It follows that: 
p is also equivalent to the proportion of 
“rat-days” at a particular age on which 
rats are pregnant by macroscopic 
observation. 
But: Except for the oldest age class con- 
sidered (438 mean days of age, ± 1 <r of 
77 days), only a portion of the rats are 
parous. 
It follows that: 
p=t 2 r/S 
Therefore: 
S—t 2 r/p, column 6, table 68 
and 
/j — T/S, column 7, table 68. 
Davis (28) calculates I\ from the equation: 
li—pTj\l [Emlen and Davis, (22)]. This 
equation assumes r=1.0, which is not 
true except for samples of older rats, 
and therefore most of the estimates 
of incidence of pregnancy listed in his 
table 3A [Davis, (28) p. 381] are too 
small when we consider only rats 
which have attained reproductive par- 
ity. Davis’ calculations are perfectly 
valid as a general incidence among 
all females of a particular age class. 
For example, using the r and Rvalues 
in table 68, the I x r by Davis’ equation 
for 74 and 144 days are respectively 
0.24 and 3.85. The shortcoming of 
such a pregnancy incidence is that it 
enables us to gain no further insight 
into the rat’s reproductive biology. 
Now let us proceed with the present 
line of reasoning to see what insights 
follow from it. 
Since: 
tJi = Number of days during the year rats 
are pregnant 
Then: 
T— tJi = Number of days available for nursing 
and since: 
t 3 =Normal minimal number of days for 
nursing a litter 
Then: 
/,= 
T-tJ u 
column 8, table 68 
It follows that: 
/ J 1 = / 2 // 1 column 9, table 68 
Since: 
r reflects the physiological and behavioral 
capacity to mate and conceive, and 
Pi reflects the adequacy of maternal be- 
havior in providing protection and 
nutrition to the preweaning young. 
Therefore: 
■Po=/V, column 10, table 68; figure 150. 
The youngest age groups (74 days) of rats which 
become pregnant conceive at a rate which during 
a year’s time would produce 15.87 litters per rat 
for those able to conceive. This means they con- 
ceive every 23 days, or immediately after each 
delivery. This leaves no time over for nursing. 
Thus, for all practical purposes primiparous wild 
rats never rear their first litter. 
Most rats are ovulating before 74 days of age. 
Failure to conceive at this early age most probably 
depends upon: 1. Disturbed reproductive physiol- 
ogy related to stress associated with learning the 
complexities of this environment. 2. Failure to 
have learned all the appropriate responses required 
to conclude mating with a male. This latter in- 
cludes not only the learning of courtship and 
copulatory behavior, but also the expansion of 
home range at estrous which insured encountering 
males. 
r as a function of age increases in a sigmoid 
fashion much as does a learning curve. However, 
r defines not how much an individual has learned 
but rather how many individuals have learned the 
required task. It is difficult to believe that heredi- 
tary differences alone can produce a year’s differ- 
ence in the ability to learn the task. Much more 
likely is it that complexities of the rats’ physical 
environment, and complexities of the hierarchical 
and class structure of rat society make it more 
difficult for some rats to learn adequate relations 
with males. All females are able to compensate 
for these difficulties and acquire behaviors essential 
to mating and conceiving provided enough time 
transpires. At least the conclusion applies to the 
Davis and Emlen (66) data on parity shown in 
column 4 of table 68. 
Achieving the ability to conceive is only a pre- 
liminary phase of the struggle for survival of the 
species. For even if the young are born they need 
to be properly taken care of or they will perish. 
Prior to parturition a well-built nest needs to be 
built in a place uninhabited by other rats. This 
means that the prospective mother must either 
seek a place unfrequented by her associates and 
268 
