SEASONAL VARIATION IN THE CONDITION OF THE TEATS 
197 TO 450 DAYS OF AGE 
NUMBER OF 
OBSERVATIONS 
27 
19 20 42 44 
it. Nevertheless the teat can usually be seen with- 
out resorting to blowing aside the surrounding 
hair. Milk can frequently be expressed. This 
stage develops near the time of parturition. It 
also characterizes the period of 1 to 4 weeks follow- 
ing weaning. + + + is the stage of lactation. 
The teats are as in the previous ++ stage of 
elongation but the hair is matted down around the 
teat by the nursing of the young and milk can 
always be expressed. This system was adopted 
from that already in use by Drs. Emlen and Davis 
of the Rodent Ecology Project. ++ and + + + 
thus characterize those rats which are or recently 
have been lactating. 
From table 36 it may be seen that none of the 
females between the ages of 83 and 115 days, which 
were handled during the breeding season, had 
attained the state of lactation. Furthermore, only 
22 percent of the females between the ages of 116 
and 196 days were in this lactating phase. Be- 
tween 6 and 12 months, around 43 percent were 
lactating, but it was not until the age of 12 to 15 
months that over 50 percent of the females were 
lactating. Judging from the fact that laboratory 
breeds of the Norway rat regularly begin breeding 
between 3 and 4 months of age, this slower attain- 
ment of sexual maturity is probably attributable 
to the slower attainment of the full pattern of 
sexual behavior, which is requisite for successful 
copulation in an open society (For a detailed review 
of the maturation of reproduction see pp. 264 to 
273). 
