 
Positive results: S'reptobacillus moniliformis was iso- 
lated from the middle ears from 15 of 33 necropsied. 
rats. This organism when transmitted to man 
produces one of the conditions termed rat-bite fever. 
It was also probably involved in damage to the 
lung tissue, associated with the infection of the lung, 
termed bronchiectasis. Many of the older rats 
showed extensive lesions of the lungs. Bartonella 
muris was found in the only two rats examined for 
this bacterium. Helminth parasites and the per- 
centage of rats examined that were infected were 
as follows: 
Trichosomoides crassicauda (6.5); Capillaria hepa- 
tica (53.3); Strongyloides ratti (16.4); Heterakis 
spumosa (60.9); Syphacia obvelata (2.2); Tricuris 
muris (2.2); Hymenolepsis spp. (42.6); Cysticercus 
fasciolaris (3.3). 
Infestation with this latter tape worm probably 
arose as a result of the invasion of the pen by a cat 
on two or three occasions before an adequate 
electric fence barrier was established. The above 
summary is given in detail by Habermann et al, 
( 21 ). 
243 
