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Litter size birth weight and early growth rate of mice (Mus musculus).

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L I T T E R SIZE, B I R T H WEIGHT, AND EARLY GROWTH
RATE OF MICE ( m sMUSCULUS)
W. H. GATES
Louisiana S t a t e EnCGersity, B a t o n Rouge, Louisiana
FOUR CHARTS
The following work was done in the Department of
Genetics of the Carnegie Institution, Cold Spring Harbor,
Long Island, during the summer of 1922, a t the suggestion
of Dr. C. C. Little, to whom thanks are due for helpful advice
and the use of his material. Dr. TV. E. Castle also has kindly
given many helpful suggestions concerning the analysis of
the data. Credit is due Miss H. Ulman for valuable aid in
the mechanical operations of weighing and recording.
The mice involved in this experiment were from the laboratory stock of Doctor Little and included the litters born from
the 21st of J u n e to the 1st of August, 1922, inclusive.
The object of the experiment was to secure individual
growth curves f o r the mice from the time of birth u p to the
time of weaning, three weeks after birth.
The method of procedure was as follows: All pens coiitaining pregnant mice were carefully gone over daily by
Doctor Little or his assistants and all new litters recorded.
The record of these was then handed to me. This record
included all young born before 9 o’clock in the morning. Ti1
a few instances, where parturition was not complete by
that hour, the litter was weighed and recorded later in the
day. TThen the record of the litters born had been received,
each litter was examined separately and each individual was
marked and then weighed.
Many different methods of marking were tried out before
a satisfactory one mas found. Staining with various dyes
183
T H E ANATOMIC4L RECORD, VOL. 29, NO. 3
JANUARY, 192.5
184
W.
H. GATES
was found not to be satisfactory, as the stains were either
worn off or licked off by the mother. Where the stain was
sufficient to stay for a day or more, it was lost soon after, due
to the shedding of the epidermis and the growth of the hair.
Finally, branding with a hot iron was tried and found to
be very satisfactory. I n fact, so much so, that several months
later Doctor I\IacDowell, who took over many of Doctor
Little’s mice, became greatly puzzled to find irregular spots
of aberrant color occurring on the pelage of some of the
mice, which were later found to have resulted from the
branding.
The number of each individual was indicated by branding
at a certain place on the body. Zero of each litter did not
receive a brand. One, two, and three were branded, respectively, on the left shoulder, left rib, and left hip; four, five,
and six were branded on the right shoulder, rib, and hip;
seven, eight, and nine between the shoulders, the middle of
the back, and at the base of the tail; ten, eleven, and twelve
were a t first branded in three successive positions on the tail,
but this was later given up as sometimes the young would
lose their tails, so that for these higher numbers double
brands were made, on tu7o of the regular positions. The
branding was done with a fine entomological scalpel, the
tip of which had been tempered and bent a t right angles. This
gave a small flat surface which could be touched t o the skin
without danger of injuring too large a surface. The scalpel
was heated almost to the red hot point on an alcohol lamp
and the mouse was quickly touched at the required spot. I n
nearly all cases the brand healed within twenty-four hours
and left a clean scar, which, of course, remained visible
throughout the life of the mouse. Frequently, as the mouse
grew and hair covered the body, the brand became covered,
but it could always be found by parting the hair. To facilitate
identification, after the second week the hair was cut around
the scar, so as to save the time in hunting the brand. Thus
every individual in the experiment retained its identity until
it was weaned or mated.
G I t O W T H O F YOTiXG MICE
185
The experiment in this particular report included 106
litters containing over 700 mice.
The young were weighed every other day-that is, at twoday intervals. Each individual was given a card on which
was recorded the weights, date of birth, litter number, the
number of dam, and also the total number in each litter, the
number of generations of brother-and-sister matings and any
other peculiarity o r abnormality. As any other interesting
coridition developed, it was also recorded.
Earlier writers, C u h o t and Schultze in mice, and C u h o t
( W ) , King ( %), King and Stotsenburg ( '15) in rats, have
noted that in these rodents the sex ratio shows an excess
of males varying from 106 to 110.9 males t o 100 females in
mice, and from 104.6 t o 107.5 males to 100 females in the rat.
King ('24) concludes that 105.2 : 100 -+ 2.00 is probably the
normal f o r albino rats, and the average for all strains of
1.35.
Nus norvegicus is 105.7 males to 100 females
I n this experiment there were 324 males and 328 females,
o r a ratio of 98.78 males to 100 females
5.21. The sex
in every case mas recorded a t the first weighing and was
later checked on the ninth o r eleventh day. As this experiment included litters born during a relatively short length of
time, this ratio may not, arid probably does not, represent
the normal f o r extended periods of time o r larger
populations.
The size of the litters shows considerable variation, the
numher in each litter varying from two to twelve. The general average, however, of the 106 litters is 7.4 -+ .14. The
size of litters with the highest frequency being six, seven,
eight, nine, and ten; these size litters containing 79 per cent
of all the individuals (table 1and fig. 1).
I n computing mortality statistics, particular care was
exercised not to include individuals which showed that death
was due to accident o r some unnatural cause.
I n reckoning time, the day of birth is considered as the
first day.
186
U
'
.
IT. GATES
A study of the mortality during growth (table 2) shows
just what one might expect-a rather high rate during the
first two days, then a slight decrease, followed by a gradual
rise up to about the seventeenth day, followed by a rather
sudden and decided drop up to weaning. This is shown
graphically in figure 2, the data for which are found in table
2. The high mortality f o r a day or so immediately following
birth is to be expected. The gradual rise in mortality during
the next tu7o weeks is to be accounted for probably by the
crowding of the young and the lack of sufficient nourishment
TABLE I
TABLE 2
Latter size
Mortality
~~~
\ I Y B F K I N FAVH
LITIFR
h i v n b H OF
I IT1 E.Rb
2
3
4
1
3
4
6
3
1
9
6
17
7
16
a
17
9
18
10
11
11
1
12
6
Average, 7.4 & .14
)
P E K CENT
IJtYS
7
9
11
13
1.i
17
19
1'
678
674
671
0
10
656
6
16
62 7
611
,579
3 .2i
rill
496
471
4
11
16
24
34
13
4
0.0
1.4
0.5
0.8
2.4
1.7
2.6
4.1
6.1
2.9
0.8
f o r all. This seemed t o be the case particularly in the larger
litters.
A study of the mortality is interesting also from the fact
that the young mice were liaiidled a good deal. On the day
of birth the young were first taken out of the cage, examined
closely by Doctor Little or his assistants, a little later they
were taken to the weigliing room, branded, and weighed individually, and then returned. All of this more or less rough
treatment and interfering with the duties of the mother did
not sccm to affect the mortality, because if it were going to
have any effect a t all it would be most likely t o show on the
very young ; whereas, in reality, the highest mortality rate
187
GROWTH O F YOUNG MICE
was at the end of the second week at a time when the young
were certainly old enough and strong enough not to be
af'fected by handling. The highest death rate came about the
time the mice were beginning to take care of themselves and
.--
LITTER SIZE
Dota in table1
15
E
s- 10
4
0
z0
5
2
3
4
5
6
7
NO in
0
9
1 0 1 1
1 2 1 3
litter
Figiire 1
Deuths
oL Mortality
t o feed independently. They quiclrly became accustomed to
the change in diet, however, and in a few days were on tlieii
feet and began t o increase in weight. Stillbirths were not
included in any p a r t of the work.
188
\V. 13. GATES
The total average rate of growth is shown graphically in
figure 3 (data in table 3). Note tlie rapid increase in weight
daily up to the ninth day, then a gradual lessening up to tbc
seventeeiitli day, and then a final pick up again, np to the
time of weaning.
Considering these increases from the standpoint of percentage of gain to that of weight, we find that the greatest
per cent of iiicrcase is from the third to the fifth day (table 3,
fig. 3 ) . As stated above, the probable reason f o r tlie subsequent decrease may be explained on the ground of insufficieiit food and perhaps crowding in some litters. Note the
T41jLE
?
Growth weaghis, totals
~
~~
D LPCI
~
51). I N D .
I\
EIGHC
GAIN
~
1
3
>
7
9
11
13
13
17
19
21
1.3ti
tii8
674
671
6.5ti
G27
611
379
1.80
4.4.5
3.21
3.94
4.;i3
5-58
G.T,G
.44
.Gti
.76
.73
.61
.R7
.34
511
496
471
.>.so
.34
G.37
6.89
.47
52
5.12
32
36
31
20,
13
12
G
G
8
8
final increase both in actual weight and percentage of gain
to body weight a s the animals begin to feed independently.
Comparing the males aiid females, we find that the general
average weight at birth is tlic same. Miller ('11) reports
that tlie average weight a t birth of males of the Norway rat
is in excess of that of the females. King ( ' G , '23) also
reports a relative weight of 5.34 grams for males and 5.09
grams f o r females in the Norway rat, and a relative weight
of 4.69 grams and 4.5 grams for the albino rat, male and
female, respectively, at birth. This experiment shows a
practical equality of weight at birth of the males and females
in mice.
GROWTH O F YOUXG MICE
189
190
W. H. GATES
The weight recorded as at birth is that after the young
liave nursed. The actual birth weight would be slightly
lower. This weight, however, is taken because it represents
a similar condition in all subsequent weighings.
It is interesting to note, however (table 4 and fig. 4), that
from the third day on the females maintain a slight advantage
over the males not only in actual weight, but i n the amount
of gain per day. From the nineteenth day, however, they
begin to lose their advantage and the males tend t o overtake
them. If the experiment had been carried further, no doubt
the males wonld sooner or later have outweighed the females.
1
3
1
7
9
11
13
1 .?
1i
19
21
324
330
328
326
320
313
“99
286’
266
262
243
1.38
1.79
2.46
319
3.89
4.50
3.06
3.49
3.80
6.22
6.78
.41
.67
.72
.70
.60
.53
.43
.31
.41
.56
29
37
29
21
15
12
8
328
323
325
320
306
298
278
266
)
048
7
9
239
228
1.38
1.88
2.32
3.27
4.00
4.60
5.17
5.64
6.00
6.43
7.01
50
.63
.7n
.72
.60
.57
.16
.33
.43
37
36
33
29
22
1.5
12
8
6
7
8
King (’1!9-’24) finds similar conclitio’ns in the early life
of both the Norway and albino rat.
Table 5 shows a n interesting relation between the weight
of the young and the size of the litter. The smaller the litter,
the greater is the first-day weight of the young. This may
he due in p a r t to their having received a greater per capita
amount of milk on the first day, in p a r t to their having received more nourishment in utero. The maximum average
first-day weight is that f o r the single litter which consisted
of two young. The first-day weight then decreases steadily,
a s the number in the litter increases, until it reaches a
minimum in litters of nine, beyond which point it is prac-
G R O W T H O F YOUNG MICE
192
W. R. GATES
tically stationary. Tlie average weight a t twenty-one days
is also largely influenced by litter size, as one might expect
from the combined influence of a greater amount of nourishment in utero and in lactation to individuals born in small
litters. No differential mortality is observed between large
and small litters. I n general, individuals born in small litters
are larger at birth and at twenty-one days than those lion1
in large litters, but it is not t o be supposed that this initial
advantage mould result in greater adult weight. (See Castle
( ' 2 2 ) , who has investigated this matter in the case of
rabbits.)
TABLE 5
Kclation between latter s w e and wezght, ut bzrth anti on twenty-first day
-~
\CVRI~ILI\
~
IIICIR
'
~~
2
~
3
4
, 5
6
7
1
8
1
9
10
11
12
~
I
'
1.87 j1.78 1.66
>kverngeaeiglrt 1st day
Average atlight 21st day 6.85 ,8.55 8.76
Kumber yonng 1st day
2
11 21
Sumt)eryoung 21stdar . 2
6 1 11
Pcr cent diminution. . . . 0 I 45 47
I
1.42
7.35
40
30
25
1.42 1.41 1 33 1.27 1 34 1.33
6 $157.20 6.92 6.47 6.54 6 30
90 96 129 134 97 11
ti5 61
91 80 74 10
27 36
29
48 23
9
I
1.32
6 i9
70
40
42
SUMMARY
The statistics in this paper involve 106 litters containing
over 700 individuals of mice born during the summer of 1922.
The sex ratio is practically 100 : 100.
The relative birth weights of males and females are equal.
The rate of growth of the females is slightly more rapid
than that of the males, at least for the first three weeks.
The mortality is greatest as the young begin to stop
nursing.
The rate of growth shows a gradual increase at first, followed by a slackening about the nineteenth day, and a final
increase as the young become independent.
Litter size influences weight of the individuals both on the
first day and on the twenty-first day after birth, through
the availahle food supply, but there is no differential mortality favoring either small or large litters.
GElOWTH O F Y O U N G MICE
193
L I T E R A T U R E CITED
CASTLE,W. E. 1!)22 Size inherit:ince in rabbit crosses. Pnbl. 110. 320. Carnegie
Inst., Wash., Pt. I.
h C x o r r , L. lS9!1 Sur 1:i (1ctennin:ition du s e w rliez les :inim:iux. Bull. Sci.
de la France e t d e la Belgique, T. 32.
KING,H E L ~ S
1)u.N
1918 The eff'ects of inbreeding on t h e growth and vari:iljility in the ljody weight of t h e nlbino rat. Jour. Exp. Zool., vol 2 6 .
1922 The growth and variability in the bocly weight of t h e Norway
rat. Anat. Rec., vol. 25.
1924 Litter production and the sex ratio i n various str:iins of rats.
Anat. liee., vol. 27.
Kisc;, 1%.D., AND STOTSENBUBG,
J . M. 1915 On the normal sex ratio and t h e
size of the litter i n the albino rat. Anat. Rec., vol. 9.
~ I I L L EIf.
R , 1911 Reproduction in the brown rat. Am. Nat., vol. 43.
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