proceeded to the South Alley Burrow and rolled at a 
burrow entry where a receptive female had recently 
been. The third male, No. 46, which approached, 
smelled all four rocks, but stayed longest on the 
rock for June 4. Female 33 then approached; 
smelled the rock for June 2; then went to that for 
June 4 and drug over it before running away as 
though startled by something. The fifth rat, 
male 69, smelled all four rocks, but drug over only 
those for June 4 and 5. Observations were dis- 
continued at this point at about 7 p.m. The 
scent of this domesticated rat attracted males 
regardless of her stage in the estrous cycle; how- 
ever, that of day of estrous and the day preceding 
it seemed more stimulating. By the next morning 
all four rocks were greasy and dirty from rats 
dragging over them. The dirt about the base of 
each, where the solution had soaked, was scratched 
up and apparently had been eaten. There was no 
detectable difference in the usage of these four 
rocks. 
These observations were repeated on the night 
of June 30. Dates of scent samples: June 7, 8, 10 
(estrous), 11. During the day small samples 
were repeatedly poured over four clean rocks, so 
that by dusk each was dry, but had an odor which 
I could detect. Five rats were noted to respond 
to these rocks. Although each rat responded 
somewhat differently I could detect no difference 
in the degree to which the four rocks elicited 
response. The only conclusions warranted by 
these observations is that the scent of this female, 
regardless of the stage of her cycle, was markedly 
stimulating. In fact, it looks as though there had 
been a chance selection toward an enhanced 
stimulating value of the scent of this domesticated 
female. Samples preserved for 9 months from 
this female elicited the full response of rubbing, 
and rolling by males, when placed at points 
where such behavior had been observed on other 
dates. 
B. Seasonal and Maturational Aspects oj Reproduc- 
tion. Domesticated rats are known to conceive 
as young as 60 days of age. Furthermore, litters 
are regularly reared by domesticated rats to 450 
days of age. With duration of pregnancy taken 
as 23 days, and a minimum interval between 
pregnancies of either 23 or 53 days, depending 
upon whether reconception accompanies parturi- 
tion or the terminal phase of lactation, it is possible 
for 7 to 17 litters to be conceived and possibly 
reared during the 390 day span prior to senescence 
setting in. And yet the reproductive performance 
of the wild Norway rats in the pen never reached 
this level. In this section we shall consider some 
of the seasonal and maturational aspects of repro- 
duction associated with lowered performance. 
Table 35. — Physical signs oj sexual maturation 
Age in 
days 
Males 1 
Females 
Number 
of obser- 
vations 
Proportion 
with 
scrotal 
testes 
Number 
of obser- 
vations 
Proportion 
with 
perforate 
vagina 
25 
15 
0 
10 
0 
35 
24 
0 
19 
0 
45 
17 
. 12 
14 
.29 
55 
24 
. 13 
23 
.83 
65 
30 
. 40 
16 
.94 
75 
20 
. 65 
73 
1.00 
85 
14 
.79 
12 
1.00 
95 
15 
1. 00 
105 
17 
1.00 
1 March to September observations only. 
The vaginal orifice of young females is sealed by 
a membrane, while the testes of young males are 
confined to the abdominal cavity. Perforation of 
the vaginal orifice or descent of the testes into the 
scrotum is the first overt sign of approaching sexual 
maturity (table 35). Not until after weaning do 
any members of either sex exhibit these conditions. 
All females have perforated vaginas by 75 days of 
age. This is before any females were observed 
exhibiting sexual behavior. Males do not attain a 
comparable state until 20 days older. By this time 
(95 days) a few individuals of both sexes exhibit 
some sexual behavior. At this age neither sex is 
fully sexually mature from a behavioral viewpoint. 
See “Period of Initial Sexual Behavior of Subadult 
Rats” (p. 151). 
Further confirmation of the slowness of sexual 
maturation was revealed through the condition of 
the teats of females. Four stages were recognized. 
These stages were represented by symbols: 0, J-, 
-f +, + + + (fig- 104). 0 is the stage in which the 
teat is flat. It is often difficult to locate such teats, 
even by blowing aside the covering hair, -j- is the 
stage when the teat is slightly enlarged and convex. 
The hair still has to be blown aside to see the teat. 
J-+ is the stage when the teat is elongated and 
cylindrical, but the body hair still closely surrounds 
158 
