population must be in part cancelled by the tend- 
ency of males and females of a local colony to have 
different origins (see p. 203). 
In the preceding brief history of the colony it is 
shown that there was considerable difficulty in get- 
ting the introduced rats adjusted to their new en- 
vironment. An accompaniment of the difficulty of 
adjustment to the new environment was a marked 
social antagonism in the form of frequent fighting. 
There was also poor survival of litters during this 
first season as well as a social stratification with the 
higher ranking rats inhabiting the southeast half 
of the pen. This regional social stratification was 
maintained through the remaining history of the 
colony, with the southeast rats characterizing a 
more favorable social status, more progenv, and 
better survival. The most rapid rate of population 
increase occurred during the second of the 3 years. 
By the third breeding season the population density, 
and the accompanying strife between rats, had so 
increased as to produce a marked inhibition of 
reproduction. 
2 . Observations Concerning Young Rats From 
Birth to Sexual Maturity 
A. Death of Litters Shortly Following Parturition. 
The number of pregnancies determined by palpa- 
tion of embryos far exceeded the known number of 
litters which survived through weaning. Probably 
most losses occurred during the first 2 days after 
birth. The following data will bring out the salient 
circumstances surrounding litter loss. 
On April 5, 1947, one of the colonizing females, 
No. 4, who was circa 426 days old, was observed 
in Box 25 with two to three newborn young. The 
lid of the box was opened only momentarily for 
this observation. On the next day she was found 
in a large new nest in Box 7, but the young were 
not with her, nor were further traces of them 
found. On September 10, 1947, she was found in 
Box 2 with 12 young bom the previous night. 
Sometime between then and September 13, when 
she was found alone in Box 8, her litter disappeared. 
Between these two dates two additional litters 
must have met similar fates since embryos were 
palpated on two intervening occasions. Although 
the minimum stimulus of momentarily opening the 
harborage boxes may have been the precipitating 
situation which led to the destruction of the litters, 
it must be noted that other females who were simi- 
larly disturbed reared their litters successfully. 
Female 4 was known to exhibit a subordinate 
status to the two original females, Nos. 10 and 11, 
who did successfully rear litters. It is therefore 
concluded that the emotional state characteristic 
of more subordinate females predisposes them to be 
poor mothers (also see p. 260). Emotional state 
is here equated with the incidence of disturbing 
situations. 
Such poor maternal care is particularly well ex- 
emplified in the histories of the three Litter 3 fe- 
males, Nos. 17, 20, and 25. These three females 
were characterized by slow growth, small mature 
size, and many wounds received in chases by other 
rats. Details of their history and that of their sibs 
are given on pages 145-147. On the basis of 
palpation of embryos these females had 3, 4, and 
5 pregnancies respectively, yet only one litter was 
reared, L-25b by female 17. As with female 4 
the actual fate of the lost litters was unknown. 
Females 17 and 20 were autopsied and found to 
have 38 and 32 placental scars respectively, thus 
verifying the relative frequency of pregnancy of 
these females. 
One of the characteristics of these and similar 
socially low-ranking females, who failed to rear 
litters, was that the increased tendency to construct 
nests during the first 4 days following parturition 
frequently continued even after the litter disap- 
peared. Thus, even though both nest construction 
and that maternal behavior concerned with ma- 
nipulation and nursing of young are normally 
accentuated by the physiological state of the rat 
following parturition, the above observation points 
to the fact that these two types of behavior may be 
independently modified. Failure to exhibit proper 
maternal care represents a failure of interacting 
with other individuals, their young. The subordi- 
nate status of these same females reflects their 
inability to develop favorable interaction adjust- 
ments with their adult associates. 
There may occur an incidental death of a litter 
shortly following parturition even though the 
mother may be one which evinces good maternal 
care. One case of this was observed. Normally 
hooded nests are constructed immediately pre- 
ceding parturition. Such a nest was constructed 
in a burrow adjoining Box 29 by female 37. She 
had a litter here on the night of June 19-20 or 
20-21, 1948. This was during a period when I 
was trapping rats and excavating some of the 
burrows. At such times there is considerable 
shifting about of rats as a result of this disturbance. 
143 
