The events described to this point cover the first 
2 days of the female’s attractiveness to males. The 
wandering, mutual courtship, and lower attrac- 
tiveness of females, which comprise the first day, 
are either omitted or highly abbreviated in time 
for females who have just given birth to a litter. 
These individuals nearly immediately fall into the 
category of receptiveness and high attractiveness to 
males, which characterize other females on the 
second day. 
On the third day for most females, or on the 
second day for females who have just given birth 
to a litter, they are still quite attractive to males, 
but no longer receptive. They are still followed 
by males who exhibit sexual rolls and rubs at 
signposts. Attempts at mounting are reduced, but 
inspection of the female is continued. Attempts at 
mounting or genital inspection is repulsed. The 
female pushes the male away with her forefeet, 
boxes them, may actually fight and chase them, 
or kicks them away with her hindfeet. The 
attractiveness of the female gradually declines 
during the 7 to 9 days following maximum re- 
ceptivity until by the 8th to 10th day males no 
longer pay any attention to her of a sexual nature 
other than that of the casual genital inspection, 
which may characterize rats of either sex upon 
meeting and passing. During this period of de- 
clining sexual attractiveness the females may still 
exhibit the rubbing at signposts of the fashion 
characteristic of the first 2 days. Stimulation of 
the male to roll and rub at signposts was not 
observed after the day following the female’s 
receptiveness. Females who are lactating also 
exhibit this declining phase of sexual behavior for 
the week following parturition. On the day fol- 
lowing maximum receptivity there is a tendency 
for the females to be quite retiring, particularly if 
they have a new litter. For comparable periods of 
observation the female may never be seen, although 
on the previous day she was frequently out and 
pursued by males. 
A number of females failed to produce litters, 
even though mountings had occurred, and intro- 
mission seemed to have occurred. If conception 
was effected, resorption must have set in rapidly 
since the embryos were not carried long enough to 
reach the size where they could be detected by 
palpating the female’s abdomen. Lack of con- 
ception, or possible early resorption of embryos, 
materially reduced the reproductive potential. 
The length of the estrous cycle, that is the num- 
ber of days elapsing between one period of recep- 
tivity to another, is inadeuqately known for these 
free-ranging rats. One period of observed recep- 
tivity frequently followed a month or more after 
the previously observed receptivity. Intervening 
dates of receptiveness may well have fallen on days 
when no observations were made. Three periods 
of receptiveness for one female were separated by 
7 and 11 days, and conception was known to occur 
on the third date. Another female was known to 
conceive on the ninth day following the loss of a 
litter on the day following parturition. These 
observations coupled with the fact that the at- 
tractiveness of females declines for a week following 
receptiveness, suggests that the cycle must, at a 
minimum, be slightly longer than a week. I am 
inclined to believe that in many cases it is con- 
siderably longer than this. Were it not on the 
average longer than a week many more signs of 
sexual activity, such as that of rolling and rubbing 
by males, should have been observed. Yet there 
were many days on which no such sexual behavior 
was noted even though there were a minimum of 
5 to 10 females who were not lactating or pregnant. 
In four instances conception occurred on what was 
probably the first period of receptivity at the 
terminal phase of lactation. These were at 26, 27, 
30, and 31 days following the birth of a previous 
litter, which was raised successfully. Another 
female was sexually attractive at 28 days. 
There appears to be an erotic implication to 
aggressive action. On several occasions a male, 
who had just attacked another male, exhibited 
sexual rolls or rubs. The reason for believing that 
this sexual behavior was not just coincidental to 
aggression was that the male might have been 
resting for several minutes at a signpost, particularly 
at a passage through the median barrier fence, 
without exhibiting any sexual behavior, but im- 
mediately following the attack he would return 
and roll, rub, or dig in the earth. One female at 
the termination of three consecutive aggressions 
toward other females immediately engaged in 
dragging of the genital region over the ground in 
front of one of the activity recorder tunnels. She 
was in the ninth day of lactation. 
Four instances were noted of behavior that might 
best be categorized as homosexual. The one case 
involving males was between two individuals, who 
were members of socially outcast all male aggre- 
gates inhabiting Area IV (sec discussion of colonies 
156 
