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The attainment of sexual maturity in the female albino rat as determined by the copulatory response.

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THE ATTAINMENT O F SEXUAL MATURITY IN THE
FEMALE ALBINO RAT AS DETERMINED BY
THE COPULATORY RESPONSE
RICHARD J. BLANDAU AND WILLIAM L. MONEY
Department of Anatomy, Harvard Medical School, Boston, Massackusetts, and
Biological Laboratory, Brown University, Providence, Rho& Island
ONE TEXT FIGURE A N D ONE PLATE (SIX FIGURES)
In the laboratory rat, many details of the first reproductive
cycle associated with the onset of puberty are either incomplete or totally lacking. The marked discrepancies in the
reported age at which the female albino rat becomes sexually
mature have been due primarily to the variations and limitations of the criteria used to determine the onset of the first
reproductive cycle. Vaginal introitus, changes in the vaginal
smear, sexual receptivity and ovulation are all integral factors
in the attainment of sexual maturity. Since it is conceivable
that there could be considerable variation in the time at
which each of these factors becomes established, it would seem
important to correlate at least several of these phenomena
in order to define the onset and attainment of sexual maturity.
To date, the most complete study of sexual maturity in the
female rat is that of Long and Evans ( '22). By investigating
the vaginal smear and the presence of the copulation plug, as
well as from data regarding the birth of the first litter, these
investigators concluded that the first estrus occurred on the
average at 92.7 days after birth, ranging from 63 to 107 days.
Vaginal introitus occurred on the average at 76.5 days after
This investigation was supported by a grant from the Committee for Research
in Problems of Sex, National Research Council.
Present location, The Biological Laboratories, Harvard University, Cambridge,
Massachusetts.
197
198
R. J. BLANDAU A N D W. L. MONEY
birth, ranging from 53 to 142 days. The authors point out that
in the majority of the females the first ovulation was delayed
approximately 5 days after vaginal introitus had occurred.
They indicated further that in their strain of rats the first
cycle was significantly longer than any of the succeeding
ones.
From observations on the Wistar strain, King ('16) states
that the first litters are dropped at about 3 months after
birth, whereas gray Norway rats, living under natural conditions, do not begin to breed, as a rule, until they are at
least 4 months of age (King, "39).
Freudenberger ( '32), using vaginal introitus as an indicator
for the onset of sexual maturity, reported that in the Wistar
strain maturity occurred on the average at 46.9 days, ranging
from 36 to 66 days after birth, whereas in the Long-Evans
strain maturity was reached at 52.7 days, ranging from 39
to 101 days after birth.
Arai ('20), from histological studies of the ovaries of rats
of the Wistar strain, came to the conclusion that ovulation
occurred between 60 and 70 days after birth. Employing the
same technique, Hargitt ( '30) reported that albino rats became sexually mature approximately 40 to 45 days after birth.
Using the revolving cage method, in which the running
activity of the aninial was recorded, Slonaker ('24) reports
that his animals became sexually mature at the average age
of 80.6 days after birth, ranging from 63 to 109 days.
The work of Slonaker ( '24), Freudenberger ( '32), Blunn
('39), and others emphasizes that strain and dietary differences have an important bearing on the time of onset of
puberty.
I n the female rat direct observation of the copulatory
response can be used as an accurate method to investigate
the periodic changes in the estrous cycles of the non-pregnant
and post-parturient animals without disturbing their cycles
(Blandau, Boling and Young, '41 ; Blandau and Soderwall,
'41). Observation of the copulatory behavior has distinct
advantages in that (1) it permits one to ascertain the time of
SEXUAL MATURITY I N T H E FEMALE RAT
199
the beginning and the end of heat, and (2) it provides a basis
for determining the temporal relationship between heat and
ovulation (Boling, Blandau, Soderwall and Young, '41).
None of the previous studies on pubertal female rats employed the copulatory response as a basis for ascertaining
the time of sexual maturity. However, we have attempted by
this method to determine more accurately: (1) the temporal
relationship between vaginal introitus and the beginning of
the first heat period; (2) the length of the first heat period
in relation to that of succeeding periods; (3) the length of
the first reproductive cycle and its comparison with subsequent cycles; (4) the time of the first ovulation and its
relation t o the beginning of the first heat, including the
number of ova shed during the first heat as compared with the
number shed in later periods; and (5) the number of females
which became impregnated by either matings or artificial insemination during the first heat period.
M A T E R I A L S AND METHODS
The data were obtained from 215 female rats all born within
an interval of 4 days. The animals were members of a vigorous
albino strain of Wistar extraction, which has been maintained
at Brown University for a number of years. I n order to
standardize conditions as much as possible, all of the male
young were discarded and each mother was allowed to suckle
six female young. The females were weaned at 21 days of
age and were then maintained on a ration of Rockland rat
diet supplemented by a green vegetable fed twice each week.
Water was provided ad libitum. Beginning at the time of
weaning, the females were examined both morning and evening
for vaginal introitus and for the copulatory response. When
the first female showing signs of either vaginal canalization or
manifestations of heat, a new routine of observations was substituted. This consis,ted of examination of the entire colony
at hourly intervals, day and night, with respect to both
vaginal introitus and the signs of heat. I n order to reduce
the influence of constant illumination on the cycle (Browman,
200
R. J. BLANDAU AND W. L. MONEY
'37 ; Hemmingsen and Krarup, '37 ; Fiske, '41), the animals
were kept in an ordinary room in which there was the normal
alternation of daylight and darkness. The only light during
the night observations and the taking of records was provided
by a small shaded bulb attached to a movable crane which
was so arranged that only one cage at a time was illuminated.
The onset of heat was ascertained by the female's response
to manual manipulation as previously described by Rlandau,
Boling and Young ( '41). That a female was about to come
into heat could invariably be foretold by the marked tumescence and discoloration of the vaginal orifice. These
changes in the vaginal orifice occur several hours before
sexual receptivity becomes apparent.
Ninety-two females were used to determine the temporal
relationship between the first heat and ovulation. Fifty-two
of these, in which ovulation had been completed, served for
ascertaining the number of ova shed during the first heat
period. Forty-one females, selected at random, were examined
f o r seven consecutive reproductive cycles. Thirty of these
were sacrificed at the end of the eighth heat period in order
to compare the number of ova shed in the first cycle with the
number shed in the more mature animal. The ovaries of
all of the experimental animals were removed and fixed in
Bouin's fluid. They were then dehydrated in dioxan, embedded
in paraffin, sectioned at 10 p and stained in Delafield's hematoxylin and eosin.
Fifteen females failed to give the copulatory response, even
though the appearance of their vaginal orifices was that of
an animal about to cume into heat. The ovaries of these
animals were subsequently removed and studied.
Thirty females were mated with mature males during the
first heat period and ejaculations observed. To witness actual
ejaculation is of importance because of the discrepancy in
size between the mature male and the pubertal female-a
discrepanoy which makes failure of insemination possible.
Thirty-seven females were inseminated artificially during
the first heat period according to the technique described by
SEXUAL MATURITY I N THE FEMALE RAT
201
Blandau and Jordan ( '41). Both the mated and inseminated
females were subsequently observed for pregnancy and birth
of the young. At the termination of pregnancy, the animals
were killed and the ovaries examined macroscopically to make
certain that pregnancy was the result of the first ovulation.
The completion of the investigation would not have been
possible without the generous assistance given by Dr. John
R. Ring and Miss Catherine Fales in the continuous day and
night observations of the animals.
OBSERVATIONS
The chronological relationship of vaginal opening to the
first heat period in 184 females is illustrated in figure 1. It
will be noted that, although there is a relatively close cor-
T I N E I N HOURS: W I T H TIME OF ONSET OF HEAT A S ZERO POINT
Fig. 1 Chronological relation of vaginal opening t o first heat in 174 female rats.
relation between the onset of the first heat period and vaginal
introitus in a large number of the females, there is, nevertheless, a relatively wide distribution in the time of vaginal
opening in a significant number of individuals. Vaginal introitus and the onset of heat took place simultaneously in
twenty-five animals. I n twenty-six females, opening occurred
from 1to 12 hours after the onset of heat, and, in one animal,
opening did not occur until 72 hours after the onset of heat.
202
R. J. BLANDAU A N D W. L. MONEY
Thus, a few females may be sexually receptive, yet their
vaginal membranes may remain unruptured at least during the
first hours of heat. I n seventy-three females, opening occurred between 1 and 48 hours before the beginning of heat,
and in the remaining females the time of opening varied from
48 to 459 hours before heat began.
Matters and MacKay ( '34), in determining the time of
sexual maturity in the black-hooded rat by histological examination of the ovaries at intervals after vaginal introitus,
found that, though vaginal introitus may have occurred as
early as 43 days of age, ovulation did not take place until
approximately 70 days of age.
We emphasize that the beginning of heat, rather than
vaginal introitus, is a truer index of the appearance of sexual
maturity in the rat, because it is more closely associated
with the time of ovulation and the ability of the animal to
reproduce. Vaginal introitus alone has only relative significance in determining sexual maturity in this animal. Long
and Evans ( '22) have further emphasized this point by showing that in females in which the ovaries are removed at about
the thirtieth day after birth the vaginal orifice is established
at about the usual time.
The age of onset of the first heat period in 200 animals
varied from 37 to 67 days, and showed a standard deviation
of 5.46 days and an average of 49.4 days. No similar data,
based upon the same method, are available in other strains
for comparison. However, using the time of vaginal opening
as a basis for determining the onset of sexual maturity,
Freudenberger ( '32) found that vaginal introitus occurred in
the Wistar strain on the average of 46.9 days, ranging from
36 to 66 days after birth. Similar results were obtained by
Blunn ( '39). By using the same method as Freudenberger
on the Long-Evans strain, he concluded that vaginal introitus
took place, on the average, at 39 days, ranging from 34 t o
45 days. Yet, as already emphasized, it must be remembered
that vaginal opening is not necessarily concommitant with
heat and ovulation.
SEXUAL MATURITY 1N T H E FEMALE RAT
203
The length of the first heat period in 138 females, which
includes the first period in the forty-one females examined
over consecutive cycles, averaged 9.07 hours, ranging from
1to 20 hours, and showing a standard deviation of 4.08 hours.
This average is approximately 5 hours shorter than the average length of heat (13.7 hours) previously reported for 609
heat periods observed in 114 mature females (Blandau, Boling
and Young, ,41). It will be shown later that with the onset of
the second reproductive cycle the length of heat increases
and becomes equal to the average length of heat in the mature
female.
It was observed in a number of animals that the copulatory
response of the first heat period does not begin as abruptly as
it does in the succeeding heat periods. Also the intensity of
the response is not as great in the pubertal female as in the
mature animal. I n nine animals “intermittent” estrous
periods were observed during the first heat period, while
none was observed in a group of forty-one animals examined
during the first seven succeeding cycles. “Intermittent ”
estrus is not, however a unique characteristic of the first
heat period, since it has been observed previously in a number
of mature individuals (Blandau, Boling and Young, ’41).
Observations were made on forty-one animals throughout
the first seven consecutive estrous cycles in order to determine whether or not the lengths of the heat periods or the
lengths of the successive reproductive cycles were altered
as the cycles became established (tables 1and 2). The length
of the 328 heat periods averaged 13.4 hours, ranging from 3
to 24 hours. This compares favorably with the average figure
of 13.7 hours obtained earlier on mature females of the same
strain.
Upon analyzing the varying lengths of consecutive heat
periods for each rat in order to test the hypothesis that there
is a zero mean difference between these heat periods, it was
found that the length of the first period shows a highly significant difference from the following seven heat periods. Beginning with the second period the mean length of heat remains
204
R. J. BLANDAU A N D W. L. MONEY
TABLE 1
Fhe length of heat in hours in rats observed over the first eight heat periods.
__
ANIMAL
5
12
13
14
15
18
20
25
28
30
34
37
41
43
44
55
57
59
69
75
76
89
90
106
141
147
148
154
170
178
192
193
200
201
205
206
238
243
253
257
261
Mean
U
1st
8
14
7
13
11
8
6
7
7
8
5
11
6
11
7
12
10
17
6
6
11
8
10
16
8
11
6
8
11
15
7
15
4
11
12
9
9
10
13
13
14
9.78
3.10
2nd
12
16
15
10
18
14
13
16
15
15
15
13
10
11
13
14
16
17
14
21
13
16
12
3
10
13
18
10
10
16
14
15
15
16
9
20
13
14
7
17
18
~
13.8
3.45
3rd
4th
11
8
13
12
16
11
10
8
11
13
13
18
12
17
15
17
13
16
10
16
10
12
20
8
17
11
18
9
7
11
17
16
18
11
12
13
14
10
9
13
18
13
12
16
13
11
12
14
14
13
11
13
10
16
24
15
6
15
14
15
16
19
17
16
15
15
16
12
14
15
15
10
16
11
18
12
17
16
11
13
17
8
9
9
15
18
20
13
15
10
12
16
16
20
17
17
18
16
18
17
11
16
6
12
7
14
11
15
14
13
14
14
16
11
19
16
14
7
10
11
15
22
15
18
10
16
14
18
11
13
16
13
16
14
14
15
16
17
13
15
17
21
20
17
21
14
15
14
21
1
2
_-__
14.9~14.4
13.5
~- __
3.52
3.05
3.55
14
11
16
9
15
15
15
16
13
17
13
18
14
17
13.6
3.89
~~
1
6th
5th
~
~~
7th
12
13
8
12
16
14
9
10
18
8
17
15
16
11
13
13
7
14
11
17
11
17
15
18
17
10
13
14
13
14
11
14
9
15
14
12
14
15
19
12
17
-~
13.4
2.97
8th
11
15
11
14
17
12
9
11
13
14
15
14
12
13
12
12
17
16
14
18
11
18
16
19
10
8
17
14
14
15
13
13
7
14
15
13
18
14
20
11
18
13.9
-.
3.00
MEAN
10.1
14.6
11.2
11.7
15.2
12.5
10.5
11.2
12.8
12.8
13.6
15.3
12.7
13.1
12.5
14.1
13.5
15.1
10.1
15.3
11.7
14.3
15.8
14.5
13.2
9.7
14.0
12.6
12.5
15.6
10.2
15.5
11.7
14.2
13.5
13.8
14.8
13.8
15.8
14.6
16.3
-~
~13.4
205
SEXUAL MATURITY IN THE FEMALE RAT
relatively constant throughout the consecutive reproductive
cycles (table 1). The mean length of heat in cycles 2 to 8
compares favorably with that previously reported (13.7 hours)
for mature animals (Blandau, Boling and Young, '41).
When the mean lengths of the first seven consecutive reproductive cycles are examined (table a), it is noted that the
mean length of the first cycle is at least twice that of any of
the succeeding six cycles. The second cycle also shows a
significant difference from the following five.
TABLE 2
T h e mean length of consecutive reproductive cycles.
NUMBER O F
ANIMALS
CYCLE
1st
2nd
3rd
4th
5th
6th
7th
11
1
1
MEAN LENGTH
IN HOURS
-
41
41
41
41
41
41
41
221.0
123.5
113.5
112.3
108.6
111.3
104.1
STANDARD
DEVIATION
-'
RANGE
I____
I
I
'
67.3
28.2
18.7
24.8
19.7
20.2
19.5
115 to 306
71 t o 191
69 to 170
83 to 194
72 to 167
69 to 169
65 to 167
The data in table 2 (from cycle 2 through 7 ) compare favorably with those previously reported for 567 cycles in mature
females of the same strain, in which the mean lengths of
consecutive reproductive cycles varied from 99.2 t o 105.2
hours (Blandau, Boling and Young, '41). The data are also
in agreement with the findings of Long and Evans ('22) in
which they observed that the first cycle, as determined by the
smear, was distinctly longer than the succeeding cycles.
The general picture of the length of the first heat period
and cycle, compared to succeeding heat periods and cycles,
indicates that the attainment of normal reproductive rhythmicity is a gradual process, but that once the hormonal mechanism is more thoroughly established, other factors being
equal, the animal will display a considerable consistency of
behavior.
An hourly record of the time of day at which the first heat
began in 200 females is shown in table 3. A comparison of these
206
R. J. BLANDAU AND W. L. MONEY
data with those previously reported by Blandau, Boling and
Young ('41) indicates that the majority of their mature
females came into heat between 4 P.M. and 1 A.M., while the
majority of our pubertal animals (165) came into their first
heat between 7 P.M. and 5 A.M. The data on the time of onset
of heat in cycles 2 through 7, in the forty-one females examined
in consecutive cycles, correspond more nearly to the time of
onset of heat in mature females in that the majority of these
animals also came into heat between 4 P.M. and 1A.M.
Since it is generally assumed that ovulation does not take
place before the establishment of the vaginal orifice, and
because there are no data in the rat on the temporal relationships between vaginal introitus, the first heat, and ovulation,
an investigation of these relationships seemed desirable.
TABLE 3
Hourly record o f the time of day at which heat was detected.
1 P.M.
2P.M.
0
1
0
'
N U N B E R OF
CASES
-
9
9 P.M.
10 P.M.
15
23
i
TIME O F
DAY
4 A.M.
5 A.M.
6 A.M.
7 A.M.
8 A.M.
i
NUMBER OF
CASES
11
13
7
5
0
1
2
2
Table 4 summarizes the temporal relationship between the
onset of the first heat and ovulation for ninety-two animals.
I n only one of eight animals examined between 3 and 4 hours
after the beginning of heat had ovulation occurred. By the
tenth hour after the beginning of heat, ovulation was in progress or had been completed in 100% of the animals. When
these data are compared with similar data for mature females
(Boling, Blandau, Soderwall and Young, '41), it is apparent
that ovulation is initiated somewhat earlier during the first
heat period. The suggestion is offered that the shorter ovulation time is directly related to the shortened total length
of the first heat period.
207
SEXUAL MATURITY IN T H E FEMALE RAT
If ovulation can be said to occur sometime between 5 and
10 hours after the beginning of heat, it is apparent that a
relatively small number of females ovulate before vaginal
introitus occurs (fig. 1).
The ovaries of all of the animals used in determining the
time of ovulation were examined microscopically in order
to make certain that only one group of eggs had ovulated.
Figure 2 is a typical example of the appearance of an ovary
removed 5 hours after the beginning of the first heat period.
Since there are no corpora lutea present from previous ovulations, the ovary as a whole appears to be relatively small.
The center of the ovary is filled with a large amount of loose
TABLE 4
Temporal relationship of heat and ovulation.
I
H O U R S AFTER THE BEGINNING O F HEAT
3-4
,
1
4-5
1
5-6
~
6-7
1
7-8
8-9
9-12
8
12
17
17
6
10
14
17
~~~~~~~
Number of animals
Number in which ovulation had begun or
was completed
8
16
1
9
~
1
connective tissue, and the maturing follicles bulge around the
periphery. Other sections show a number of follicles, both
large and small, undergoing atresia. The preovulatory follicular growth is progressing rapidly. The cells of the
cumuli are undergoing separation and in several follicles the
ova, surrounded by their cumuli, are floating free in the
antra. The secondary follicular fluid is making its appearance
and there is evidence of the infolding of the granulosa with
subsequent “pocketing.” I n all the ova the first polar bodies
had formed and the second maturation spindles were being
organized.
Figure 3 is a fortunate section through one of eight follicles
in an animal killed 7 hours after the beginning of the first
hea.t. The rupture area is just breaking loose and the ovum
with the surrounding cumulus and follicular fluid can be seen
208
R. J. BLANDAU A N D W. L. MONEY
streaming toward the rupture point. The first polar body is
formed and the second maturation spindle is completed.
Figures 4, 6 and 7 are sections through freshly ruptured
follicles from ovaries removed 10 hours after the beginning
of heat. I n figures 4 and 6 the ova surrounded by their cumuli
are adjacent to the point of rupture. The tension lines in
the follicular fluid at the time of rupture are clearly visible,
The observations of Minot (1891) on the guinea pig, and
Hammond ('14) on pigs and rabbits, showed that fecundity
is low during the early life of the female. The fewer offspring
born to young animals is due, according to Hammond, to the
smaller number of ova shed.
On the other hand, in swine, McKenzie ('28) found that
early breeding increased the litter size throughout the reproductive life of the animal.
Since it is possible to determine with considerable accuracy
the time of the first ovulation, we were interested in comparing
the number of ova shed during the first ovulation with the
number ovulated after at least six cycles had elapsed. I n
addition, data have been obtained on matings and artificial inseminations made during the first heat period.
I n fifty-two females killed at the end of the first heat period
the average number of ova recovered from the periovarial
sacs was 7.8 ova per animal, ranging from 5 t o 12. Thirty
females were killed at the end of heat after at least six
reproductive cycles had elapsed. The average number of ova
recovered from these sacs was 9.2 per animal, ranging from
6 to 13.
Of the thirty females in which observed matings were made,
twenty-one or 70% became impregnated. The average litter
size was 6.0, ranging from 3 to 10. Thirty o r 81% of the
thirty-seven females artificially inseminated during the first
heat period were impregnated. The average litter size in this
group was 6.5, ranging from 4 to 11. The average number
of corpora lutea observed macroscopically on the surface of
the ovaries of both mated and inseminated females was 8.5. In
the majority of cases parturition was observed. The only
SEXUAL MATURITY IN THE FEMALE RAT
209
respect in which parturition in young animals differs from
that of the mature animal is the greater tendency of the young
mother to kill and eat the young during and immediately
following parturition.
Fifteen females, in which the vaginal membranes ruptured,
failed to come into heat. I n all of these animals vaginal
tumescence was obvious at varying intervals during the period
of observation. The ovaries were subsequently removed and
examined. Figure 5 is a section of an ovary removed from
one of these females 15 hours after the first vaginal tumescence appeared. Five fresh corpora lutea are seen in the
section and five fresh ova were recovered from the periovarial
sac. Ovulation was definitely of recent occurrence. The
ovaries of eight of the fifteen animals revealed at least two
sets of corpora lutea, indicating that several ovulations had
taken place without the females coming into active heat. I n
six females the sections revealed only one set of corpora
lutea and follicles of varying sizes.
A number of females, in which vaginal tumescence occurred
without heat, were placed with vigorous males. Numerous
mating attempts, without intromission, were observed.
DISCUSSION
The time of attainment of sexual maturity in the female rat
may be defined as the earliest period at which the animal is
capable of being impregnated. Maturity involves the development of the genital tract to a point at which vaginal introitus,
sexual receptivity and ovulation occur in the proper sequence
so that fertilization and implantation are possible.
From the data which have been presented it appears that
in the majority of females the temporal relationship between
the onset of vaginal introitus, heat and ovulation is correlated
in such a way that normal reproduction is not delayed. However, we are in agreement with the conclusion of Long and
Evans ('22) that it would be a mistake to regard any single
phenomenon of puberty a.s a wholly reliable criterion of normal
reproductive ability. These authors were of the opinion that
210
R. J. BLANDAU A N D W. L. MONEY
in the majority of instances where vaginal introitus and the
first heat occur simultaneously, the vaginal membrane does
not rupture until estrus proper has ensued. I n only a very
few animals, in the present investigation, was vaginal introitus delayed until near the end of sexual receptivity or
longer.
The possibility exists that in some females the shortened
length of heat may not provide a sufficient opportunity for
mating, especially if sexual receptivity is of low intensity.
I n the present series low intensity of sexual receptivity was
observed only during the first heat period. I n the subsequent
heat periods the intensity of elicitable copulatory responses
was in every respect comparable to that of mature animals.
It is commonly acknowledged that pregnancy may not occur
in some females of an unselected group which is confined continuously with vigorous males. I n guinea pigs, observed by
Young, Dempsey, Myers and Hagquist ( '38), the occasional
failure of mating was attributed either to poor follicular development which resulted in failure of ovulation, or, in females
in which ovulation occurred normally, to a high threshold t o
the conditioning action of estrogen.
I n a more comprehensive study on the failure of mating
in the rat (Boling, Blandau, Rundlett and Young, '41), it
was revealed that in some females the ovaries appeared
normal, although mating was not successful. I n other animals
the histological appearance of the ovaries indicated a deficiency in hypophyseal stimulation. When these abnormal
females were tested for their sensitivity to estrogens, the
conclusion was reached that the basic factor in the failure
of mating was due to the decreased sensitivity to estrogens.
That age is an important factor in the degree of response to
estrogenic treatment is shown by the work of Hemmingsen
( '33) and Wilson and Young ( '41). According to Wilson and
Young the female rat attains a high sensitivity to estrogen
about the thirtieth day after birth, irrespective of the presence
or absence of the ovaries and uterus since birth.
SEXUAL MATURITY I N THE FEMALE RAT
211
In females which do not come into heat the present experiments show that vaginal tumescence and ovulation nevertheless occur. One might conclude that a higher estrogen level
is necessary to condition the animal for sexual receptivity
than to call forth the remaining reproductive phenomena.
Further emphasis is given this speculation by the observation
that all of the ovaries removed during the first heat period
showed normal follicular development, and ovulation was
either in progress or had been completed. However, in the
observations of Boling, Blandau, Rundlett and Young ( '41),
female rats were described in which follicular development
was poor o r abnormal but matings nevertheless occurred.
Many similar observations on other species have led to the
general conclusion that in the majority of animals heat and
ovulation occur concomitantly. I n certain individuals, however, heat is not dependent upon ovulation, nor ovulation
upon the manifestation of sexual receptivity.
The comparative data on the number of ova shed during the
first and sixth heat periods and on the matings and artificial
inseminations made during the first heat period are to a
certain extent in accord with those of Hammond ('14) for
the sow and the rabbit, as well as those of Asdell, Rogart and
Sperling ('41) for the rat. It is true that the number of ova
shed during the first heat period is significantly lower than
the number liberated in more mature females. However, the
average litter size of rats mated or inseminated artificially
during the first heat period does not vary greatly from those
obtained by normal matings or artificial inseminations of
mature females (King, '39; Blandau and Jordan, '41). I n
order to elucidate this point completely, it would be necessary
to compare the number of corpora lutea found in the ovaries
with the actual number of young born after first matings in a
large number of females.
SUMMARY
Two hundred and fifteen albino female rats were observed
from the time of weaning to sexual maturity. The onset of
212
R. J. BLANDAU A N D W. L. MONEY
sexual receptivity was used as a basis for the determination
of the temporal relationships between vaginal introitus, heat
and ovulation. I n addition, data were obtained on (1) the length
of the first period of heat and the first reproductive cycle
compared with succeeding heat periods and cycles; (2) the
time of day that the first period of heat began; (3) the number
of ova shed during the first and sixth cycles; and (4) the
effects of matings and artificial inseminations during the
first period of heat.
1. The temporal relationship between vaginal introitus and
the onset of the first heat in 174 females varied from 459 hours
before heat began to 72 hours after the onset of heat. In the
majority of animals vaginal introitus occurred from 48 hours
before the onset of heat to 12 hours after heat began.
2. The average age for the onset of heat in 200 females was
49.4 days, ranging from 37 to 67 days, with a standard deviation of 5.46 days.
3. The length of the first heat period as observed in 138
females averaged 9.07 hours, ranging from 1to 20 hours, with
a standard deviation of 4.08 hours. Nine cases of “intermittant” estrus were encountered.
4. Forty-one females were observed throughout the first
seven consecutive estrous cycles. The first heat period was
significantly shorter than the succeeding periods, and the
first and second reproductive cycles were significantly longer
than the remaining cycles.
5. The first period of heat began in the majority of females
between 7 P.M. and 5 A.M.
6. The first ovulation occurred between 5 and 10 hours after
the onset of the first period of heat.
7 . The average number of ova shed during the first heat
period was 7.8, ranging from 5 to 12. The average number
of ova recovered after at least six cycles had elapsed was 9.2,
ranging from 6 to 18.
8. Seventy per cent of the thirty females mating during the
first heat period were successfully impregnated. The average
size of the litters was 6, ranging from 3 to 10. Eighty-one
SEXUAL MATURITY I N THE FEMBLE RAT
213
per cent of thirty-seven females inseminated artificially bore
litters which averaged 6.5, ranging from 4 t o 11. The average
number of corpora lutea for both groups was 8.5.
9. Fifteen females failed to become sexually receptive even
though vaginal introitus and ovulation occurred.
LITERATURE CITED
ARAI, H.
1920 On the postnatal development of the ovary (albino r a t ) , with
especial reference t o the number of ova. Am. J. Anat., rol. 27, pp.
4 0 5-46 2.
ASDELL, S. A., R. BOGART
AND G. SPERLING
1941 The influence of age and rate
of breeding upon the ability of the female r a t to reproduce and raise
young. Cornell Univ. Agr. Exp. Sta., Memoir 238, pp. 3-26.
BLANDAU,R. J., A N D E. S. JORDAN
1941 The effect of delayed fertilization on
the development of the rat ovum. Am. J. Anat., vol. 68, pp. 275-291.
1941 A technique f o r the artificial insemination of the white rat.
J. Lab. and Clin. Med., vol. 26, pp. 1361-1363.
BLANDAU, R. J., A N D A. 12. SODERWALL
1941 Post-parturitional heat and the
time of ovulation i n the albino rat. D a t a on parturition. Anat. Rec.,
vol. 81, pp. 419-431.
BLANDAU,
R. J., J. L. BOLINGAND W. C. YOUNG 1941 The length of heat in
the albino r a t a s determined by the copulatory response. Anat. Rec.,
v01. 79, pp. 453-463.
BLUNN, C. T. 1939 The age of r a t s at sexual maturity as determined hy their
genetic constitution. Anat. Rec., vol. 74, pp. 199-213.
BOLING,J. L., R. J. BLANDAU,
B. RUNDLETTAND W. C. YOUNG 1941 Factors
underlying the failure of cyclic mating behavior i n the albino rat.
Anat. Rec., vol. 80, pp. 155-171.
BOLING,
J. L., R. J. BLANDAU,
A. L. SODERWALL
AND W. C. YOUNG 1941 Growth
of the granfian follicle and the time of ovulation in the albino r a t .
Anat. Rec., vol. 79, pp. 313-331.
BROWMAN,
L. G. 1937 Light i n its relation t o activity and estrous rhythms
in the albino rat. J. Exp. Zool., vol. 75, pp. 375-388.
EVERETT,
J. W. 1939 Spontaneous persistent estrus in a strain of albino rats.
Endocrinol., vol. 25, pp. 123-127.
FISKE,V. M. 1941 Effect of light on sexual maturation, oestrous cycles, and
anterior pituitary of rat. Endocrinol., vol. 29, pp. 187-196.
FREUDENBERGER,
C. B. 1932 A comparison of the Wistar albino and LongEvans hybrid strain of the Xorwiy rat. Am. J. Anat., vol. 50, pp.
293-349.
HAMMOND,
J. 1914 On some factors controlling fertility in domestic animals.
J. Agr. Sci., vol. 6, pp. 263-277.
HARQITT,G. T. 1930 The formation of the sex glands and germ cells of
mammals. J. Morph., vol. 49, pp. 277-331.
HEMMINOSEN,
A. M. 1933 Studies on the oestrus-producing hormone (oestrin).
Skad. Arch. Physiol., vol. 65, pp. 97-250.
214
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A. M., ~ K D
N. B. KRARUP1937 Rliythniic diurnal variations in
the oestroiis phenomena of the r a t and their susceptibility to light
and dark. K. Daiiske Videnskab. Selskab. Eiol. Meddel., vol. 13,
pp. 1-61.
KING, 11. U. 1916 Tlic relation of age t o fertility in the rat. Anat. Rec.,
vol. 11, pp. 269-287.
1939 Life processes in gray Norway r a t s during fouiteeu gears in
captility. Wistar Institute. Philadelphia.
I,ON(;, d . A., A N D IT. 31. ELINS 1922 The oestrous cycle in the r a t and its
associated phenomena. Mein. Unir. Calif ., vol. 6, pi’. 1-148.
MATTFRS, R. F., A N D 111. E. MacKkY 1934 A note on the maturation of the
female, black hooded r a t (Alus noraegicus). Austi:tlian J. Exp. Eiol.
and Mcd. Sci., 101. 1 2 , 1111.151-154.
M ( > I < m z i r ,F. F. 1028 Gronth and reproduction in sniiie. Missouii Agr. Exp.
Sta. R e ~ e x r c hBull. 118, pp. 1-67.
MINOT, C. H. 1891 Senescenre and rejuvenation. I. On the neight of guinea
pigs. J. Pligsiol., ~ o l 12,
. pp. 97-153.
SLON.U\ER,
J. R. 1924 The effect of pubescence, oestiuation slid nienopause on
tlie voluntary activity in the albino rat. Am. J. Pliysiol., vol. 68,
PI). 294-315.
WILSON,J. G., ~ N DW. C. YOUNG 1941 sensitivity t o estrogen studied by
nieaiis of experimentally induced mating response in the feinale guinea
pig and rat. Endocrinol., vol. 29, pp. 779-783.
The
YOI~SG,
W. C., E. W. DEXIPSEY,H. I. MYERSAND c. W. H ~ G Q U I S1938
T
ovarian conclition and sexual behayior in the f(ma1e guinea pig. Am.
J. Anat., vol. 63, pp. 457-487.
HLYXINGSEN,
PLATE 1
rxPL.m.\TIoN
OF FIGURES
2 Section through ovary removed 3 hours a f t e r the beginning of the first heat.
X 18.
3 Follicle i n wliieh ovulation is just beginning, remored 7 hours after the
onset of the first heat. x 75.
4, F and 7 Recently ruptured follicles removed 1 0 hours after the onset of
the first heat. The e x t r u d d ova :ire still near the point of rupturc in figures 4
a n d 6. X 75.
5 Section through ovary removed 15 lioiirs a f t e r the aplmmincc of vaginal
tumescence without subsequent heat. The condition of tlie five corpora hitea
indicates a n ovulation of rerent occurrence. X 18.
XUAL MATURITY I N TIIE F E M A L E RAT
R. J . I3LANDAll A N D W . 1,. M O N R Y
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