whose members rarely engaged in any sexual 
activity. 
A behavioral complex which contributed to the 
accentuation of social interaction was that relating 
to the construction and use of trails. Each pair of 
major goals, such as a burrow or a site for procuring 
food became connected by trails. These trails also 
interconnected with each other. However, ex- 
pansion of the meshwork of trails was limited by 
some response which precluded the minimum diam- 
eter of the polygon enclosed by a group of trails 
becoming less than 5 feet. Once the pen became 
filled to this extent further increase in the number 
of rats merely increased the frequency of contacts 
between rats inhabiting the same locality. The 
opportunity for increased contacts with other 
individuals became greater among the socially 
lower ranking groups of rats inhabiting the north- 
west half of the pen. This resulted from the greater 
number of rats inhabiting this half of the pen 
following its invasion by rats forced to emigrate 
from the higher ranking colonies inhabiting the 
southeast half of the pen. 
In addition to spatial isolation of colonies 
according to their social rank, there was isolation 
in time. All rats visited the single central source 
of food and water. Members of higher ranking 
colonies tended to maintain the bimodal nocturnal 
rhythm of visiting this source that was charac- 
teristic of the early history of the colony when the 
population density was low. In contrast to this, 
members of lower ranking colonies tended to visit 
this source at times when most members of higher 
ranking colonies had retired to their burrows. 
Thus, they were more active in the dusk and dawn 
hours and between the two peaks of activity 
exhibited by their higher ranking associates. 
One of the impacts of social interaction was the 
alteration of growth. Inhibition of growth was 
greater among individuals born into socially low- 
ranking groups. Regardless of the relative rank 
of the group into which an individual was born, 
growth was inhibited in proportion to the number 
of other rats with which the individual associated 
following weaning. Thus, as the population in- 
creased more individuals were characterized by 
inhibited growth. 
Social rank was judged on the basis of success 
in combat, growth, place of birth, migration, re- 
productive success, and associates. Some of these 
served as both cause and effect of social rank. The 
ly-st criterion for judging social rank was the overall 
picture presented by the group of adults with which 
an individual associated during the breeding 
season. 
The secondary objective of this study was to 
describe how rats interact with their physical en- 
vironment in fulfilling their needs for food, shelter, 
and reproduction. Fulfillment of these needs was 
considered from the viewpoint of how the behaviors 
contributed to the survival of the individual and 
the species. 
Most rats stored food. This behavior was accent- 
uated in lactating females and in some socially low- 
ranking rats. Dispersal of food through the en- 
vironment was enhanced by caches serving as 
secondary foci from which other rats transported 
food to neighboring localities. The value of stored 
food was not recognized. Caches were frequently 
covered over while enlarging the burrow, or they 
were excavated to the surface and allowed to dete- 
riorate by the action of rain, etc. 
The frequency of responses from a point of 
orientation declines roughly exponentially with 
distance. In the construction of nests, this leads 
to the utilization of materials close to the nest site. 
In the construction of burrows this leads to a 
diminution in frequency of longer tunnel segments. 
Orientation in space is initially governed by the 
presence of goals such as sources of food or places 
of retreat. Shortest routes are generally not 
traveled exactly. Instead orientations are from 
one vertical object to another or along a continuous 
vertical object or narrow smooth plane. Repeti- 
tion of traversing the same routes leads to the for- 
mation of trails. These trails are kept clear of 
vegetation. Once this happens, tactile responses 
to the sides of the trail facilitate orientation. Such 
tactile responses appear to be innate or are learned 
very early. Young rats on their first independent 
excursion rapidly move along established trails. 
Thus in a very few hours they have found sources 
of food and water as well as other burrows which 
may serve as places of retreat. Alternate routes 
between goals are learned. When an introduced 
physical barrier, or the presence of an aggressive 
rat, bars travel, the individual immediately resorts 
to an alternate route toward its goal. 
This biological modification of the environment 
as well as the social structure of the existing group 
forms a cultural mold which guides the develop- 
ment of the young rat. Overt aggression of adults 
against the juveniles is infrequent. Aggression 
250 
