Observations were made on Albino rats over a period of four
years (1933-37); 238 litters were obtained comprising 2043 young at
birth; 1789 (872 males and 917 females) were reared and weighed
(a) Oestrus and service.
The length of the oestrous period was 4 days in 60 per cent of the
cases observed, 3 days in 20 per cent, and 5 days in the remaining
20 per cent cases. The time that vaginal plugs were visible varied
from 6 hours to 10 days after service, the average being 1 to 2 days.
Nearly all females were served during the night or early morning.
Quite a large number of the females that had bred, had infected
uteri. Here the ovaries were inactive except when only one horn of
the uterus was affected; then the ovaries were active but the females
did not become pregnant when served.
(b) Weights of pregnant females.
Different classes of females were used in studying the changes
in weight of pregnant females: (1) Those served when the vaginas
opened, (2) old virgins 8 to 11 months old and weighing about 200 g.
and (3) females that had already had one or more litters. Only those
pregnant for the first time showed an appreciable gain during the
first two weeks of pregnancy, while rats that had had litters showed
practically no gains. The weights of all females, however, increased
rapidly during the last 7 to 8 days of pregnancy which is mainly due
to the increase in weight of the developing foetuses. Only those rats
pregnant for the first time showed an appreciable gain (56 per cent.)
after parturition as compared with weight at service. This increase in
permanent gain was equal to the gain during the first two weeks of
Litter size at birth had no influence on the gains of the females
during the first two weeks, hut during the third week the gains of
the females increased with increase in litter size.
(c) Weight and number of foetuses.
Females of the three classes mentioned above were killed at
different stages of pregnancy. The pregnant uteri showed an increase
in weight only from the 6th or 7th day onward after service. By
means of the method we used, foetuses could be weighed only from
the 13th day onward, whereas the number of foetuses could be
determined macroscopically from the 7th to the 8th day of pregnancy.
The three different classes of rats showed large differences in the
number of prenatal deaths. In the case of the young females which
were about 3 months old and pregnant for the time, there were only
two dead foetuses out of 190, average number of living foetuses per
litter 9.4. In the case of the old rats 8 to 11 months, pregnant for the first time and of those of more or less the same age but which had
already had one or more litters, 38.5 per cent of dead foetuses were
found during the last two weeks, in the former class the average
being 4 living foetuses per female, in the latter 5.
LITTER SIZES AND WEIGHTS AT BIRTH.
During the first 24 hours the birth weights remained more or
less constant, or could show a slight decrease for young born during
Mortality at birth was 1.4 per cent. This figure may be
slightly higher since females often eat the stillborn. Average birth
weights of males and females born alive were 5.19 g. and 4.96 g.
respectively, and of stillborn 4.88 g. and 4.8 g. respectively. Average
litter size was 7.4 but for litters in which there were stillbirths
average size was 8.8.
Sex ratio for all rats born was 94.5 per cent. It decreased from
163 to 77 as litter size increased from 3 to 9, then increased again to
129 as litter size increased to 12.
Birth weights decreased with increase in litter size. After a litter
size of 9 was reached, birth weights showed a tendency to remain
Birth weights increased with age of mothers while litter size
decreased. When litter size remained constant then age of mothers
did not show any regular effect on birth weights.
Females 2 to 3 months old had average litter sizes of 8.4 and
those 12 to 13 months old 4.7. This continuous decrease in average
litter size from the youngest age class to the oldest, instead of the
expected increase to a maximum followed by a decrease, may have
been due to the greater prenatal mortality in the older females.
Total litter weight decreased from 43.3 g. to 25.0 g. Age of mothers
was more important in determining litter size than parity.
The weight of the mothers immediately after parturition did not
appear to influence the birth weights of the young when litter size
LITTER SIZES AND WEIGHTS AT WEANING.
The average weaning percentage of all litters was 84.0. There
was no pronounced drop with increase in litter size as was found in
the pig for instance. The average birth weights of young weaned was
5.11 g., but for young that died shortly after birth only 4.87 g.
Greatest postnatal mortality occurred during the first week after birth.
Litter size at birth showed the same influence on weights at 2,
3 and 4 weeks as at birth, i.e., first a decreasing decline in average
weight as litter size increased up to 9, then weights remained more
or less constant up to the largest litter size (12).
Although litter size at birth and at weaning showed practically
a straight hue, the influence of litter size at weaning on the average
weight was quite different and peculiar. There was first an increase,
then a decrease, another increase and finally a pronounced decrease.
The most likely explanation of this appears to be the effect on milk
supply of rats of different ages and weights, which were not kept
constant in this case.
Rats in litters of which all were reared were heavier than rats in
litters of which some were destroyed by the females.
The young of females 8 to 9 months old were the heaviest at
weaning, indicating that the milk supply was the highest at this
period. This coincided more or less with the production of the third
As mentioned the weight of the mothers did not influence the
average birth weights of the young. The weights of the mothers at
parturition, however, had a decided influence on the average weight
of the young at weaning. As the weight of the mother increased up
to 260 g. the average weight of the young decreased, and after that
increased again. This is probably due to difference in milk supply.
It appeared that only in the case of young females did increase in
weight of the mother accompany a decrease in the average weaning
weight of the young, whereas in the case of the old females there was
a tendency for the young to increase in weight with an increase of the
weight of the mother. Young heavy females therefore appear to be
poor milkers, probably due to excessive fat, while weight in old
females is more likely due to greater muscle and bone development.
Although the best females, i.e., those rearing the largest and
heaviest litters, were retained, it does not appear that the fertility
of the population had been raised during the four years. Selection
was probably mainly for good mothers, poor mothers being discarded.
This strain of albino rat has been in bred for so many generations that
the stock is probably nearly homozygous for the number of eggs shed,
showing that variation in litter size is probably only due to factors
such as foetal atrophy.
(1) Length of time females took to become pregnant.
(2) Litter sizes and weights at birth and weaning.
(3) Sexual maturity of females.
(4) Weights and tail lengths.
(1) From March to July, autumn and winter, the females remained
long with males before they became pregnant, but from
August to February it took a shorter time. After February there
was a sudden increase which may have been caused by the decrease
in length of days. Females appear to have been affected more than
(2) The average birth weights decreased from 5.19 g. during
the first year to 4.96 g. during the fourth year. Our average birth
weights were appreciably lower than those obtained at the Wistar
The average litter size was higher during the summer months,
November to January (7.8), than during the winter months, May to
July (6.8). Birth weights, when effect of litter size was eliminated,
did not appear to have been affected.
Weaning percentages showed a decided seasonal trend, being highest
between May and June, then decreasing until August and
remaining low until October, after which there was an increase.
During the four years the weights at 2, 3 and 4 weeks have
decreased. This may have been caused by the effect on milk production of the mothers as well as by the environmental influence
directly. There were decided seasonal fluctuations which appeared
to be about the same at 2, 3 and 4 weeks. The highest weights were
obtained during the winter, with a decline during the spring and the
lowest level coinciding with the warm months, October and November.
The weights of the suckling females showed about the same
seasonal trends as the weights of the young, so that milk supply may
also have been affected.
(3) The sexual maturity of the females was affected as follows.
During the first year the vaginas opened when the females were 47.2
days old and weighing 100 g . and during the fourth year when they
were 42.7 days old and weighing 95 g.
(4) When the average weights of our rats are compared with
those of the original stock at the Wistar Institute, our males are
appreciably heavier at the different ages up to 20 weeks, while the
females show practically no difference.
There was a marked seasonal effect on the weights. The highest
and lowest points shifted about 3 to 4 months as the rats become
older - 4 to 16 weeks - indicating the influence of the month of birth,
also that an early influence may persist until maturity. The highest
weights for all ages were obtained for those born during May to
August, then there was a decrease with the lowest weights from
about October to February. It appears that after 8 weeks the
ultimate weight of the rat is not affected any more, or only to a
small extent, by the environment under which it grew up.
Tail length was affected less than weight by the environment.
Although temperature may have influenced the weights of the
rats, it appears more likely that there may have been an effect by a
combination of factors, probably temperature and humidity, while
a change in the length of the days probably also affected the growth
of the animals through the effect on the pituitary.
(5) Lung trouble occurred in 12.2 per cent. of the males and
6.8 per cent. of the females. This trouble occurred mostly when rats
were l6 to 20 weeks old - later cases were not recorded. The disease was most prevalent during the winter months, 35.1 per cent. of the cases, spring following with 30.5 per cent. and summer and autumn
much lower (18.3 and 16.0 per cent. respectively).
GROWTH AND FOOD INTAKE.
Daily gains of suckling young showed an increase from birth
to 13 days, milk supply being then adequate, a standstill or decrease
in daily gains from 14 to 18 days, milk supply being the limiting
factor, and a continuous increase in daily gains from 19 to 28 days
after the young have become accustomed to solid food.
As litter size increased from 4 to 7, females required 40 g.
additional food for each additional young for the 28 days period and
98 g. for each additional young from 7 to 11.
Food utilisation of females was lower than that of males and
showed a much greater variability. When the same amount of food
was given individually to pairs of males and females of the same
weight, then there was no difference in the food utilization. The
difference was, therefore due to the more rapid growth of the males
when both sexes were fed ad lib.
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