the harem. There were instances where a female 
alone was rather effective in keeping away other 
rats of both sexes from her territory both during 
and following lactation. Dominance and the 
social rank ordering of local groups did occur, but 
most particularly it occurred among groups which 
did not exhibit territorial defense. Aggression and 
the formation of hierarchies without the concomi- 
tant development of territoriality is detrimental. 
This is documented in the later sections, Stress and 
Reproduction (pp. 214 to 216) and the Develop- 
ment of Local Colonies during the spring of 1949 
(pp. 203 to 213). 
E. Conditions Influencing the Frequency and Outcome 
of Aggression. In addition to the conditions which 
influence aggression mentioned in the previous 
discussion there are others which operate: 
a. Sex. There were 88 paired conflict situations 
in which each member of the pair was known, and 
in which both rats were born in the pen. The 
number of instances and the sexes involved are 
listed as follows. The sex of the winner is listed 
first and the expected number is given in paren- 
thesis: male-male: 53 (26); female-female: 20 (18); 
female-male: 12 (22); male-female: 3 (22). Ex- 
pected numbers in each catagory were derived on 
the basis of their dependence upon chance inter- 
actions among the 87 males and 72 females which 
were born during 1947 and 1948 and lived for 
more than 1 50 days. The proportions of the inter- 
action will be male-male (a 2 ): 0.299; male-female 
plus female-male (2ab): 0.496; and female-female 
(b 2 ): 0.205. Differences between the observed and 
that expected, on the basis of proportionality to 
the chance encounter rate, are of such magnitude 
as to indicate significance without recourse to a 
statistical test. 
From the fact that the frequency of intermale 
antagonistic encounters is so much greater than 
expected it is concluded that males actively “seek” 
competitive situations. Fights between females 
were in proportion to a chance contact rate. 
Intersex fights, regardless of the sex of the winner 
were so reduced that one might say that males and 
females “avoid” competitive situations. 
“Seek” and “avoid” are understandable in 
terms of alterations in extent of home range and the 
attitude of the sexes toward each other as goals. 
Females maintain a more stable extent of home 
range than do males. Also, female members of 
the same local colony are remarkably tolerant of 
each other. That is, they exhibit neither attraction 
or repulsion of each other as goals. Thus, fights 
between females primarily involved contact be- 
tween members of different colonies when one 
chanced to approach the place of residence of 
another. Under such circumstances fights approxi- 
mating a chance contact rate is logical. 
When females are in estrous, males are attracted 
to their vicinity. Some males depart from their 
normal home range. This and their simultaneous 
attraction to the female or her scent posts increases 
the contact rate between males above the cus- 
tomary rate. The frequency of fighting certainly 
increases at these times of increased contact rates. 
In a few males this temporary increase of aggressive- 
ness toward other males becomes transformed into 
a conditioned territorial behavior, in which in- 
creased aggression about the burrow is elicited 
even in the absence of estrous females. In such a 
system antagonistic encounters among males would 
be expected to exceed the proportionality to chance 
contact rate. 
Contact between males and females are reduced 
because of the development of local colonies, whose 
adult members are either largely male or largely 
female. See the section “The Development of 
Local Colonies” (pp. 203 to 213). This alone 
would account for a reduced frequency of fighting 
between the sexes, if fighting is proportional to 
contact rate. Furthermore, the attitude of the 
sexes toward each other, particularly of males 
toward females, must further reduce fighting. 
Were there not some such ameliorating attitude 
the act of copulation would rarely be consummated. 
The complete data on the observed frequency of 
intersex and intrasex encounters is given in table 
41. This includes encounters involving the 
introduced rats. From this table sexual differences 
in the number of wounds are apparent. Males 
which win receive more wounds than do females. 
This difference is probably completely dependent 
upon the greater frequency of fighting by males. 
In fact, the more dominant males (the winners) 
frequently were characterized by more wounds 
than were their lower ranking associates. Those 
individuals with the most wounds represent the 
extremes of the social scale. The few most domi- 
nant males received a large number of wounds 
because they engaged in frequent combat with a 
larger number of other males who fought back. 
At the other extreme there were a few males 
which were frequently attacked, but which prac- 
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