in its expulsion. A much greater number of ob- 
servations pertaining to expulsion following inva- 
sion of harborages was made at the time of marking 
and releasing rats. The reason for this was that 
following their capture rats normally avoided re- 
entering the harborage box from which they were 
removed, or from entering any place of harborage 
in the immediate proximity to the place where they 
were captured. This behavior frequently led the 
rats to enter harborages inhabited by rats with 
whom they normally did not have close associa- 
tions. The events recorded in the following four 
examples are typical of a form of territorial defense 
which is certainly of equal importance to that 
exhibited by a few socially dominant individuals. 
1. September 28, 1948. One-hundred eighty- 
eight-day-old female 75 escaped near Box 11 
in Area II where I had been handling all rats 
captured that day. She was still slightly 
under the effects of anesthesia. She first went 
through two burrows along the northeast fence 
of Area II. Her departure from these two 
burrows was not observed to be associated 
with any antagonistic action toward her. 
From here she went up Path 3 to the Food 
Pen, from which she departed through Passage 
5 and proceeded to the South Alley Burrow. 
Although this was the place of her birth she 
had not inhabited it since just before her first 
pregnancy 3 months earlier. She entered at 
the first hole she encountered and emerged 8 
feet away being chased by an older female 
(one born in 1947) who trailed her to Passage 
1. Female 75 was lost sight of as she headed 
through Area IV toward her home in the 
burrow opposite Box 28. This record is inter- 
esting from two viewpoints. In the first place, 
she was excluded from the place of her birth 
by an older female with whom she had close 
associations as a juvenile rat. In the second 
place she exhibited a retrogressive orientation 
when she arrived near the center of the Food 
Pen on her way home. Instead of departing 
through Passage 8 and going through the West 
Alley and Passage 4 to Area IV, as she had 
done for the past 3 months, she elected a route 
customarily utilized when she was a juvenile. 
This is one of the types of observation which 
has led me to believe that adults are more 
likely to exhibit juvenile behaviors when 
disturbed. 
2. September 24, 1948. One-hundred forty- 
seven-day-old male 683 (born at the SAB) 
was released near the burrow at Box 29 where 
he was caught. He ran into Box 36 where a 
fight occurred. He was chased out by adult 
male 30. He then ran southeast along the 
fence. This male, who at this time had two 
large sores resulting from warble emergences, 
was destined by the spring of 1949 to become 
a member of the socially low-ranking all- 
male aggregate inhabiting the borderline 
region between Area I and IV (colony i, 
p. 212). 
3. December 7, 1948. Two-hundred thirty-two- 
day-old male 97 (born in Area I) was in- 
habiting Box 25 in Area III. This was by 
far the worst place of harborage in the whole 
pen at this time. A large section of one side 
of the box had rotted away leaving any 
inhabitant exposed to inclement weather. 
Male 97 was inhabiting this place even 
though most rats were joining together in 
particularly large harborage aggregates, pre- 
sumably for the purpose of conserving body 
heat. Upon my opening the lid of Box 25 
he ran out into the Box 24 burrow, but was 
chased out. He then ran into the Box 19 
burrow, where after a squealing fight it also 
ran out. By the next day he was again back 
in the exposed Box 25. During the winter 
and spring of 1949 this male joined the 
aggregate of outcasts and socially lowest rank- 
ing males inhabiting the collapsing burrow 
in the North Alley by Passage 3 as well as 
the extremely exposed harborage under the 
ladder in the East Alley (colony k, p. 213). 
4. April 13, 1949. Three-hundred sixteen-day- 
old male 662 (born in Area III) was trapped 
and released at the burrow in the North 
Alley by Passage 3. He then entered this 
burrow momentarily, but left followed by 
another rat. It then entered the North 
Alley Burrow where it was chased out by a 
resident rat. The resident rat left its burrow 
only momentarily. Male 662 then entered 
the burrow at Passage 4, but remained there 
only a few minutes before going to and enter- 
ing the South Alley Burrow. There it was 
chased out, fought by, then chased for about 
25 feet by male 49, who dominated that 
burrow and its vicinity. The exclusion of 
662 from burrows was so great that during 
676-768 O — 63 13 
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