Growth rate and pattern of gut development in mammals and birds

Livestock Production Science, 11 (1984) 461--474

461

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

GROWTH RATE AND PATTERN OF GUT DEVELOPMENT IN MAMMALS AND BIRDS

J A M E S K. K I R K W O O D

Department of Pathology, University of Bristol Medical School, University Walk, Bristol BS8 1TD (Gt. Britain) NICOLA J. PRESCOTT

Meat Research Institute, Langford House, Langford, Bristol BS18 7DU (Gt. Britain) (Accepted March 27, 1984)

ABSTRACT Kirkwood, J.K. and Prescott, N.J., 1984. Growth rate and pattern of gut development in mammals and birds. Livest. Prod. Sci., 11: 461--474.

Selection for a reduction in time taken to reach slaughter weight and adult weight whilst adult weight remains unaltered would improve the biological and economic efficiency of meat production. However, time taken to grow is strongly correlated with adult weight and attempts to select for the former have usually led to changes in the latter so that efficiency of growth to adult weight has remained largely unchanged. The animal kingdom includes m a n y examples of species which, after accounting for differences in weight, grow more rapidly than the domestic meat animals. A model is developed which describes how, if time taken to reach adult weight is to be reduced whilst adult weight remains unchanged, then energy intake per unit body weight m a y increase throughout growth. It is postulated that relatively high intakes are associated with relatively large digestive systems. Gut sizes and intake rates during growth in four species whose growth rates differ widely are examined in relation to this hypothesis. It is suggested that little progress will be made in reducing time taken to grow without changes in pattern of development.

INTRODUCTION

Much of the variation between species in the time t taken to reach adult body weight A can be accounted for by difference in A. Generally, the larger the animal the longer it takes to reach A. To be more precise, it has been found that t tends to increase in parallel with the quarter power (or thereabout) of A (Taylor, 1965; Ricklefs, 1968; Linstedt and Calder, 1981). Considerable variation in t remains, however, after scaling to account for differences in A. For example, it has been found that birds, especially altricial birds (those reared by their parents in nests), generally reach A more rapidly than mammals (Ricklefs, 1973; 1979), and that among mammals,

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462 some primates, notably the apes and Old World monkeys, take an exceptionally long time to grow to A (Kirkwood and Webster, 1984; Kirkwood, in press). There has been considerable interest among ecologists as to whether t in an animal reared under optimum conditions is constrained by the design and developmental pattern of the animal (Ricklefs, 1969; 1973) or whether it has evolved, independent of design, to that conferring greatest survival benefit (Case, 1978). This discussion about whether or not growth rate is inextricably linked to organisation is of relevance to the meat production industry since the efficiency of meat animal production would be increased by successful selection for a decrease in t in relation to A. Attempts to achieve this in mice, sheep, cattle and fowl have so far met with rather little success (Kirkwood and Webster, 1984), which lends support to the hypothesis that growth rate is linked to design and developmental pattern of the animal. In this paper the relTa~ionship of the pattern of development of the gut to differences of rates of food intake and growth rate in mammals and birds is discussed. The case presented supports the argument that differences in developmental strategies underlie the substantial differences in t seen among mammals and birds, that cannot be attributed to A. The approach pursued in this study was first to consider how relative food consumption, RFC (MJ/kg/day) might be expected to change during the course of growth and how it might differ between species showing differences in t not attributable to differences in A. Secondly, records of the weight of the empty digestive tracts Wg of various species were examined for evidence that might support or refute the hypothesis that relative gut size RGS (Wg/body weight) might be associated with RFC and might thus influence the time taken to reach A. A MODEL OF ENERGY INTAKE DURING GROWTH A model is described below which shows how the pattern of RFC with growth may be expected to differ between relatively fast and slow maturing species. The energy requirement of a growing animal can be considered as the sum of two components; (1) the requirement for maintenance, Em and (2) the requirement for growth Eg. Let us assume that Fan is related to body weight W within a species during growth as it is between species (Kleiber, 1975), and as a first approximation that; Era (MJ/day)= 0.5 Ws~

(1)

and thus Era (MJ/kg/day) = 0.5 W-~

(2)

Fan, per unit weight, thus declines with increasing stage of maturity u (where u is proportion of adult weight attained; u ffi W/A). Since u = W/A, then W = u' A and therefore Em (MJ/kg/day) = 0.5 u- IA.A_~A

(3)

463 Thus the general pattern of the change in Em with stage of maturity can be described Em ( M J / k g / d a y . A 1/~) = 0.5 u-1~

(4)

In this equation (4), Era in units of MJ/kg/day has been scaled for differences in A by multiplying by A la (Taylor, 1980). The pattern of weight gain in animals can be approximated by a number of mathematical expressions (Wilson, 1977), for example the Gompertz curve; W = Ae_e-~ ( t - M)

(5)

where A is the asymptote (adult body weight), B is the rate constant, t is age in days and M is age in days at the inflexion point of the sigmoid curve. The absolute growth rate d W / d t (kg/day) at a given time is therefore: d W / d t = A e -e-~ Ct- M~ • e-B ( t - - M ) .

B

and since; u = W~

= e - e -B ( t - M )

~d; _logee_e-B (t -- M) = e- B (t--M) then, the absolute growth rate at a given value of u can be calculated; d W / d t = - - B W loge u

(6)

The energy density of tissue deposited typically increases with u (Ricklefs, 1974; MacDonald et al., 1981) but for the purpose of simplicity we will assume here that it remains constant at 10 MJ/kg throughout growth. We will also assume that the cost of tissue deposition remains constant at 1.33 MJ/MJ (Webster, 1981). These figures have been chosen as reasonable first approximations (at least for monogastric animals) and are adequate for the purpose here of generating a rough guide as to the pattern of energy intake during growth in homeotherms. Then; Eg (MJ/day) = --13.3 B W . logeu

(7)

and Eg (MJ/kg/day) = --13.3 B logeu

(8)

Multiplying through by A % to scale for differences between species due to size (Taylor, 1980); Eg ( M J / k g / d a y . A ~ ) = --13.3 B A ¼ logeu

(9)

The growth rate constant B is largely a function of A (Ricklefs, 1968; Taylor, 1965) so that

464

(10)

C = BA ¼

where C is a constant, describing the variation in B that can be attributed to variation in A. Substituting into equation (9) we get; Eg ( M J / k g / d a y . A ~ ) = --13.3 C loge u

(11)

The total energy requirement for growth in units of MJ/kg/day and scaled for differences in A between animals is (Era + Eg): Etotal ( M J / k g / d a y ' A ~ ) = 0.5u -*~ -- 13.3 C loge u

(12)

02s~s is a useful index for comparing time taken to grow between species (Kirkwood and Webster, 1984). It is equal to the time taken in days to grow from 0.25 to 0.75 of adult body weight, t25-~s, divided by A '~ to scale for differences in A. It is a property of the Gompertz curve that (13)

t2s-+s = 1.57/B

and thus the relationship of O2s-Ts to C can be calculated from equations (10) and (13) to be 82s-~s =

(14)

1.57/C

4

3

•

C •

metabolisable energy intake

0.10

15.7

o.m

52.3 157

2 -1

-1

MJ.Kg .d .A

e.lo

14

o;2o stage of maturity

"

Fig. 1. A model of the pattern of energy intake (MJ/kg/day) during growth in homeotherms. Intake has been scaled for differences in adult weight A by multiplying by A ~ . 82+.~5 is an index of time taken to grow scaled to account for differences in A (see text). The model shows how, as time taken to grow decreases in animals of comparable adult size, so rate of energy intake per kg is expected to increase.

465

This model (equation (12)) predicts that Etotal, the RFC, will be greatest in the earliest stages of growth and also that a decrease in time taken to mature in relation to A is associated with a higher rate of energy intake particularly in the early stages of growth. For example as C increases from 0.01 to 0.10, which is consistent with a decrease in 82s-Ts from 157 to 15.7 days/A 1A (which are typical for the times taken by rather slow-growing mammals and altricial birds respectively) (Kirkwood and Webster, 1984), E t o t a 1 increases threefold when u = 0.2 (Fig. 1). The model does not account for costs of activity or thermoregulation above those covered by the maintenance component (equation (1)). If one or both of these costs is raised then an animal must either increase food intake in order to maintain growth, or else, if this cannot be done (because further food is unavailable or because of a limit to the amount of food than can be ingested; Kirkwood, 1983), energy must be diverted away from growth to fuel activity and thermoregulation if the animal is to survive. A high RFC may thus be a consequence of high thermoregulatory or activity costs and may not necessarily be associated with rapid growth. We should not therefore expect the relationship between RFC and time taken to grow among animals to be as rigid as it is in this simple model. 20

~s j,

15

J S S

S S

10

S S

e es

LM B--

z

n

I

20

I

I

I

40

FOOD CONSUMPTION

i

60 g.d "1

Fig. 2. Relationship of intestine weight to food consumption rate of rat~ during pregnancy and lactation. (The data are from Cripps and Williams, 1975). The dotted line was drawn b y eye.

0

/

3F

I

broiler

I

1

10

Stage

38

of m a t u r i t y

I

I

u se

I

I

53

I

i

macaque

~

i

~0

0.2 0.4 0.6 0.8 1.0

I

of adult w e i g h ~

i 0.2 0.4 0.6 0.8 1.0 0

I

mo

[proportion

I I I I I _ I I I I J 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0

cormorant

3;F

(~25 t o 75

~cn

m

o'J o~

467 G U T SIZE A N D F O O D C O N S U M P T I O N IN A D U L T A N I M A L S

Between species gut weight Wg in adult animals varies in parallel with W I"° (Calder, 1974) being always around 5--10% of body weight. Since energy requirements F-an (MJ/day) increase in parallel with l~A between species, and Wg is proportional to W I"°, then E m will scale with Wg s~ and, conversely, Wg will scale with Fan 4/3. This means that, between species, for each twofold increment in Fan, Wg will increase by 2.5 times. Within species also, gut weight increases with increasing food consumption. For example, the trebling of food intake that occurs during lactation in the rat is associated with a doubling of the weight of the intestine (Fig. 2; Cripps and Williams, 1975; Remesar et al.,1981), and in the sheep a trebling of food intake was found to cause a doubling of the mass of the rumen epithelium in about a week (Fell and Weekes, 1975). Both within and between species an increment in the rate of food intake is associated with an increase in gut mass. This is not surprising since the amount of food that can be eaten and digested in a day must be, in part, related to the capacity of the gut. The model described above predicts that relative food intake (RFC, kJ/kg/day) declines throughout growth and we might expect that R G S would decline in parallel. Furthermore we might expect from the model that R G S would be greatest in those animals that grow most rapidly and that the difference would be most marked in the early stages of growth. GROWTH

RATE, F O O D I N T A K E A N D G U T SIZE IN S O M E M A M M A L S

A N D BIRDS

In Fig. 3 values for 025-7s. are given for the double-crested cormorant Phalacrocorax auritus, the domestic broiler fowl, the domestic mouse and the rhesus macaque Macaca mulatta. These are, in terms of growth rates, fairly typical representatives of the. altricial birds (those that are reared by their parents in nests), precocial birds (those that walk and feed themselves from hatching), domestic production mammals and Old World primates respectively (Kirkwood and Webster, 1984; Kirkwood, in press). The relative food intake (MJ/kg/day) of all these animals shows a tendency to decrease during growth as predicted, and the pattern for that of the broiler is very similar to that of the model. Intake is initially low in the mouse whilst it is sucking milk, but after weaning, is roughly of the pattern and Fig. 3. Relative gut size and relative energy intake (in MJ/kg/day scaled for differences between species in adult weight) during growth in two birds and two mammals. 0=s.~s is an index o f time taken to grow which accounts for differences due to variation in adult size. (Data for cormorant: Dunn, 1975a and 1975b; for broiler: Prescott et al., in preparation; for mouse: Altman and Dittmer, 1962, preweaning intake estimated from Enzmann, 1933 and Jenness, 1974, postweaning intake from Kirkwood and Webster, 1984. F o r rhesus macaque: Bourne, 1975; RiopeUe, 1979; National Research Council, 1978; metabolisability of diet was estimated at 0.75).

468

magnitude expected. The initial intake of the cormorant is rather low in view of its extremely rapid growth rate. This is because the chicks of altricial birds do not achieve e n d o t h e r m y for several days after hatching and prior to this maintenance requirements are lower than assumed in the model (Ricklefs, 1974). Nevertheless relative intake is higher at all stages of maturity during growth in the cormorant than in the broiler and the mouse. The patterns of RGS with stage of maturity in these species broadly reflect the differences in RFC and growth rate (Fig. 3). RGS declines with stage of maturity and is greatest in the species which grows the m o s t rapidly, the cormorant. RGS is initially low in the mouse and this is presumably associated with its low intake whilst drinking milk. Data on gut sizes in other mammals and birds are rather few and incomplete. The RGS of the sheep (82s-Ts = 70) follows a pattern very like that of the mouse during growth (Altman and Dittmer, 1962), whilst that of the herring gull Larusargentatus (Dunn and Brisbin, 1980) (82s-Ts = 14) is similar to that of the cormorant. It is, perhaps, instructive to consider the relationship of E intake to ~2s-Ts more closely. When u = 0.2 (equation (12)) the model of the pattern of E intake during growth becomes: Etoud (MJ/kg/da¥'~4 ~ ) = 0.748

+

21.4 C

(15)

-3o0 4 <

gT

.i ,-4

>i

n-

°i L

!

01

0

I

I

I

200

l

400

I

600

~25 to75

Index

of t i m e

token

to motur

Fig. 4. The predicted p a t t e r n o f the relationship between relative energy intake in mammals and birds when 20% of adult size (u = 0.2) scaled to account for differences in size between species, and 8 , . ~ , , a size-independent index of time taken t o grow (see text). Relative gut sizes (gut w e i g h t / b o d y weight) when u ffi 0.2 for a n u m b e r o f species are also plotted against e2s.~s o n this graph for comparison. Data: 1, double-created cormorant (Dunn, 1975a); 2, herring gull (Dunn and Briabin, 1980); 3, kestrel (Kirkwood, unpublished), 4, broiler (Prescott et al., in preparation); 5, mouse (Kirkwood, unpublished); 6, sheep (Altman and Dittmer, 1962); 7, rhesus macaque (Bourne, 1975).

469

Since 02s-~s = 1.57/C (equation (14)) then 025-7s = 33.6/(Etotal --0.748)

(16)

This relationship of Etotal to 025-7s is demonstrated graphically in Fig. 4. According to this model Etotal when u = 0.2, increases very steeply with decreasing 02s-Ts values below 100. This pattern is quite closely approximated by the relation of relative gut size to 02s-~5 in the animals for which data are available (Fig. 4). Relative gut size and SMR

Poczopko (1979) has remarked u p o n the difference in patterns of standard metabolic rate SMR per Ws~ between the rat and the domestic fowl (Fig. 5), but offered no explanation. The patterns are strikingly similar to the patterns of relative food intake and gut size in the mouse and broiler. It is therefore possible that these differences in SMR reflect differences in the proportion of gut tissue, the latter having a high metabolic activity compared with the rest of the body (Webster, 1981). This hypothesis is supported by the findings of Koong et al. (1982) who found that, within species, differences in fasting metabolic rate in animals of equal size correlated with differences in relative gut and liver sizes.

0.6

0.4 SMR

:" N~.______"~,,.r..~... ~ hen I : rat

MJ. Kg-~d -1

0.2

i/

.'J

a

•

J

•

02

0.4

0.6

08

•

stage of maturity Fig. 5. Changes in standard metabolic rate SMR (MJ/kg~/day) during growth of the rat and the hen, (adapted from Poczopko, 1979).

470

Altricial birds have a low S M R in the early stages of growth which rises to the levels more typical of homeotherms around the time the nestlings attain endothermy (Ricklefs, 1974). This initially low S M R is likely to be due to a low rate of metabolism in tissues other than those associated with digestion. The metabolic rate of the gut itselfmust be high as a result of the relatively high food intake rate, and thus rapid rates of digestion, absorption and metabolism. DISCUSSION

The data presented here are consistent with the argument that R G S during growth reflects R F C and is thus linked with growth rate strategy. The implications is that gut size m a y limit intake rate and thus growth rate. To increase growth rate without increasing adult body size m a y require an increase in R G S during growth and thus a decrease in the relative size of the rest of the body or some component of it. The relatively large gut of the young altricialbird m a y be possible only because there is no need for functional legs or wings in the nest. Such a large gut might be incompatible with a functional body design in mammals, which require full powers of locomotion at a relatively early stage of maturity. Altricialbirds may be capable of growth rates greater than those achieved by other animals because they, more than other species, forego the capacity for mature function whilst growing and do not compromise adaptation for growth with adaptation for functional maturity to the same extent as do precocial birds and mammals. S o m e of the difference in growth rate between precocial birds and m a m m a l s m a y be due to the former being able to have relatively larger guts in the early stages of growth by delaying the development of their wings, which are not essential for survival at that time. Interestingly, in the growing broiler, as the breast muscles are increasing in size relative to the rest of the body the gut is decreasing, so that throughout growth the combined weights of gut and breast muscles represent a remarkably constant proportion of the whole body (Fig. 6). This illustrates the point that gut can only be relatively large as long as some other organ or organs are relatively small. It also points to what may be an ultimate limit to muscle meat production, namely that rapid growth in relation to adult size (and thus a large gut) and a large, early-developing muscle mass may be mutually exclusive. Ricklefs (1979) demonstrated a negative correlation between relative leg size and growth rate (in relation to A) among precocial birds. That is, those species which when adult have the largest legs, and thus the largest leg muscles in relation to A, tend to be those that take longest to reach A. The evidence that SMR may be higher in those animals with larger relative gut sizes points to a disadvantage in possessing the capacity for rapid growth, namely that if maintenance costs are higher, there will be a lower growth

471 rate and perhaps reduced viability when f o o d is scarce. An animal evolved to grow in a habitat in which f o o d was scarce would be expected, if gut maintenance is relatively expensive, to keep gut size to a minimum and thus to keep maintenance requirements low so that growth can proceed, albeit slowly, on a low food intake. Such a strategy may, b y restricting gut size, preclude the capacity for rapid growth when f o o d is abundant.

A ",..

gut + breast muscles

0.2

0.4

0.6

0.8

1.0

STAGE OF MATURITY

Fig. 6. Weight of empty gut and breast muscles in relation to live weight during growth of broiler cockerels to adult size. The dotted line shows the relative weight o f the gut and breast muscles combined. (Data from Prescott et al., in preparation). The evidence p u t forward in this paper shows h o w growth rate m a y be inextricably linked to m o r p h o l o g y and pattern of development. The implication of this in selection of m e a t animals for improved growth rate in relation to adult size (and thus for improved efficiency of growth), is that little progress may be possible w i t h o u t changes in the pattern of development. ACKNOWLEDGEMENT We thank Professor A.J.F. Webster for criticising this paper.

472 REFERENCES Altman, P.L. and Dittmer, D.S. (Editors), 1962. Growth, Including Reproduction and Morphological Development. Biological Handbooks Series; Federation of American Societies for Experimental Biology, Washington, D.C. Bourne, G.H. (Editor), 1975. The Rhesus Monkey, Vol. I. Academic Press, New York. Calder, W.A. Jr., 1974. Consequences of bodysize for avian energetics. In: R.A. Paynter (Editor), Publication of the Nuttall Ornithological Club No 15, Cambridge, MA, pp. 86--151. Case, T.J., 1978. On the evolution and adaptive significance of post natal growth rates in the terrestrial vertebrates. Q. Rev. Biol., 53: 243--282. Cripps, A.W. and Williams, V.J., 1975. The effect of pregnancy and lactation on food intake, gastro-intestinal anatomy and the absorbtive capacity of the small intestine in the albino rat. Br. J. Nutr., 33: 17--32. Dunn, E.H., 1975a. Growth, body components and energy content of nestling doublecrested cormorants. Condor, 77: 431--438. Dunn, E.H., 1975b. Caloric intake of nestling double-crested cormorants. Auk, 92: 553--565. Dunn, E.H. and Brisbin, I.L. Jr., 1980. Age specific changes in the major body components and caloric values of herring gull chicks. Condor, 82: 398--401. Enzmann, E.V., 1933. The milk production curve of albino mice. Anat. Rec., 56: 345--358. Fell, B.F. and Weeks, T.E.C., 1975. Food intake as a mediator of adaptation in the ruminal epithelium. In: I.W. McDonald (Editor), Digestion and Metabolism in the Ruminant. University of New England Publishing Unit, Armidale, N.S.W., pp. 101--118. Jenness, R., 1974. The composition of milk. In: B.L. Larson and V.R. Smith (Editors), Lactation, Vol. III, Academic Press, London, pp. 3--108. Kirkwood, J.K., in press. Patterns of growth in primates. J. Zool., Lond. Kirkwood, J.K., 1983. A limit to metabolisable energy intake in mammals and birds. Comp. Biochem. Physiol., 75A: 1--3. Kirkwood, J.K. and Webster, A.J.F., 1984. Energy budget strategies for growth in mammals and birds. Anita. Prod., 38: 147--155. Kleiber, M., 1975. Fire of Life. R.E. Kreiger, New York, pp. 453. Koong, L.J., Ferrell, C.L. and Nienaber, J.A., 1982. Effects of plane of nutrition on organ size and fasting heat production in swine and sheep. In: A. Ekern and F. Sundst~l (Editors), European Association for Animal Production No. 29. Agricultural University of Norway, pp. 245--248. Linstedt, S.C. and Calder, W.A., 1981. Body size, physiological time and longevity of homeothermic animals. Q. Rev. Biol., 56, 1--16. McDonald, P., Edwards, R.A. and Greenhalgh, J.F.D., 1981. Animal Nutrition. Oliver and Boyd, Edinburgh. National Research Council, 1978. Nutrient requirements of non-human primates. Nutrient Requirements of Domestic Animals, Series No. 14. National Academy of Science, Washington, D.C. Poczopko, P., 1979. Metabolic rate and body size relationships in adult and growing homeotherms. Acta Theriol., 24: 125--126. Prescott, N.J., Wathes, C.M., Kirkwood, J.K. and Perry, G.C., in press. Growth, food intake and development of broiler cockerels raised to maturity. Anita. Prod. Remesar, X., Arola, L., Palou, A. and Alemany, M., 1981. Body and organ size and composition during the breeding cycle of rats (Rattus norvegicus) Lab. Anita. Sci., 31: 67--70. Ricklefs, R.E., 1968. Patterns of growth in birds. Ibis, 110: 419--451. Ricklefs, R.E., 1969. Preliminary models for growth rates in altricial birds. Ecology, 50: 1031--1039.

473 Ricklefs, R.E., 1973. Patterns of growth in birds II. Growth rate and mode of development. Ibis, 115: 117--201. Ricklefs, R.E., 1974. Energetics of r e p r o d u c t i o n in birds. In: R.A. Paynter (Editor), Avian Energetics. Publication of the Nuttall Ornithological Club No. 15, Cambridge, MA, pp. 152--297. Ricklefs, R.E., 1979. A d a p t a t i o n , constraint and compromise in avian postnatal development. Biol. Rev., 54: 269---290. Riopelle, A.J., 1979. Primates in nutritional and developmental research. In: K.C. Hayes (Editor), Primates in Nutritional Research. Academic Press, London, pp. 341--371. Taylor, St C.S. (1965). A relation between mature weight and time taken to mature in mammals. Anita. Prod., 7 : 203--220. Taylor, St. C.S., 1980. Genetic size-scaling rules in animal growth. Anita. Prod., 30: 161--165. Webster, A.J.F., 1980. Energy costs of digestion and metabolism in the gut. In: Y. Ruckebush and P. Thivend (Editors), Digestive Physiology and Metabolism in Ruminants. MTP Press, Lancaster, pp. 469---484. Webster, A.J.F., 1981. The energetic efficiency of metabolism. Proc. Nutr. Soc., 40: 121--128. Wilson, B.J., 1977. Growth curves, their analysis and use. In: K.N. Boorman and B.J. Wilson (Editors), Growth and Poultry Meat Production. British Poultry Science, Edinburgh, pp. 89--115. RESUME

Kirkwood, J.K. et Prescott, N.Y., 1984. Croissance et d~veloppement du tube digestif des mammif~res et des oiseaux. Livest. Prod. SCi., 1 1 : 4 6 1 - - 4 7 4 (en anglais). La s~lection en r u e d'une r~duction du temps n~cessaire pour atteindre le poids d'abattage ou le poids adults sans que ce dernier soit modifi~ devrait am~liorer l'efficacit~ biologique et ~conomique de la p r o d u c t i o n de viande. Cependant, la dur~e de la croissance est fortement corr~l~e avec le poids adulte, et les tentatives de r~aliser une telle s~lection ont g~n~ralement conduit i des modifications du poids adulte, si bien que l'efficacit~ de la croissance jusqu'~ ce poids est demeur~e pratiquement inchang~e. Le r~gne animal cornporte de nombreuses esp~ces qui, lorsque l ' o n tient compte des diff4rences de poids, croissent plus rapidement que les animaux domestiquas destines ~ la boucherie. Nous avons ~tabli un module qui dt.~crit c o m m e n t l'ingestion d'~nergie/unit~ de poids vif doit ~tre accrue p e n d a n t la croissance pour que le temps n~cessaire pour atteindre le poids adulte soit r~duit sans que ce dernier soit affect& 11 est postul~ que des appareUs digestifs relativement d~velopp~s sont associ~s aux niveaux de consommation ~lev~s. La taflle de l'intestin et le niveau de c o n s o m m a t i o n p e n d a n t la croissance ont ~t~ ~tudi~s dans le cadre de cette hypoth~se pour 4 esp~ces d o n t les vitesses de croissance diff~raient grandement. II est sugg&~ que peu de progr~s seront r~alis~s en ce qui concerne le temps n~cessaire ~ la croissance, sans modifier le d~veloppement normal des individus. KURZFASSUNG Kirkwood, J.K. und Prescott, N.J., 1984. Wachstumsrate und Entwicklung des Verdauungsapparates bei Siiugtieren und VSgeln. Livest. Prod. Sci., 1 1 : 4 6 1 - - 4 7 4 (auf englisch). Die Selektion auf eine Reduzierung der Zeitspanne zwischen dem Erreichen des Schlachtgewichtes und der ausgewachsenen K~rpergr~sse, bei konstanter ausgewachsener

474 K/SrpergrSsse, wiirde die biologischen und 5konomischen Aspekte der Fleischproduktion verbessern. Allerdings sind die Wachstumsrate und volle KSrpergrSsse eng korreliert und Selektionsversuche fiir die erstere resultieren gewShnlich in Ver~nderungen der letzteren, so dass die Wachstumseffizienz his zum adulten Gewicht weitgehend unver~ndert bleibt. Das Tierreich enth/ilt viele Beispiele yon Tierarten, die unter Beriicksichtigung der unterschiedlichen K~rpergrSsse verh~tnismiissig schneller wachsen als die domestizierten Fleischtiere. Ein Modell wird entwickelt, das beschreibt, wie die Energieaufnahme pro Einheit KSrpergewicht w~hrend der Wachstumsperiode zunehmen muss, wenn die Zeitspanne zwischen den beiden genannten Z e i t p u n k t e n reduziert werden soil, ohne das ausgewachsene KSrpergewicht zu ver/indern. Es wird postuliert, d a u verhifltnism~mig g r o u e Futteraufnahme mit relativ grossem Verdauungsapparat assoziert ist. Die Grosse des Verdauungsepparates und die F u t t e r a u f n a h m e w~ihrend der Wachstumsperiode von vier Tierarten mit unterschiedlichen Wachstumsraten werden im Zusammenhang mit dieser Hypothese gepriift. Es wird geschlossen, dass Fortschritte in der Reduzierung der Wachstumsperiode ohne Auswirkungen auf die allgemeine Entwicklung schwer mSglich sind.