pages 143 to 152 and on figure 103. Seitz (59) 
summarizes the present state of our concepts of the 
importance of experiences during infancy, partic- 
ularly with the mother, on adult behavior. 
Wiesner and Sheard (69) have presented extensive 
experimental studies of the mother-young bond as 
reflected in the behavior of retrieving by the mother. 
Wistar albino rats served as subjects. Their 
results will be reviewed for their bearing on the 
question of early experience. 
Retrieving of her own young by the mother 
declines to about day 19-20 following parturition, 
beyond which time she no longer retrieves. This 
is the age by which the eyes of all young have 
opened, and at which I observed the young to 
engage in independent movements outside the 
nest but within the burrow. Beginning with day 
20 and extending to day 40 three marked transi- 
tions in behavior of the young take place. These 
are (1) initial explorations outside of but near the 
home burrow; (2) first extensive wanderings 
coupled with incomplete expression of food trans- 
port; and (3) termination of nursing. We may 
now pose a question: How can the mother-to- 
young relationship be manipulated in the light of 
some of Wiesner’s and Sheard’s findings? 
They found that the maxium age of young 
retrieved declines exponentially from about 30 days 
old on the day of parturition to 1 day of age on 
day 40 of lactation. At least this trend held when 
small young were given first during a set of re- 
trieving trials followed by successively larger ones. 
By removing a mother’s own young or her foster 
young at 19 to 25 days of age and replacing them 
with foster new born ones lactation and retrieving 
can be continued for as much as a year. Further- 
more, such “trained” mothers continue retrieving 
after lactation ceases. These results indicate the 
feasibility of intensifying and prolonging the re- 
trieving behavior to the point where the mother 
prevents or interferes with any independent behav- 
ior of her young entailing moving away from the 
nest cavity. In terms of a rat’s way of life, such 
procedures enable studying the phenomena of 
maternal overprotection and mother-child conflict. 
For example: A female could be trained as a 
continuously lactating efficient retriever. When 
he had arrived at this stage 2 members of one of 
her consecutively “adopted” litters could be al- 
lowed to mature rather than removing them after 
5 or 7 days of age as is done with members of the 
other adopted litters. During their maturation 
the mother could be kept in continuous lactation 
by the regular replacement of the other young she 
is nursing by several newborn ones. Now if the 
situation were designed to provide the possibility 
for independent movement by these two older 
young to a goal away from the nest compartment — ■ 
one would anticipate that the mother would inter- 
fere with the attempts by her older adopted young 
to leave the nest and reach the goal. In contrast 
to young of normal mothers, there would be a 
delay in the expression of independent activity by 
these young. Furthermore, the young of such 
overattentive mothers should be weaned much 
later. Age of weaning could be delayed further by 
feeding the mother outside the nest cage and ex- 
perimental situation, while at the same time 
preventing access to adequate solid food by the 
young. Perhaps weaning could be prolonged to 
the age of first expression of sexual behavior. Such 
rats should develop an ambivalent attitude toward 
their foster mother. They should be highly attract- 
ed toward her because of their long attraction 
toward her as a source of nourishment. On the 
other hand, they should have an aversion for her 
as a result of her interference with their attempts 
at independent behavior. When such rats in turn 
became adults one would anticipate significant 
deviations in behaviors involving interaction with 
other rats. 
Continuation of closer maternal bonds between 
23 to 40 days of age (the usual weaning period) 
must satisfy some need felt by the young but no 
longer supplied by their mother. This tentative 
deduction follows from the observations (pp. 147 
and 151) of my free-ranging rats. Rats of this 
age range, and even older sexually immature young 
(i.e., under 80 days of age), are found in the nest 
cavity with females, other than their mother, who 
are still lactating. 
G. Aggressiveness during lactation. Chitty and 
Southern (31) cite a record of a wild gray female 
who gave birth to a litter in a large cage containing 
17 other adults. On the day of parturition the 
female and young occupied the usually occupied 
end of the cage while the remaining adults resided 
at the opposite, and usually unoccupied, end of 
the cage. This is the only record, other than my 
own observations, that I have been able to find 
indicating an increased aggressive state by a lac- 
tating female rat. Beach (37) states that aggres- 
siveness is a general characteristic of vertebrate 
mothers with newborn young, although he makes 
274 
