Body composition of mice subjected to genetic selection for different body proportions

Comp. Biochem. Physiol., 1972, Vol. 42B, pp. 679 to 691, Pergamon Press. Printed in Great Britain

BODY C O M P O S I T I O N OF M I C E S U B J E C T E D T O G E N E T I C S E L E C T I O N F O R D I F F E R E N T BODY PROPORTIONS N. J. DAWSON, 1. S. K. STEPHENSON * and D. K. FREDLINE 2 1Depariaa~ent of Physiology and *Department of Agricultural Biology, University of New England, Armidale, N.S.W. 2351, Australia

(Received 11 October 1971) A b s t r a c t - - 1 . House mice (Mus muscuhts) were subjected to genetic selection for

different body proportions. 2. The gross chemical composition of the body of animals from each selection line was determined after twelve and fifteen generations (Experiments 1 and 2, respectively). 3. Analysis of variance demonstrated highly significant between lines differences in all constituents in Experiments 1 and 2. 4. Analysis of eovariance (body weight is eovariate) demonstrated highly significant between lines differences in water and fat, but not in fat-free combustible matter or ash in Experiment 1, and highly significant between lines differenees in all constituents in Experiment 2. 5. The physiological and genetic implications of these results are discussed. INTRODUCTION IT hAS been stated that knowledge of " . . . the composition of the body and its t i s s u e s . . , remains the foundation upon which most other biological studies must be built". (Widdowson & Dickerson, 1964.) The study of body composition can make considerable contributions to animal and human biology (Garn, 1963; Pearson, 1963), to physiology (Kyle, 1963) and to medicine and surgery (Moore, 1963). Such knowledge is essential for the study of energy exchange ( Gopalan et al., 1955; Bailey et al., 1960; Joy et al., 1967; Dawson, 1970a). A relatively recent review of current advances in the study of body composition (Bro~.ek, 1961) did not, however, note any work relating to the effects of genetic selection and inbreeding. There have been a number of investigations on breed differences in anatomical (i.e. dissected) composition in domestic animals, and these have been reviewed by Berg (1968), Bichard (1968), Bowman (1968), Lucas (1968), Spedding (1968) and Taylor (1968). The relationship between inbreeding and gross chemical composition of the body has been studied recently by Dawson (1970a, b). The present paper deals with an investigation of the effect of genetic selection for different body proportions on the gross chemical composition of the body of the house mouse; a preliminary report of this work has appeared (Dawson et al., 1969). # Present address: Department of Physiology, School of Medicine, University of Auckland, Private Bag, Auckland, New Zealand. 679

680

N . J . DAWSON, S. K. STEPHENSONAND D. K. FREDLINE MATERIALS AND METHODS

Animals T h e animals used were adult male and female house mice (]Flus musculus) housed in an air conditioned mouse colony at 22°C, with a 14 hr light and 10 hr darkness r6gime; they had ad lib. access to water and to pelleted ration (Drug Houses of Australia Ltd.). There were five lines: an outbred line unrelated to the selection lines, named Quackenbush; four selection lines, namely a random bred control line (CL), and high (HL), low (LL) and "kangaroo" (KL) lines.

Genetic selection of lines of mice Four of the lines referred to above were produced by two of the authors (S. K. S. and D. K. F.) using genetic selection based on external measurements made on the living animals. T h e selection was carried out using an index of the form I = 5"0677RU + 0 " 3 6 7 7 T F - z~/(BW), where R U is radius-ulna length, TF is tibia-fibula length and B W is body weight. T h e regression coefficients were calculated in such a way as to maximize the relationship of RU , TF and B W to the proportion of muscle in the body, so that a high index number indicated a high proportion of muscle and vice versa. T h e correlations used to calculate the above relationships were phenotypic, not genotypic. Selection using the index I was carried out as follows : (1) A high index number as the basis of selection (HL), with 25 mated pairs. (2) A low index number as the basis of selection (LL), with 25 mated pairs. (3) A control line (CL) maintained as far as possible by taking one male and one female from each litter and mating them randomly, subject to the restriction that full sib matings were excluded. In practice, because some matings in every generation were sterile, some litters had to provide an extra male or female to the parents of the next generation. There were 100 mated pairs. (4) T h e ratio tibia-fibula/radius-ulna length was used as the basis for selection in one line called "kangaroo" (KL), where individuals with the highest ratio were selected for breeding. T h er e were 25 mated pairs.

Experimental design T h e work described in this paper was carried out in two stages, which will be designated "Experiment 1" and "Experiment 2". Experiment 1 studied Quackenbush mice, and the CL, H L , L L and K L mice after twelve generations of selection. T h e animals were all adult males. Experiment 2 studied CL, H L , L L and K L mice after fifteen generations of selection. Half the mice from each line were females and half were males.

Chemical analyses T h e animals were starved overnight, but allowed access to water, and then killed by cervical dislocation. About 15 min was allowed to elapse to ensure intravascular clotting of the blood. T h e bodies were analysed for water, fat and ash. T h e change in weight of the dried extracted body on ashing was designated "fat-free combustible matter" ( F F C M ) and was taken as largely an indication of protein content. T h e methods of analysis were as follows : Water. T h e body was dried in a Pyrex container in an oven at 90°C for 72 hr. (It had been found in a preliminary test that it was necessary to dry for this period in order to attain constant weight.) T h e change in weight was taken as the water content. Fat. T h e dried body was transferred to a glass mortar and crushed using a glass pestle. All material, including expressed low melting-point fat, was transferred to a Whatman double thickness extraction thimble, which was then dried overnight and weighed. T h e

BODY COMPOSITION OF MICE SUBJECTED TO GENETIC SELECTION

681

thimble was placed in Soxhlet's Apparatus and extracted for 24 hr using as solvent a 2 : 1 (v/v) mixture of methyl alcohol and diethyl ether. The thimble was dried overnight and weighed. The change in weight on extraction was taken as the fat content. Ash. The extracted material was carefully removed from the thimble and placed in a porcelain crucible. The crucible was then placed in a muffle furnace and the material was ashed to completion at 400-600°C. The crucible and contents were allowed to cool, dried for at least 3 hr and weighed as rapidly as possible. Whenever dried material was transported from the drying oven to the balance it was conveyed in a desiccator over calcium chloride. This was necessary because it was observed that dried tissues, especially after extraction, were very hygroscopic. Statistical analyses

The results were analysed by the standard techniques for analysis of variance and regression. One- and two-way factorial analysis of covariance was performed (Winer, 1962) in order to determine if there were significant differences between lines and sexes in body constituents after the differences due to body size per se were removed. RESULTS T h e results from Experiment 1 are presented in Tables 1-4, and of Experiment 2 in Tables 5-9. In all tables statistical significance is indicated by asterisks as follows: * 0.010 < P < 0.050; ** 0.001 < P < 0 . 0 1 0 ; *** P < 0 . 0 0 1 . DISCUSSION It was noted in the Introduction that breed differences in body composition have been studied extensively in domestic animals. These comparisons differ from the present one in that the body composition differences studied were anatomical, not chemical, and although the breeds studied had been produced by genetic selection, this selection was not carried out on a consistent, quantitative basis. Because of the possible inaccuracies introduced by the use of ratios when used to remove the effect of a concomitant variable (see Tanner, 1949; H6roux & Gridgeman, 1958; Angervall & Carlstr6m, 1963) all computations have been carried out on the absolute values of the chemical constituents and not on the amounts expressed as a percentage of body weight. Where it has been necessary to remove the effect of body weight per se in the analysis, this has been carried out by analysis of covariance. Both Experiments 1 and 2 may be discussed together because the only real difference between them was that the Quackenbush mice in Experiment 1 constituted an extra control line entirely unconnected with the selection lines. It appears from Tables 1 and 5 that genetic selection was accompanied by a change in body weight, and this impression is confirmed by analysis of variance (Tables 3 and 6). This result is interesting, because in selecting individuals with a high index ( H L ) there was a negative weighting on body weight, while the low index selection ( L L ) produced a positive weighting on body weight. T h a t is, high index selection was for mice with long legs but a light body weight, while low index

Ash

Fat

FFCM

Water

(Bw)

Fat-flee

BW

S.D.

CV (%)

20.493 (10) 4.942 (10) 5.229 (10) 1"034 (10)

(10)

8-51

0"612 59.19

3.254 62.23

2.684 54.31

1.744

33.391 5.027 15.05 (10) 28.162 2.481 8.81

Mean Constituent (n)

CL

T A B L E 1 - - E X P E R I M E N T 1.

S.D.

CV (%)

21.752 (10) 5.075 (10) 6.828 (10) 1"096 (10)

(10) 8.80

0"601 54"84

3.070 44.96

2.733 53.85

1.915

36.612 4.852 13.25 (10) 29-784 2.740 9-20

Mean (n)

HL S.D.

14.046 (9) 3.667 (8) 2.296 (9) 0"833 (8)

(9) 5.89

6.12

7.87

CV (%)

0"387 46"46

0.834 36.32

1.512 41.23

0.828

21.616 1.706 (9) 19.319 1.183

Mean (n)

LL

Line

S.D.

20-597 (9) 4.718 (9) 4.750 (9) 1"066 (9)

(9) 4.84

5.26

6.64

CV (%)

0"633 59"38

1.755 36.65

2.700 57.23

0.996

33.080 2.198 (9) 28.330 1.490

Mean (n)

KL

S.D.

16.751 (10) 4.894 (9) 1.974 (10) 1"000 (9)

(10) 9.10

8.92

8.00

CV (%)

0"198 19"80

1.092 53.32

0.596 12.18

1.524

24.504 1.960 (10) 22.566 2.013

Mean (n)

Quackenbush

M E A N WEIGHTS ( g ) OF WHOLE BODY AND GROSS CHEMICAL CONSTITUENTS OF EACH LINE

U .~

>

~9

~n

z

O~ CO tO

Fat-free BW Water FFCM Fat Ash

Fat-free BW Water FFCM Fat Ash

Fat-free BW Water FFCM Fat Ash

Fat-free BW Water FFCM Fat Ash

Fat-free ash Water FFCM Fat Ash

CL

HL

LL

KL

Quackenbush

d.f. = degrees of freedom.

Fat-free BW Water FFCM Fat Ash

Overall

8 8 7 8 7

7 7 7 7 7

7 7 6 7 6

8 8 8 8 8

8 8 8 8 8

46 46 37 46 37

d.f.

1-021 -0-621 2"552 - 1-261 0"720

14"730 12"154" 17"526 - 14-730 3-906

5.892 4.656" 2.192 - 5-892 -0.935

12-967" 10"732" -5"235* -12"967" - 0"989

14"394"* 11.341 * * 9.180 -14"394"* 2"270

6"786 *** 5.372*** 1"462"** -6.756*** 0.374**

a

4"606 2-769 2"854 4"704 0"984

6"752 4"686 14"252 6"752 3"362

2-528 1.773 7.781 2.528 1"864

4"288 3"330 2"077 4-288 0"560

3-236 2" 538 5.835 3"236 1"300

0"990 0.775 0.310 0.990 0"164

S.E. of a

0"879 * * 0"708*** 0"094 0"132 0"011

0"411 0"255 -0"376 0"588* - 0"083

0.621 ** 0.434"* 0.068 0"378" 0"081

0"459" * 0"309" 0"310"** 0-540** 0"062"*

0"412"* 0"274" * -0.124 0"587*** -0"036

0-631 *** 0.448*** 0-137"** 0"367*** 0"028***

b

0"187 0"112 0"114 0"191 0"040

0-203 0"141 0"418 0-203 0"098

0.116 0.081 0.358 0-116 0-086

0"116 0"090 0"061 0-116 0"016

0-096 0"075 0.169 0"096 0-038

0"032 0.025 0-010 0"032 0"005

S.E. of b

0"8565 * * 0"9120"** 0"2968 0"2369 0"1070

0"6064 0"5636 -0"3222 0"7376* - 0"3046

0.8955 ** 0.8950"* 0.0773 0-7753 * 0-3618

0"8131 * * 0"7626" 0"8737*** 0"8544** 0"8029"*

0"8353 ** 0.7898 * * -0"2514 0"9078*** -0"3216

0"9448"** 0"9339*** 0.9111"** 0.8590*** 0.6427***

r

CONSTANTS FOR THE SIMPLE LINEAR REGRESSION OF CONSTITUENT ON BODY W E I G H T , OVERALL AND FOR EACH LINE, WHERE CONSTITUENT ( g ) ~- a q- b (BODY Vc'EIGHT)

Constituent

1.

Line

TABLE 2--EXPERIMENT

oo

o

0

,-1

o o

0

684

N. J. DAWSON, S. K. STEPHENSON ANDD. K. FREDLINE

TABLE 3--EXPERIMENT

1. SUMMARY OF ANALYSIS OF VARIANCE OF GROSS CHEMICAL CONSTITUENTS, INDEX (I) AND RATIO (R), BETWEEN LINES

F between lines (d.f.)

Variate BW Fat-free BW Water FFCM Fat Ash I R TABLE 4----EXPERIMENT I.

35"53** 43"01 ** 47"74** 20"28** 7"99** 6"10"* 114"48"* 56"40**

(4, 43) (4, 43) (4, 43) (4, 34) (4, 43) (4, 34) (3, 34) (3, 34)

SUMMARY OF ANALYSIS OF COVARIANCE OF GROSS CHEMICAL

CONSTITUENTS, INDEX (I) AND RATIO (R), BETWEEN LINES

Criterion Fat-free BW Water FFCM Fat Ash I R

Covariate BW BW BW BW BW BW BW

F between lines (d.f.) 4"64** 7"53 * * 1"79 4"62* * 0"77 36"64** 53"49**

(4, 42) (4, 42) (4, 33) (4, 42) (4, 33) (3, 33) (3, 33)

selection was for mice with short legs but a relatively heavy body weight. Apparently a high genetic correlation of both radius-ulna and tibia-fibula length with body weight has overridden the phenotypic relationship in the selected mice. That the selection on bone length itself was successful is demonstrated by the significant differences between lines in index and ratio of RU to T F (Tables 3 and 4). Quantitative inheritance of body weight itself in mice has been adequately proven (Roberts, 1965 ; Timon & More O'Ferrall, 1966; More O'Ferrall, 1968) ; some work in domestic animals suggests a heritability for adult body weight of about 0.58 in cattle (Taylor, 1968) and 0.27 in sheep (mean value calculated from a table in Bowman, 1968). In both Experiments 1 and 2, analysis of variance demonstrated significant between lines differences in all constituents (Tables 3 and 6). Analysis of covariance demonstrated significant between lines differences in water and fat but not in F F C M or ash in Experiment 1 (Table 4), but in all constituents in Experiment 2 (Table 7), when body weight was used as covariate. The difference between Experiments 1 and 2 may be explained partly by the continued progress that resulted from an extra three generations of selection, by the greater number of animals analysed in Experiment 2, and by the inclusion of female animals in Experiment 2. Both analysis of variance and of covariance showed significant differences between lines in index

Ash

Fat

FFCM

Water

Fat-free BW

BW

Constituent

TABLE

24"828 (20) 18.662 (20) 4-987 (19) 5.448 (20) 1"104 (19)

(20)

30.275

Mean (n)

5--EXPERIMENT

0"088

1-646

0-845

2"462

3"259

3.793

7"97

30.21

16.94

13"00

13-13

12-53 26.810 (20) 20"082 (20) 5.340 (19) 4.944 (20) 1"239 (19)

(20)

31.754

Mean (n)

0"123

2.660

0.986

2"995

4"143

5.549

S.D.

HL

9-93

53.80

18.46

14"71

15"45

17-47

CV (%)

19.463 (20) 14.554 (20) 3.914 (20) 3.002 (20) 0.906 (20)

(20)

22-426

Mean (n)

0"108

1-309

0.543

1-722

2.402

3-244

S.D.

LL

11"92

43.60

13.87

11-83

12"34

14.47

CV (%)

24"853 (20) 18"934 (20) 5.073 (19) 4.535 (20) 1"132 (19)

(20)

29.738

Mean (n)

0"086

2-645

0.656

2"355

3"138

4.402

S.D.

KL

W E I G H T S O F W H O L E B O D Y A N D GROSS C H E M I C A L C O N S T I T U E N T S F O R E A C H L I N E

CV (%)

MEAN

S.D.

CL

2.

7"50

58"32

12.54

12"44

12"63

14-80

CV (%)

O~ Oo

M

e~ r~ r-

o

,q

0

Z

N. J. DAWSON,S. K. S~I~PHF2~SONAND D. K. FREDLINE

686

and ratio in Experiment I (Tables 3 and 4); these parameters were not re-measured in Experiment 2. The possible reasons for the above observed differences may be speculated upon. It is possible that the genetic selection has affected growth rate, and the differences were due to this, because age changes in body composition of the mouse are well known (Bailey et al., 1960; Cheek & Holt, 1963). It is true that the lines were compared at the same age, but as Bailey et al. (1960) point out, chronological age need not bear any fixed relationship to physiological maturity. The difference between lines in water content after correction for body weight is somewhat surprising, particularly in Experiment 1 where there is no significant T A B L E 6 - - E X P E R I M E N T 2. SUMMARY OF FACTORIAL ANALYSIS OF VARIANCE OF GROSS CHEMICAL CONSTITUENTS BETWEEN LINES AND BETWEEN SEXES

Variate

F between lines (d.f.)

BW Fat-free BW Water FFCM Fat Ash

27'46** (3, 72) 37"46**(3, 72) 44.96** (3, 72) 41"38"* (3, 69) 4-61"* (3, 72) 39"89** (3, 69)

F between sexes (d.f.) 43.17"* 76.36** 94.29** 143.00"* 0.71

(1, (1, (1, (1, (1, 8.32** (1,

72) 72) 72) 69) 72) 69)

F interaction lines x sexes (d.f.) 0.60 1.45 1.45 4.18"* 0.06 1.41

(3, (3, (3, (3, (3, (3,

72) 72) 72) 69) 72) 69)

difference in F F C M after the same analysis. This perhaps indicates some genetically based dissociation between water and protein content, if F F C M can be taken as a good approximation to the latter. There has been some work at the cellular level on differences in muscles between mice selected for large and small body size (Luff & Goldspink, 1967), and during growth in the domestic fowl (Moss, 1968) and the mouse (Rowe & Goldspink, 1969). Moss (1968) and Rowe & Goldspink (1969) observed constancy of fibre number, which may suggest a change in cell size with growth, but Luff & Goldspink (1967) concluded that for most of the muscles they studied a large part of the between line difference in size could be explained on the basis of difference in fibre number. Work on fibre number is required in the mice used in the present experiment to help resolve the question of the reasons for the differences in water and F F C M between lines. Of all constituents, the amount of fat exhibits the widest variation with respect to body weight, yet in both experiments there were significant differences between lines. Hull (1960) found that in mice selected for high 3-week body weight there was an increase in the proportion of fat in the body, but not in those selected for high weight at 4{ or 6 weeks of age; this conflicts with the results reported here, where selection was at 8 weeks of age. It has been found by experiments using extirpation and grafting of gonadal fat depots in mice that there appears to be some physiological mechanism regulating the total mass of adipose tissue in the body (Liebelt et al., 1968), but this work contains no information on the effects of body

X

Body wt. Fat-free body wt. Fat-free dry wt.

Variables

2"465 0"918 0-255

Lines Sexes Interaction

(3, 71) (1, 71) (3, 71)

d.f.

(3, (3, (3, (3, (3, (3, (3, (3, (3,

71) 71) 71) 68) 68) 71) 71) 68) 68)

20"46** 32"26** 43"05** 74"54** 1"24 9"53** 3"06 18"08"* 4"63*

(1, (1, (1, (1, (1, (1, (1, (1, (1,

Sexes 71) 71) 71) 68) 68) 71) 71) 68) 68)

2"37 0"92 1"43 5"37** 1"43 1"52 2"22 0"70 0"48

-0"009 0"156" 0-169"**

a

0"009*** 0"004 0"013

b

0-002 0"003 0"007

S.E, of b

r

F 8"809"* 40"788*** 1"348

Source Lines Sexes Interaction

(3, 70) (1, 70) (3, 70)

d.f.

Factorial analysis of covariance (covariate : body weight)

0"053 0.068 0.044

S.E. of a

Regression analysis (d.f. = 78)

71) 71) 71) 68) 68) 71) 71) 68) 68)

0.4801 *** 0"1522 0"1974

(3, (3, (3, (3, (3, (3, (3, (3, (3,

Interaction

R A T I O OF FAT TO WATER : SUMMARY OF STATISTICAL ANALYSIS

5"93"* 9-12"* 10"42"* 7"92** 10"50"* 3-83" 1"17 2"90* 2"93"

Lines

Note: In no case is the within-lines regression of fat/water on body weight or fat-free body weight statistically significant.

F

Source

Factorial analysis of variance

Fat/water Fat/water Fat/water

Y

wt. wt. wt. wt.

TABLE 8--EXPERIMENT 2.

Body wt. Body wt. Body wt. Body wt. Body wt. Fat-free body Fat-free body Fat-free body Fat-free body

Covariate

F (d.f.)

SUMMARY OF FACTORIAL ANALYSIS OF COVARIANCE OF GROSS CHEMICAL CONSTITUENTS BETWEEN LINES AND BETWEEN SEXES

Fat-free BW Water Fat FFCM Ash Water Fat FFCM Ash

Criterion

TABLE 7--EXPERIMENT 2.

O~ OO

Overall Overall Overall CL HL LL KL

Lines

(Fat + water)/fat-free (Fat + water)/fat-free (Fat + water)/fat-free (Fat + water)/fat-free (Fat + water)/fat-free (Fat + water)/fat-free (Fat + water)/fat-free

Y wt. wt. wt. wt. wt. wt. wt. Body wt. Fat-free body wt. Fat-free dry vet. Fat-free dry wt. Fat-free dry wt. Fat-free dry wt. Fat-free dry wt.

78 78 78 18 18 18 18

d.f. 3"338*** 3"936*** 4"336*** 5"200*** 5"230*** 4-910"** 3"450***

a 0"252 0"286 0"250 0"634 0"548 0"376 0"838

S.E. of a

F 4"148" 0"057 0-542

Source Lines Sexes Interaction

(3, 70) (1, 70) (3, 70)

d.f.

b 0"015 -0"006 -0"093* -0"205 -0"216" -0"260** 0"046

Factorial analysis of covariance (covariate : fat-free dry weight)

dry dry dry dry dry dry dry

X

Regression analysis

0"008 0"011 0"040 0"102 0"080 0"076 0"132

S.E. of b 0"1950 0'0648 -0"2515" -0"4289 -0"5372* -0"6265** 0"0819

T H E RATIO (FAT-t-WATER)/FAT-FREE DRY W E I G H T : SUMMARY OF STATISTICAL ANALYSIS

Variables

TABLE 9--EXPERIMENT 2.

t7

2:

O0 O0

BODY COMPOSITION OF MICE SUBJECTED TO GENETIC SELECTION

689

size. There is some evidence that the number of mature adipocytes in the adult rat is fixed (Hirsch & Hun, 1969). Work is necessary to determine if the physiological mechanisms regulating the total amount of adipose tissue have been altered in the selection lines. The difference in ash content between lines on covariance analysis in Experiment 2 (Table 7) suggests the degree of calcification may be different in different lines, possibly due to a growth effect (Dickerson & Widdowson, 1960), or to genetically controlled changes in the sizes of bones. The results discussed so far are sufficient to make the point that differences in the relative proportions of metabolically active (protein) and metabolically inactive (largely fat) components of the body unconnected with body size per se can occur, thereby leading to marked errors when attempts are made to compare metabolic heat production between animals. If the amount of fat in the body, for example, could be measured directly a correction could be made. Some workers have estimated fat content by means of regression equations relating fat content to water content; however, this practice has been discussed unfavourably at times by Keys & Bro~ek (1953). In the present results there is a very highly significant relationship between body weight and the relative proportion of fat to water (Table 8), but nevertheless this relationship accounts for only 23 per cent of the variance. The relationship of metabolically inactive to metabolically active components of the body was examined by analysis of the ratio (Table 9): (fat + water)/fat-free dry weight. Covariance analysis indicates that differences in the ratio of metabolically inactive to metabolically active components of the body exist that are unconnected with the amount of metabolically active tissue. However, because of the lack of a significant regression of the above ratio on body weight it is not even possible to correct for differences in it due to body size, let alone those not due to size. It is concluded that differences in body composition can be a source of error in metabolic rate studies on mice when metabolic heat production is expressed on a body weight basis, and this error cannot be removed because some of the differences are not due to a direct relationship between amounts of metabolically inactive constituents and body weight. The genetic selection carried out in this work approximates the type of selection that has been practised in livestock bred for meat production. In the case of the high index line (HL) the selection approximates that where body weight has been emphasized in animal breeding but with little or no attention paid to body conformation. With the low line (LL), selection approximates the situation where animals have been selected for type, that is, with an emphasis on body conformation, rather than for body weight. The existence of these lines will make possible experimental testing of a number of strategies of importance to animal breeders. For example, a large number of small animals of the same total body weight as one large animal may exhibit the same weight gain from the same amount of food, but at a much faster rate (Kleiber, 1961). However, any decision to select for a small

690

N . J . DAWSON,S. K. STEVHENSONAND D. K. FImDUNE

animal because of its high rate of production of food compared to a larger animal would have to be taken in the light of knowledge of the effect of such selection on body composition.

Acknowledgemems--The authors thank the University of New England for financial support; Imperial Chemical Industries of Australia and New Zealand Ltd. for a Research Fellowship to one of us (N. J. D.) during part of this investigation; Mr. T. W. Field for making available computer programmes; and Professor C. W. Emmens, Department of Veterinary Physiology, University of Sydney, for stocks of Quackenbush mice. REFERENCES ANGERVALL L. & CARLSTROM E. (1963) Theoretical criteria for the use of relative organ

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