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Variability in body length body weight and organ weights of the rat.

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VARIABILITY I N BODY LENGTH, BODY WEIGHT,
AND ORGAN WEIGHTS O F THE RAT1
CLAY B. FREUDENBERGER
Department of Anatomy, University of Utah, Salt Lake City
Jackson ('13) made a rather extensive study of the variability of the body and the various organs in the albino rat.
The albino rats which he used were obtained from various
fanciers and fed a varied diet. A supply of chopped corn
was kept constantly in the cages, a liberal amount of wheat
bread soaked in whole milk was supplied daily, and fresh meat
(beef) once a week. While it would seem that this diet should
contain the essentials for good nutrition, it is well known that
the rat thrives much better on our more modern diets. The
rats which I have used in the present study are from strains
which havc been caged and well cared for for many years.
It is of interest to see how domestication, close breeding, a
well-balanced diet, and excellent care f o r a number of years
will affect the variability.
MATERIALS AND METHODS
Rats of both the Wistar albino and the Long-Evans hybrid
strain were used. All rats were given the same excellent
care and fed the same diet. The diet consists of: casein, 15
per cent; whole milk powder, 10 per cent; sodium chloride,
0.8 per cent; calcium carbonate, 1.5 per cent; butter (unsalted), 5.2 per cent; and whole ground wheat, 67.5 per cent.
A constant supply of the diet was kept in the cages, and a
constant supply of clean tap water was provided.
Contribution from the Department of Anatomy, University of Ninnesota,
Minneapolis, and from the Department of Anatomy, University of Utah, Salt
Lake City.
47
48
CLAY B. FREUDENBERGER
All young weighing less than 35 gm. were usually discarded
at weaning time ( 3 weeks). Five hundred and eighty-five rats
were completely autopsied with organ weights, and forty-five
other animals were incompletely autopsied t o get certain
organ weights to fill out incomplete groups. Animals were
autopsied at birth and (approximately) at 3 weeks, 12 weeks,
and 1year of age.
The number of rats of each sex, strain, and age group
(30 to 50 in each group) was given in detail in a previous
paper (Freudenberger, ’32). The details a s to the origin
of the two strains of rats used, method of autopsy, etc., were
also given in the paper just mentioned, so they will not be
repeated here.
The stomach and intestines (with their contents), mesentery, and pancreas are spoken of as the ‘alimentary group.’
OBSERVATIONS
The coefficients of variation for body length, body weight,
and each of the organ weights are given in tables 1 and 2.
The probable error of each coefficient has been computed, but
they are not included in the tables. Since there are no consistent sex differences, the values for the two sexes have been
averaged in the following discussion.
B o d y lemgth
Variability in body length for the Wistar strain is lowest
at 3 weeks (the coefficient being 2.9) and is only slightly
higher at 1 2 weeks (3.2). Body length is more variable at
1year (3.6) and is most variable in the newborn group (4.7).
The situation is different in the case of the Long-Evans strain.
Here, variability in body length appears highest at 3 weeks
(4.2), and it is only slightly less in the newborn group (4.0).
It is lowest at 12 weeks (2.9). The coefficient at 1 year is
the same as it was for the Wistar strain at the same age (3.6).
The chief difference in the variability in body length between the two strains is at 3 weeks, the body length in the
Wistar strain being least variable at this age and the body
m
-1
X
Body length
Body weight
Head
Integument
Humerus
Femur
Brain
Spinal cord
Eye balls
H ypoph ysis
Suprarenals
Thyroid
Thymus
Alimentary group
Stomach
Intestines
Submaxillary glands
Liver
Spleen
Kidneys
Heart
Lungs
Ovaries
Uterus
Testes
Epididymides
Average
of viscera
_.-
I
1
I
iS.99
17.16
....
1 ... .
j
19.43
47.78
24.82
22.64
15.06
12.27
33.39
18.45
18.07
15.36
I
I
I
~
'
1
1
'
17.64
.. ..
1
15.22
14.19
1
i
'
~
.... 1
4.16
13.00
6.83
13.28
7.63
13.48
3.57
6.35
4.07
1,5.46
13.33
16.68
17.98
20.81
14.32
19.85
12.66
12.49
25.83
13.89
14.48
11.98
__
Males
....
.'..
. . . . ' 18.98
21.51
57.45
20.08
21.92
14.13
9.81
27.16
15.77
21.47
10.56
21.81
....
9.55
10.32
11.51
30.76
18.25
8.57
12.95
12.75
33.73
20.20
I
3.50
7.65
10.43
5.92
9.85
9.10
13.92
....
TABLE 1
11.98
....
....
1.64
9.04
5.16
12.62
7.49
9.41
3.61
6.34
4.29
18.36
9.58
21.99
16.56
17.81
12.07
16.84
11.93
8.58
18.32
11.33
12.68
8.02
11.09
12.09
Females
,
~
11.74
20.35
13.15
....
. . ..
3.57
9.77
8.41
12.95
8.02
9.02
4.17
7.67
3.95
14.32
8.48
24.84
26.89
15.88
9.78
15.58
11.00
13.26
13.68
13.66
11.89
10.37
I
1
I
,
'
12.49
....
....
2.86
6.71
5.45
5.72
5.12
6.70
3.56
6.75
3.89
13.77
15.11
17.07
18.01
13.07
11.62
15.84
9.36
10.43
11.60
10.17
7.23
17.60
13.85
26.37
Coe.ficienls of variation in the Wistar strain
17.89
17.50
17.30
....
....
4.29
14.13
9.53
15.39
9.92
10.90
4.46
7.19
3.51
17.48
12.74
18.89
39.73
18.99
13.10
17.46
16.70
17.32
21.58
12.98
12.11
42.42
2.85
9.94
6.09
10.80
7.74
9.65
4.78
5.35
2.37
19.46
19.68
21.92
31.46
17.47
10.60
13.20
10.96
15.12
17.05
12.71
11.57
43.01
3.60
10.01
7.63
11.59
8.67
3 1.62
5.28
7.87
5.79
20.42
14.92
20.40
23.95
26.16
14.55
17.92
12.73
12.66
21.08
13.87
13.69
19.92
*H
m
i
M
50
CLAY B. FXEUDENBERGEZ
. -
.":
'' 01
C
. .
. .
. .
.0.1
. m,
' ( 3
'ri
m
a
VARIABILITY I N T H E RAT
51
length in the Long-Evans strain being most variable. As has
been shown by Freudenberger ( '32), the Long-Evans animals
grow much more poorly from birth to 3 weeks of age than
they do later. This may account for some of the high variability in body length at 3 weeks of age in the strain. The
mean coefficient of variation f o r body length (all age groups
combined) is 3.6 in the Wistar strain and 8.7 in the LongEvans strain.
B o d y weight
Variability in body weight for the Wistar strain is lowest
at 12 weeks (the coefficient being 8.2) and is only slightly
higher a t birth (8.8). It appears highest at 1 year (12.0)
and is only slightly less at 3 weeks (11.0). I n the Long-Evans
strain, variability in body weight is lowest a t birth (6.5) and
is highest at 1 2 weeks (8.6). I t is highest at 1year (14.2) and
slightly less at 3 weeks (10.6).
Jackson ('13) found variability in the body weight of the
albino rat t o be lowest at birth and highest at 3 weeks. This
agrees fairly well with these results. The highest variability
in this study was found to be at 1 year. However, Jackson
did not have any figures for rats at this age. Although variability in body weight of the Wistar animals was found to be
less at 12 weeks than at birth, the difference in the coefficients
is very small (statistically insignificant).
The mean coefficient of variation for body weight (all ages
combined) was found to be 19 by Jackson. I n this study
the mean coefficient of variation for body weight is 10 in the
case of both strains. Thus, the animals used in this study
are only about one-half as variable in body weight a s those
studied by Jackson. This is probably due to the more homogeneous stock and more uniform environment in the present
colony of rats.
The results of this study do not agree with those of King
( 'E),
since she found variability in body weight of the Wistar
albino rat to be greatest at 60 days. They do agree more
closely with King's results than those of Jackson as to degree
52
CLAY B. FREUDENBERGER
of variability for the body weight in general. She found the
mean coefficient of variation (all ages) for the male rats to
be 13.6 and that for the females to be 12.1. The mean coefficient of variation for the body weight (four age groups combined) for our Wistar males is 11.7; Wistar females, 8.3;
Long-Evans males, 10.5 ; and Long-Evans females, 9.5. These
results also agree well with those of Smith and Bing ('28),
who found the mean coefficient of variation for body weight
at all ages for albino rats to be 11.9 for the males and 10.5
for the females. Jackson ( '13) likewise found the variation
higher in the males, but concluded that the apparent sex difference is of doubtful significance. The present data are in
accordance with this conclusion.
Head
The coefficient of variation for the weight of the head in
the Wistar strain is highest at birth (9.8) and lowest at 3
weeks (6.0). It is slightly higher at 12 weeks (6.9) than at
3 and higher at 1year (7.8) than at 12 weeks. These same
relations hold in the Long-Evans strain. Jackson ( '13) found
the coefficient of variation for the weight of the head to be
highest at 3 weeks of age. The average coefficient of variation (four ages combined) for the weight of the head in the
Wistar strain is 7.6, while it is 7.7 in the Long-Evans strain.
Jackson found the average coefficient of variation f o r the
weight of the head in albino rats to be 12. Thus the head is
much less variable in weight in the rats used in this study
than those studied by Jackson 20 years ago.
Tnt egumemt
The coefficient of variation for the weight of the integument
in the Wistar rats is lowest at 1 2 weeks (9.3) and highest at
1 year (13.1). It is higher at birth (11.0) than at 12 weeks
and higher at 3 weeks (13.0) than at 12. These same relations hold in the Long-Evans strain. The average coefficient
of variation (age groups combined) for the weight of the
integument in the Wistar strain is 11.6 and in the Long-Evans
strain is 12.7.
VARIABILITY IN T H E RAT
53
Skeleton
The coefficient of variation for the weight of the left
humerus of the Wistar rats is highest at birth (11.7) and lowest at 12 weeks (6.6). It is higher at 3 weeks ('7.6) than at
1 2 weeks and higher at 1 year (8.8) than at 3 weeks. I n the
Long-Evans strain the coefficient is highest at 1 year (11.6)
and lowest at 12 weeks (8.0). The average coefficient of variation (age groups combined) for the weight of the left humerus
is 8.7 in the Wistar strain and 9.6 in the Long-Evans strain.
The coefficient of variation for the weight of the left femur
is highest in the Wistar strain at birth (16.9) and lowest at
12 weeks (8.4). It is higher at 1year (10.3) than at 12 weeks
and higher at 3 weeks (11.5) than at 1year. This same relationship holds in the Long-Evans strain. The average coefficient of variation (age groups combined) for the weight of
the left femur is 11.6 in the case of both strains.
There is, thus, a definite tendency in the case of both the
humerus and femur for the coefficient of variation to be highest at birth and lowest at 12 weeks. The femur appears to
be more variable than the humerus in every case.
Alimentary group
The coefficient of variation for the weight of the alimentary
group of the Wistar rats is highest at birth (52.6) and lowest at 12 weeks (14.5). It is higher at 1 year (18.2) than at
1 2 weeks and higher a t 3 weeks (19.3) than at 1 year. The
coefficient is also highest in the Long-Evans strain at birth
(37.2) and lowest at 12 weeks (15.6). It is highest in the newborn, since only a part of the rats had suckled. The average
coefficient of variation (all ages combined) for the alimentary
group in the Wistar strain is 26.2 and 23.1 in the Long-Evans
strain. Jackson ('13) found an average coefficient of 35 for
the weight of the intestinal canal (plus contents) in the albino
rat.
54
CLAY B. FREUDENBERGER
Viscera
The coefficients of variation f o r the weight of each of the
individual organs are given in tables 1 and 2. The average
figures only will be discussed here. The average coefficient of
variation f o r the viscera of the rats of the Wistar strain is
highest at birth (18.3) and lowest at 12 weeks (12.8). I t is
only slightly higher at 3 weeks (13.1) than at 1 2 weeks, and
it is higher at 1 year (17.3) than at 3 weeks. I n the LongEvans strain the average coefficient of variation for the
viscera is highest at 1 year (19.9) and lowest at 12 weeks
(14.4). I t is somewhat higher a t 3 weeks (15.4) than at 12
and slightly higher a t birth (15.7) than at 3 weeks. The
tendency, therefore, in both strains is f o r the coefficient of
variation for the viscera to be highest a t either birth or 1
year and f o r it t o be lowest at 12 weeks. The average coefficient of variation (all ages combined) for the viscera is
15.4 in the Wistar strain and 16.3 in the Long-Evans strain.
Jackson ('13) found the average coefficient of variation for
the viscera in the albino rat to be 25. He did not include a
study of all the viscera reported here. Therefore, I computed
the average coefficient of variation considering only those
viscera which he reported. The results gave practically the
same average a s that found for the complete group. It is,
therefore, evident that the viscera are much less variable in
the rats included in this present study than those reported
by Jackson ('13).
I n the Wistar strain, the brain, spinal cord, and eyeballs
form a group of low variability (average coefficient, 5 to 8).
The suprarenal glands, stomach, intestines, submaxillary
glands, liver, kidneys, heart, lungs, and ovaries form a group
of moderate variability (average coefficient, 13 to 20). The
hypophysis, thyroid, thymus, spleen, and uterus form the
most variable group (average coefficient, 20 to 26). The organs of the Long-Evans animals fall in the same groups, with
the exception of the ovaries which in this strain fall in the
group of highest variability.
VARIABILITY I N T H E EAT
55
DISCUSSION
Jackson ('13), in his study of the albino rat, found variability in body weight a s well a s the average coefficient of variation f o r the viscera t o be lowest at birth and highest at
3 weeks. As will be noted, there is a definite tendency f o r
the coefficient of variation for body length, body weight, and
the organ weights studied here to be highest at either birth
o r 1 year and to be lowest at either 3 or 12 weeks. It i s
difficult to say why Jackson found variability to be highest at
3 weeks and I found it to be lowest at that age. Jackson's
rats grew very poorly from birth t o 3 weeks. They averaged
only 19.55 gm. in weight a t 20 days, while my Wistar albino
rats averaged 39.03 gm. at 3 weeks of age. The diet used
by Jackson may not have supported lactation properly, resulting in some of the young getting enough food and others
not being properly nourished. This would, of course, result
in high variability a t 3 weeks. Jackson's albino rats aver,aged 112.6 gm. in weight a t 10 weeks, while mine averaged
171 gm. at this age. It is apparent that his animals grew
quite poorly. This may be the cause of high variability at
3 weeks and at later ages. I discarded all rats at 3 weeks
of age which did not weigh 35 gm. This probably is a factor
tending t o lower the variability at 3 weeks.
The coefficients of variation recorded here are much lower
than those found by Jackson for albino rats in 1913. Undoubtedly, such factors as domestication, improved diet, uniformly good care, close breeding, etc., have been very important factors in bringing about this lowered variability.
Variability in many cases is only about one-half that found
by Jackson. These reduced coefficients of variation indicate
clearly that a great advance has been made in the task of
producing a standard r a t for use in experimental work.
Much credit for this is, of course, due to the constant efforts
of The Wistar Institute of Anatomy and Biology, Philadelphia.
The fact that no consistent differences were found between
the coefficients of variation of the two strains of rats used
56
CLAY B. FREUDENBERGER
is not surprising, since both have been maintained as pure
strains under excellent conditions for many years.
SUMMARY
The coefficients of variation for body length, body weight,
weight of head, integument, humerus, femur, alimentary
group, and viscera have been computed f o r both the Wistar
albino and the Long-Evans hybrid strain of the Norway rat.
Animals were autopsied at birth and a t (approximately)
3 weeks, 1 2 weeks, and 1 year of age. The average coefficient
of variation (all ages and sexes combined) for each in the
Wistar strain is as follows: body length, 3.6; body weight,
10; head, 7.6; integument, 11.6; humerus, 8.7; femur, 11.6;
alimentary group, 26.2 ; and viscera, 15.4. Variability tends
to be highest at either birth or 1 year and to be lowest at
either 3 or 1 2 weeks.
Considering the individual viscera of the rats of the
Wistar strain, the brain, spinal cord, and eyeballs form a
group of low variability (average coefficient, 5 to 8). The
suprarenal glands, stomach, intestines, submaxillary glands.
liver, kidneys, heart, lungs, and ovaries form a group of
moderate variability (average coefficient, 13 to 20). The hy.
pophpsis, thyroid, thymus, spleen, and uterus form the most
variable group (average coefficient, 20 to 26).
There are no consistent differences in coefficients of variation between the two sexes or the two strains.
The coefficients of variation are found to be consistently
much lower than those reported by Jackson ('13) for albino
rats. This would seem to indicate that some success in
standardizing the rat has been made.
L I T E R A T U R E CITED
FREUDENBERQER,
CLAY B. 1932 A comparison of the Wistar albino and the
Long-Evans hybrid strain of the Norway rat. Am. J. Anat., vol. 50,
pp. 293-349.
JACKSON,
C. M. 1913 Postnatal growth and variability of the body and of
the various organs in the albino rat. Am. J. Anat., vol. 1.5, pp. 1-68.
KING, HELENDEAN 1915 The growth and variability in the body weight of
the albino rat. Anat. Rec., vol. 9, pp. 751-776.
SMITH,A. H., AND F. c. BING 1928 Improved rate of growth in stock albino
rats. J. Kutrition, vol. 1, pp. 179-189.
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