VOL. 32., NO. 3 September, I957 THE QUARTERLY REVIEW of BIOLOGY QUANTITATIVE LAWS IN METABOLISM AND GROWTH T BYLUDWIGVON BERTALANFFY Los Angeles Mt. Sinai Hospital,and University BiologicalResearch, ofSouthern California, INTRODUCTION _ HE work reviewed in this paper is aimed at establishingconnectionsbeaspectsofliving tweentwofundamental and growth. their metabolism organisms, What we call growthof even a simple complexphenomenon organismis a tremendously fromthe biochemical,physiological,cytological, viewpoints.Thereare,however, and morphological certainaspects that are amenableto quantitative analysis,and such an approach appears to lead to some insight into the connectionsbetween metabolismand growth,and to some answer to the seeminglytrivial,but in fact hardlyexplored question, "Why does an organismgrow at all, and why, after a certain time, does its growth come to a stop?" QUANTITATIVE RELATIONS BETWEEN BODY SIZE AND METABOLIC RATE it may be In orderto begin this investigation, emphasizedthat,in manyphysiologicalactivities, the absolutesize of the body is a mostimportant the rate of processes.Whether factordetermining we take total metabolism,heart or respiratory rate, the chemicalcompositionof the organism, excretion,or the enzymecontentof the cells-we vary always will findthat theycharacteristically with body size, this being true even thoughthe the organismscomparedin such respectsshow a tremendous diversityin theiranatomy,physiological mechanisms,adaptations to certainenvironments,and so forth(cf. Adolph, 1949). To give just one example:pulse rate in mammalsclosely correspondsto the Y powerof body weightover seven orders of magnitude,from a dwarf bat weighingsome four grams, to an elephant of 2000 kilograms,in spite of the fact that the animals under comparison belong to different orders,and are adapted to all sorts of climate and ways ofliving(Fig. 1). The relationbetweenmetabolicrate and body size belongsto the classical topicsof physiology. It goes back over more than a hundredyears to the time when Sarrus and Rameaux, Bergmann and Leuckart, and Richet noticed that weightspecificmetabolicrate, that is, the intensityof metabolismas measuredby oxygenconsumption or calorieproductionper kilogramofbodyweight, decreases with increasingbody size. A classical example is provided by Rubner's experiments size (Table 1). It appears with dogs of different that metabolic rate per kilogramdecreases. If, however,metabolismis calculated per unit of body surface, approximatelyconstant values appear. The comparisonof metabolic rates in mammals led Rubner to the contentionthat warm-blooded animals produce daily about 1000 Cal. per square meterof body surface.This is the originof the famous surfacerule, which was explainedbyRubnerin termsofhomeothermy. All warm-bloodedanimals heat theirbodies to a of ca. 37?C. Heat outputtakesplace temperature throughthebodysurface.Hence, thesamenumber of caloriesmust be producedper unit surfacein orderto maintainthe body temperature constant. There are, however, considerable difficulties 217 218 THE QUARTERLY REVIEW OF BIOLOGY 1000 ~~~~80O L IL 3 = ctS0O OVS~~~~~~~MQMTO-' P co DD o - X C ED6Eh'0G 4W" .39 5Sg log !50Q ioOs RABBIT - -A SS -~~~~~~~~~~~~~_T EUOSELjEP 509 gI kg SW I tk!g X , 50k(s l0kg 200O kg 500kq IOQOV9 BODY WEIGHT FIG. 1. ALLomETRic DZEPENDENCE OF PULS5E FEREQuENcY ONBODY WEIGHTIN MAMMLS It rate, pulse basal may be assumed that the volume of blood transported per minute is proportional to the basal metabolic as the oxygen consumed must be transported by the blood. This volume is equal to stroke volume (S) X frequency (F). In a rough first approximation, S may be taken as proportional to body weight (W). The metabolic rate follows interspecifically, in the series of mammals, the W! rule. Hence: S.F. = CW.', and = C'W = Thefigmreshows that the allometryconstantof pulse frequency,a =-.28. Notwithstandingthe gross over whichneglectsanatomical,physiological,ecologicaloand otherdifferencesabsolute body size is the simplirication dominatingfactorin the controlof pulse frequency,in a rangefromthe dwarfbat (4 g. body weight)to the elephant (2000 kg.). ModifiedafterBertalanffy(1951a). TABLE 1 Metabolismin dogs AfterRubner (1902). in kg. Weight 3.1 6.5 11.0 17.7 19.2 23.7 30.4 Cal.per production Cal.m. production per kg. sq. bodysurface 85.8 61.2 57.3 45.3 44.6 40.2 34.8 1909 1073 1191 1047 1141 1082 984 in measuring the outer surfaces of animals exactly,but a simplemathematicaldevice can be applied. If two bodies are reasonablysimilarin shape, their surfacescan be expressedas a 23 powerofweight,sincethecubicrootofthevolume or weightis a lineardimension,and therefore its square has the dimensionof a surface.Hence, the surfaceareas of geometrically similarbodies can be obtained by multiplyingthe 23 power of the weightby a suitableconstant.This is seen in the well-known formulaofMeeh: S =bW (1) The surface rule of metabolism accordingly states that the basal metabolic rate is proportional to the /% powerof the weight.In the case of man, the determinationof the basal metabolism is a clinicalroutine,in orderto diagnose thyroiddisordersand the like.Here thesomewhat more complicated Dubois formula is applied. Dimensionally,however,the Dubois formulais identical with the surface rule. The Dubois formulais: S = kW0425X L0 725.As, presupposing geometricalsimilarity, lengthL = cW', this can be written:S = kWO-420. cW0 725(033) = bW The relationbetweenmetabolicrate and body size can be studiedeitherintraspecifically, i.e., by comparinganimalsofthesamespeciesand different body size, or interspecifically, i.e., by comparing adult animals of differentspecies. We are at present mainly concerned with intraspecific comparison. A grave objection can be raised against the surfacerule as foundin textbooksof physiology. In consideringthe quantitativerelationbetween metabolic rate and body size, homeothermic vertebrates and, in particular, mammals are almost solely taken into account (e.g., Brody, 1945; Kleiber, 1947; Krebs, 1950). However,the case ofmammalsis by no meanssimplebut rather QUANTITATIVE LAWS IN METABOLISM AND GROWTH is intricate.Moreover,as we shall see presently, many familiar conclusions and explanatory hypothesesfall flatif not onlymammalsbut also vertebratesand invertebratesare poikilothermic necessary It is therefore takeninto consideration. to considerthe problemon the broaderbasis of comparativephysiology.A considerablepart of thisworkhas been carriedthroughin the author's laboratories. In orderto understandtheseresults,one more mathematicalformulais necessary.The dependence of the metabolicrate of an animal on body size can be expressedin the equation: 219 TABLE 2 CO2production pallasii ofArmadillidium (Temperature 21?C.) AfterMUller (1943b). 15 33 50 100 160 Weightin mg. 3.0 5.2 7.2 11.2 15.2 Cmm.C02/hr. 200 174 144 112 94 Perg./hr. Perunitsurface (WI)/ 48.5 54.2 53.0 49.8 51.6 hr. of animals in which the surfacerule does not hold. 3. Thus we come to the statementthat several M = bWG, (2) metabolictypesexistwithrespectto the relation whereM is the metabolic rate per unit time, betweenmetabolicrate and body size. In view of what was said previously,three W the body weight,and a and b are constants. metabolic types,that is, three different ways of This is a special case of the so-called allometric fornula (Huxley, 1932) which expresses the dependenceof the metabolicrate on body size this classificationapplying, dependenceon body size foran enormousamount can be distinguished, of morphological, biochemical,physiological,and as was emphasized, to intraspecificallometry, evolutionarydata. This formulacan furtherbe that is, to individuals of differentsizes or to growinganimalswithinone species. way: writtenin thefollowing In thefirsttype,metabolicrate is proportional (3) log M = log b +o? log W to a surfaceor the 23 powerof theweight.RepreThat is to say, if metabolic rate is plotted sentatives of this type include fishesbut also such as crustaceans,clams, we certaininvertebrates, against body weight double-logarithmically, obtaina straightline the slope of whichindicates and ascaris. Table 2 presentsone example, the theconstanta. Ifa = 3, themetabolicratefollows metabolic rate in the sowbug, Armadillidium. the surfacerule. If a = 1 or the slope is 450, the As can be seen, its oxygenconsumptionper unit metabolic rate is proportionalto weight. With weightdecreaseswith increasingbody size, but remainsconstantper unit surface.Subsequently 1 > a > 23, an intermediary case obtains. values are taken, that is, if it will be seen that sowbugand companyreveal If weight-specific metabolicrate per unit weightis plottedinstead quite a bit about human growthas a central of thatof the total animal,the equationbecomes: problemofphysiology. Here the The second type is quite different. M =bW (4) metabolic rate is proportionalnot to surface area, but to weightitself,so oxygenconsumption metabolicrates, in an animal of double size is simplydoubled,in weight-specific Correspondingly, as a generalrule,decreasewithincreasingweight, an animal fourtimesas large is quadrupled,etc. and the slope of thelogarithmic plot is negative. Direct proportionality of metabolicrate to weight After these preliminaries,we can summarize is found in growinginsect larvae and hemimethe experimentalresults in the followingway tabolous insects, as well as interspecifically, in 1941b,et seq.): (Bertalanffy, comparingimagos of differentrelated species. 1. The surfacerulealso holdsforpoikilothermic Table 3 shows metabolic rates in the walking The rule is, stick, Dixippus morosus.Oxygen consumption vertebratesand certaininvertebrates. of a wide application;but the explana- per gramand hour appears to be constantover a therefore, tion given by Rubner is too restricted,for in wide range,coveringall body sizes and the entire animals thereis no thermoregula- development.Other groups belonging to this poikilothermic tion, and thus the latter cannot be the basic type are land snails of the familyHelicidae, factor in the relation between body size and intraspecifically as well as interspecifically, and metabolicrate. annelidssuch as the earthworm. 2. On the otherhand, there are many classes Finally, in the thirdtypemetabolicrates are 220 THIE QUARTERLY REVIEW OF BIOLOGY TABLE 3 Oxygen consumption ofDixippusmorosus (Temperature 20?C.) AfterMuller(1943a). 8 130 250 450 630 850 Weightin mg. 2.0 30.6 60.7 113.2 154.8206.6 Cmm.02/hr. Perg./hr. 250 236 243 252 245 242 TABLE 4 Oxygen consumption ofPlanorbis sp. (Temperature 23?C.) AfterBertalanffy and Muller(1943). Weightin mg. Cmm. 02/hr. 30-3558-6290-100140 190-200 2.3 3.9 5.4 69 65 56 Perg./hr. Per unit surface 22.9 25.1 26.1 (WI)/hr. 7.3 9.5 52 48 27.0 28.2 intermediatebetween proportionalityto weight to surfacearea. To this type and proportionality belongsuchpond snailsas Planorbisand Lymnaea like Planaria. Table 4 gives data and filatworms fortheramshorn snail,and showsthatitsmetabolic rate decreases with respect to its weight,but increaseswithrespectto its surfacearea. The relationsmentionedare typicaland characteristicof the species concerned.Table 5 gives a survey of available observations.A few minor discrepanciesneed elucidation,but in generalit can be said that the "metabolictype," i.e., the relationofmetabolicrate to bodysize,is a physiological characteristicof the species or group of speciesconcerned. INTERPRETATIONS OF THE SIZE DEPENDENCE OF METABOLISM We now come to the question,what is at the basis of the relationbetweenmetabolicrate and body size and, in particular,of its most familiar form,the surfacerule? (Cf. Kleiber, 1947; Bertalanffy,1951a; Bertalanffyand Pirozynski,1953; We must admit that we withfurtherreferences). do notknow.What can be shown,however,is that the explanationsusuallygivenare insufficient. There seemsto be, first,thealternativewhether the dependenceof metabolicrate on body size is based upon cellular or upon organismic factors. That is to say, thedecreaseof metabolicrate with increasingbody size may be due to intrinsic in themetabolismofthecellsofsmaller differences and larger individuals,in which case it should also be foundin the respirationof tissuestaken out of the organism;or else, it may be regulated by factorspresentand active onlyin theorganism as a whole. Let us start with the organismic hypotheses. The most familiarone has already been mentioned, namely, thermoregulation. There is no doubt that energyexpense for thermoregulation formsa considerablepart of the total metabolism in homeothermic animals.However,this explanation cannot be generalsince the surfacerule also applies,and in factis moreaccuratelyestablished, in cold-bloodedvertebratesand even in certain invertebrates wherethereis no thermoregulation. Anotherinterpretation assumes that the surfaceruleis based upon theanatomyand physiology of the circulatory system.The supply of oxygen and nutritivematerialsto the tissuesis naturally a functionof the intensityof the blood current. The latterdependson factorssuch as the size and stroke volume of the heart, the frequencyof heart beat, the diameterof the blood vessels,the degree of capillarization,and the like. As has already been indicated, there are rather strict quantitativerelationsbetween body size, metabolic rate, and pulse rate. Thus, in interspecific comparison"fromthe mouse to the elephant", pulse rate decreases approximatelyproportional to the % power of the weight(Fig. 1). So does basal metabolicratein theinterspecific comparison of mammals,if adult specimensof corresponding species are plotted (Brody, 1945; Kleiber, 1947). However, hemodynamicscannot offera general explanation.Remember,for example,the clams, where the circulatory system is completely fromthat foundin vertebrates,or recall different ascaris, which has no blood circulationat allanimalswhosemetabolicrate nevertheless follows the surfacerule. Recent investigations of the Ludwiglaboratory (Ludwig, 1956; Kienle and Ludwig, 1956; Sattel, 1956) give some supportto the hypothesisproposed by Ludwig and by Bertalanffy(1951a, p. 252f.) that the "metabolictypes" are connected withtypesof respiratory apparatus.Gill-breathing animalsappearto followthe surfacerule;henceits validityin fishand certaininanimateclasses. On the otherhand, the surfaceof tracheasin insect larvae developsproportionalto body volume,as was shownby Sattel (1956) in Bombyxmori;hence the proportionality of metabolicrate to weight. TABLE 5 Relationbetween metabolicrateand bodysize Species PLATYHELMINTHES Dugesia gonocephala NEMATHELMINTHES Ascaris lumbricoides ANNELIDA Lumbricussp. Eisenia foetida MOLLUSCA Lamellibranchiata Anodontacygnaea Dreissensiapolymorpha Prosobranchia Lithoglyphus,Paludina fasciata and P. vivipara Pulmonata Lymnaeastagnalis Lymnaeastagnalis Lymnaeaacuricularia Planorbissp. Planorbiscorneus Planorbiscorneus Isidora proteus Pulmonata and Operculata,15 species intra-andinterspecific Helicidae Helix, Chilotrema,and Cepaea (interspecific) Cepaea vindobonensis CRUSTACEA Branchiopoda Daphnia pulex Artemiasalina Isopoda Asellus aquaticus Asellus aquaticus Armadillidiumpallasi Porcellioscaber Oniscusasellus Ligia oceanica Decapoda Astacus astacus Potamobiustorrentiuin Pugettiaproducta Homarusvulgaris INSECTA Hemimetabola Dixippus morosus Holometabola Various species,intra-and interspecific Tenebriomolitor PISCES Lebistesreticulatus Various species (Scorpaena,Abramis, Cyprinus,etc.) REPTILIA Lacerta Reference to Respirationproportional W2'8(surface),W (weight) or intermediate Bertalanffyand Muller, 1943 Intermediate Kruger,1940 Surface Muller, 1943b Kruger,1952 Weight Surface? Weinland,1919 Ludwig and Krywienczyk,1950 Surface Surface Krywienczyk,1952a Surface and Muller, 1943 Bertalanffy Fusser and Kruger,1951 Krywienczyk,1952b and Maller, 1943 Bertalanffy Fusser and Kruger,1951 Krywienczyk,1952b Krywienczyk,1952b v. Brand, Nolan and Mann, 1948 Intermediate Intermediate Weight? Intermediate Intermediate Intermediate Intermediate Surface (high tenperature[30?C.]?) Liebsch, 1929 Weight and Muller, 1943 Bertalanffy Weight 1948 Jan6aroik, and Krywienczyk,1953 Bertalanffy Surface Surface Muller, 1943b Will, 1952 Muller, 1943b Will, 1952 Will, 1952 Ellenby, 1951 Surface Surface Surface Intermediate Surface Probably surface Kalmus, 1930 Wolsky,1934 Weymouthet al., 1944 Thomas, 1954 Surface? Weight? Intermediate Surface Muller, 1943a Weight Kittel, 1941 Weight and Muller, 1943 Bertalanffy Weight and Muller, 1943 Bertalanffy Jost,1928 Surface Surface Kramer,1934 Surface 221 222 THE QUARTERLY REVIEW OF BIOLOGY Intermediatecases wouldresultfromthe presence and Campbell, 1952), of succinodehydraseand malicodehydrase (Fried and Tipton,1953). apparatus. oftwo typesof respiratory The picture, however, is differentin intraStill anotherexplanationof the surfacerule is based upon anatomical or chemicalchanges in specificcomparisons,as betweenrat tissuesfrom body size and age. Seven compositionwithincreasingbody size. "Metaboli- animals of different callyactive" organssuchas theviscera,thebrain, main organsof the rat have been investigatedby etc., are relativelylargerin small as comparedto Bertalanffyand Pirozynski (1951, 1953), and large animals. So it can be assumed that they skeletalmusculature,whichis particularlyimporconsumerelativelymore oxygenand are respon- tant because it formsa highpercentageof body metabolic mass, was studied by Bertalanffyand Estwick sible for the higher weight-specific decline rate in smallerorganisms.However,the relative (1953). As Fig. 2 illustrates,no significant fromone of average Qo2withincreasingbody size is found growthof innerorgansis verydifferent organto the other,and so it is improbablethat it in brain, lung, and kidney; a slight decline in can yield the simplerelationof the surfacerule skeletal muscle,liver, and heart; and a marked 1951a). A quantita- declinein thediaphragm.So thereis no systematic ofmetabolism(cf.Bertalanffy, and Pirozynski,1953) decrease of Qo, in the various organs consistent tive estimate(Bertalanffy to account with, and responsiblefor the decrease of the shows that this factoris not sufficient weight-specific metabolic rate with increasing fortheactual variationsofbasal metabolicrate. in termsof body size. These results have been essentially Now we come to the interpretations intracellularfactors.This amountsto sayingthat confirmedby other workers and with other metabolic rate materials: in growingchicken by Crandall and the decrease of weight-specific with increasingsize, as expressedin the surface Smith (1952), in the heart muscle of the guinea decrease in the pig by Wollenberger and Jehl(1952), in theteleost rule, is due to a corresponding is measured brain by Vernbergand Gray (1953), and in rat oftissues.Tissuerespiration respiration as Qo,, that is, ,l 02/mg. dry weight/hr.,as testes by Homma (1953). Similarly,Fried and determinedwith the Warburg apparatus. A Tipton (1953) did not find a decrease in the enzymes. considerableamount of work has recentlybeen contentof respiratory From this it would appear that genetic,and done along these lines,partlystimulatedby our in the enzymatic differences own work now to be presented,just as we may hencespecies-specific, also say that the interestin comparativemetab- activityand Qo, oftissuesare foundin interspecific however,in animals of olism as classifiedin the metabolictypes men- comparisons.Differences, weightare irregular tionedhas been stimulatedby the investigations the same speciesand different The ques- withrespectto thevarioustissuesor are absent. on growthlawsto be explainedhereafter. is a So we have to assume factorswhich,within tionofthe size dependenceoftissuerespiration controversialone, but the statementsto follow the intact organism,regulate the respirationof the tissues,the sum total of whichis the metabappear to be a fairpresentationof the case. In interspecificcomparison of mammalian olism of the entire animal, but which do not species of different sizes, rangingfromthe mouse show up in the isolated tissueused forWarburg and Pirozynski,1951, (Bertalanffy to the horse, a decrease of Qo, with increasing determination body size is generallyfound, as a number of Schmidt-Nielsen,Bertalanffy,and Pirozynski, observershave established(Kleiber, 1941; Wey- 1951). We have alreadysaid that the organismic mouth, Field, and Kleiber 1942; Krebs, 1950; factors usually contemplateddo not offer a Martin and Fuhrmann, 1955). This decrease, satisfactoryexplanation.What one may expect however,is not parallelin the variousorgansand, can be illustratedby the action of thyroxin.It is as a generalrule,is less thanwouldcorrespondto easy to induce an increase of metabolismby into the animalin vivo; but thesurfaceruleor the 3 powerruleofmetabolism. injectionof thyroxin way,a decreasewithincreasing in spite of many efforts In a corresponding made, nobodyhas been this effectby an body size was found in enzymaticsystemscon- able to reproducesatisfactorily such as in the concentra- administrationof thyroxinto a tissue in vitro. nectedwithrespiration, tion of glutathione (Gregoryand Goss, 1933; On the otherhand, chronichormonalconditions Patru'sev,1937), of cytochromec (Rosenthaland are manifestedby significantchanges of the Drabkin, 1943), of cytochromeoxidase (Kunkel tissue Qo, as has been shownwith tissuesfrom QUANTITATIVE LAWS IN METABOLISM AND GROWTH 223 15 7.~~~~~~~~~~~~~~z 6 t--Y?US 4 31 a 1o 20 30 40 50 60 eo MO l_ no 300 J 400 BODY WEi6rT IN G. FIG. 2. TIssuE RESPIRATIONor VARious ORGANSOF TIE WHITE RAT IN RELATIONTO BODY WEIGHT Qo2 = .dl02/mg.drywt./hr. Only regressionlines are shown; for individual data and statisticalevaluation cf. the originalpaper. After and Pirozynski(1953). Bertalanffy hypophysectomizedanimals and in pituitary thegrowthcurvesoftheseveralspecies.It appears dwarfmice,whichlack somatotrophin (Bertalanffy that we have been successfulin establishinga and Estwick,1954). definiteand strictconnectionbetweenmetabolic The writerdoes not feelhappy about thisstate types and growth types, in consequence of a of affairs,and the situationwould be much more generaltheoryofgrowthwhichestablishesrational satisfactoryif a straightforward relationbetween quantitative laws of growthand indicates the the decrease of the weight-specific metabolic physiologicalmechanismupon which growthis rate and the tissue respirationcould be found. based. Indeed, the Ottawa study was startedwith this Let us startwitha ratherobviousdeliberation, was not borne firstindicatedby Putter (1920). Animal growth expectation,which,unfortunately, can be considereda resultof a counteractionof out by thefacts. The explanationof the surfacerule and of the synthesisand destruction,of the anabolismand size-dependenceof metabolismin general thus catabolismof the buildingmaterialsof the body. remainsratherunsatisfactory. We have,at present, There will be growthso long as buildingup preto take the metabolictype,in the sense defined, vails over breakingdown; the organismreachesa as an empiricaldatum of the species concerned. steady state if and when both processes are However, even this cautious attitude leads to equal. We may expressthisin a generalformula: certainremarkableinferences with respectto the -KWn. dWIdt = nWWm (5) problemof growth. METABOLIC TYPES AND GROWTH TYPES It has already been stated that, among the various animal classes, so-calledmetabolictypes can be distinguishedby virtue of the relation between the metabolicrate and the body size. Now as thereare different metabolictypes,there are also different growthtypeswhich are distinguishedby the course of growthas expressedin In words: The change of body weight W is given by the difference betweenthe processesof building up and breakingdown; v and K are constants of anabolism and catabolism respectively,and the exponentsm and n indicatethat the latterare proportionalto some power of the bodyweightW. Obviously the growthof any organismis of an enormouscomplexity,whetherwe considerit 224 THE QUARTERLY REVIEW OF BIOLOGY froma biochemical,physiological,morphological, physiologicalfacts (cf. Bertalanify,1951a), that of it is directlyproportionalto weight.On the other or any otheraspect. However,the justification an overallformulaand thesimplemodelit implies hand, mathematicalconsiderations(Bertalanffy, Our equationstates that the 1941b) show that our basic equation is rather lies in the following. grossresultofsyntheticand degradativeprocesses insensitiveto smallerdeviationsof the exponent withinthe organismfollowsthe law of allometry, n fromunity.So we may put, withoutany conthat is, that the rate of these processescan be siderableloss of generality,the exponentn equal expressedas a power functionof body mass. to 1. This makesthesolutionofourbasic equation But this assumptionis justified,because at least mucheasier. therateofall physiological The solutionofequation5 (n = 1) is (Bertalanffy, in a firstapproximation processeshithertoinvestigatedcan be expressedin 1941b): allometric or power formulas (Adolph, 1949). W = {87/K - [vq/K- Wo (1-m)]e-(1_-tn)Kt}1-m (6) The intrinsiccomplexityof the phenomenon concerneddoes not preclude it from following withWo = weightat timet = 0. forexample, sucha simple,generallaw. Remember, The case is somewhatdifferent withrespectto what has been found in the dependenceof the anabolism. The synthesisof high-molecular cell basal or restingmetabolismof the intact animal. componentsneeds, on the one hand, building Of course,what is called the basal metabolicrate blocks such as amino acids, sugars,phosphates, is, in fact,the outcomeof innumerableand to a and so forth,and on theotherhand energywhich, large extentunknownprocessesof intermediary in aerobic animals, is provided by oxidative metabolism. Not only this, but the growing processes. Both can be taken into account as organismundergoeschanges at the biochemical, limitingfactors.The experimental resultsindicate physiological,cellular,and morphologicallevels. that,so faras higheranimalsare concerned,there that a is a lawfulconnectionbetweenrespiration,anabNevertheless,we can state quite definitely certain organismobeys, let us say, the surface olism,and growthwhich works out in the folrule; that is, that the rate of metabolismof the lowingway. entireanimal,whateverits size or developmental The exponentn in our basic equation denotes age, can be expressedas a functionof the -' the dependenceof anabolismon body weight.If powerofits respectivebodyweight. we insertform thatvalue whichis experimentally We have nowmorecloselyto definetheprocesses foundforthe size dependenceof restingmetaboappearing in our basic equation. The catabolic lism,thegrowthlaws forthe organismin question processesmean, of course,the continuousloss of follow automatically.Thus we can predict the buildingmaterialas it takes place in any living growth type of an animal from its metabolic organism.Biochemically,thismeans the turnover type,and thispredictionhas provedto be correct of buildingmaterialsand particularlyof proteins, in a largenumberof cases, oftenin a quite unexas demonstratedby the isotope techniques. pectedway. Cytologically,it means the renewal of cells, as In a firsttype,respirationis proportionalto the found in many tissues and organs,oftenat an 23 power of weight,according to the surface the law of growthassumes the unexpectedlyhighrate (cf. Leblond and Stevens, rule. Accordingly, 1948; Storeyand Leblond,1951; F. D. Bertalanffy following form: and Leblond, 1953; Leblond and Walker,1956; a dW/dt - W2 -KW. (7) table of the rates of cell renewalas foundby the Leblond school is given in Bertalanffy,1957). We shall not bother with the mathematical The isotopeand othertechniqueshave shownthat elaboration,but show immediatelythe results. theanimalorganismmaintainsitselfin a so-called Fig. 3 gives metabolismand growthin the small dynamic or steady state (Schoenheimer,1947), aquariumfish,Lebistesreticulatus. Metabolicrates, chemicalcomponentsas well as cells being con- measuredas oxygenconsumption,are presented tinuallywornout or degraded,and on the other in the log-logor allometricplot. As will be rehand being replaced by way of resynthesisand membered,in the case of the surfacerule the the formationof new cells. So far as the rate of allometricregressionline has a slope of 23. So catabolism is concerned,we may assume, as a far as the growthcurvesare concerned,the solufirst approximationand based upon various tion of the growth equation gives theoretical QUANTITATIVE LAWS IN METABOLISM AND GROWTH 225 200 __ _ curves with the followingmain characteristics. First, growthrates are decreasingand growth eventuallyattains a steady state. Secondly,the a) -0 curves for weightgrowthand linear growthare 4o 0~ The curve of weight characteristically different. growthis sigmoid,with a point of inflexionat - ---40 ---about one-thirdof the finalweight.The curveof -e00 30 lineargrowthis a decayingexponentialwithouta - 20 60 80 100 200 300 500 800 30 40 turningpoint. This is what we actually find Weight In mg. experimentally. This is themostcommonformofgrowthcurves, foundin fish,in a numberof invertebrateclasses and also, with certainrestrictions, in mammals. 160 40 -The validityof these growthequations has been shown in many examples (Putter, 1920; Berta_ 140 5 3 lanffy,1934, 1951a), and theyhave been adopted 120 6-30 in applied biology. It appears that the "Bertalanffy growthequaE E 00 b) 125----_It tion" is widelyappliedin international fisheries. has been foundto fitthe commercially exploited fishspeciesstudiedby the FisheriesLaboratoryof 80 4~~~~the Ministryof Agriculture, Fisheriesand Food at -0 ?' 2 I 0 Lowestoft(England), withthe possible exception ofthehake (Wimpenny, pers.commun.).A comprehensivetheoreticalmodel of the dynamicsof exploitedfishpopulationshas beendeveloped,where-0 0 in growthof the speciesconcernedis represented 12 10 6 0 4 2 8 by equation 7 (Beverton, 1954; Beverton and Time in weeks Holt, 1957). Discussionof this populationmodel FIG. 3. IHE FIRST METABOLIC AND GROWTH TYPE (which,apartfromfisheries, maywellbe adaptable curves(b) in the Metabolicrate (a) and growtlh to otherpopulations)is beyondthe scope of the Growthcurvesfor,d: Guppy(Lebistesreticutatus). presentreview.It shouldbe mentioned,however, lengthgrowth;- - - - weightgrowth;calculated to equation(7). AfterBertalanffy and Millthat examinationof the variousgrowthfunctions according proposed led to the conclusionthat "von Ber- ler (1943). talanify'sgrowthequationis the mostsatisfactory ofanythathave hithertobeen developed"(Bever- weightis continuallyshiftedin disfavorof the ton and Holt, l.c.). Ampledata as well as descrip- surface.Consequently,so long as the animal is tionof mathematicalanalysiscan be foundin this small, surface-proportional anabolism prevails work. over weight-proportional catabolism, and the A relationsimilarto that stated by Bertalanffy animal grows.The largerit grows,the more the et al. for the surfacedependenceof respiration surplus remaining for growth decreases, and was found by Yoshida (1956) in food intake. eventuallya steady state will be reached where The quantityofplanktonconsumedby thesardine anabolism and catabolism balance each other, is proportionalto the square of body length,and and growthcomesto an end. the same appears to be true for assimilating Now we come to the second type. We have and the gut. organs,such as the gill-rakers said that in certain animals, for example, in This characteristiccourse of growthis easily insects, respirationdoes not follow the surface understood.If a body,withnot too muchchange rule but ratheris proportionalto weightitself. of shape, increasesin size, its surfacesincrease Let us see whathappensin thiscase (Fig. 4). The approximatelywith the second power of the log-logplot of metabolicrate againstbody weight length,but its volume and mass with the third will give a line witha slope of 45?. On the other power. Hence, the ratio between surface and hand, we have to insert1 for the exponentm in C',j -- 226 THE QUARTERLY REVIEW OF BIOLOGY 80 60 a)~~ 0-0 -j Of course,an insect larva does not grow to any indefinitesize. However,growthis stopped here by an altogether differentmechanism. It is metamorphosiswhich abruptly intercepts the exponentialincrease,even causing a decrease in body weightas largeamountsof tissueare broken down in order to develop the imago. The same metabolicand growthtype also applies to hemimetabolousinsectslike the walking-stick, where there is no apparent metamorphosis,but the hormonal mechanismsresponsiblefor development appear to be similar.Again,in land snails, whichalso belongto thistype,exponentialgrowth is interceptedby seasonalcycles. Finally, we have described a thirdmetabolic type,one where metabolic rate is intermediate betweensurfaceand weightproportionality, and whichis exemplified by pond snails.Again we can calculate what growthcurvesare theoretically to be expected.If we inserta value 23 < m < 1 into the basic equation, it appears that the growth should followa thirdtype. The curve of weight growthdoes not differvery much fromthat in - - - 0 -A 10 5--20 60 80 100 30 40 WeightIn mg. (Tenebrio) 200 300 5 E 1: .43 > - -?- 20 b) 2--f a) 0 8jII7 6 = 30 40 o 20 40 60 80 60 80 100 200 300 Weight in mg. 500 700 Timqe in hours (Drosophila) METABOLICAND GROWnTYasPE FIG. 4. THE SECOND (b) culrve(exponentMetabolicrate(a) and growth and Muller ial) in insectlarvae. AfterBertalanify (1943). E8 = '- --V4 {> the growthequation, and thereuponget a comcm e _ _ growthcurveand a secondgrowth pletelydifferent 40 type.In contrastto the firsttype,anabolismand 0 run catabolism,both being weight-proportional, at the same pace. The morecatabolismincreases, the more does anabolism. Therefore,growth 4 12 24 8 0 16 20 rates will not decrease but always increase,and Time in weeks the largertheanimalbecomes,the fasterit grows. FIG. 5. THE THIRD METABOLIC AND GROWTH TYPE Growthis not limitedbut exponential,and no rate (a) and lineargrowth (b) (diameter steady state is reached.This seems to be a para- ofMetabolic snail (Planorbissp.). After shell)in the ramshorn butis exactlywhathappenls. Bertalanffy stateofaffairs, doxrical and Muller(1943). QUANTITATIVE LAWS IN METABOLISM AND GROWTH 227 TABLE 6 Metabolictypesand growth types Metabolictype Growthtype Examples I. Respirationsurface-proportional (a) Linear growthcurve: attainingwithout Lamellibranchs,fish, mammals inflexiona steady state. (b) Weight growthcurve: sigmoid,attaining,with inflexionat ca. 13 of final weight,a steady state. II. Respirationweight-proportional Linear and weightgrowthcurves exponen- Insect larvae, Orthoptera, Helicidae tial, no steadystate attained,but growth or seasonal interceptedby metamorphosis cycles. III. Respirationintermediatebetween (a) Linear growthcurve: attainingwithin- Planorbidae flexiona steady state. surface- and weight-propor(b) Weight growthcurve: sigmoid,similar tionality to I(b). thefirsttype.Lineargrowth,however,is different, developmentrevisions and the introductionof as its curveis S-shapedwith an inflexion.Again, complicatingfactorswill be necessary(forrecent discussions of the theory,cf. Duspiva, 1955; ourpredictionis verified(Fig. 5). animalclasses thereare different Harms, 1955; Linzbach,1955; Zeuthen,1955; BerSo in different 1957). metabolic types and differenttypes of growth, talanffy, A case in point is growthin mammals.From agreeing with theoreticalexpectation.Table 6 givesa surveyof examplesinvestigated,and may the viewpointof metabolism,it may be said that, be consideredas a firstdraftfora new chapterin in a firstand crude approximation,mammals physiologyof appear to belong to our firsttype, where the physiology,namely, a comparative surfacerule of metabolismapplies. Indeed, the growth. From the theoryof growthjust outlined,many surfacerule was firststated by Rubnerformamconsequences can be derived which have been mals, and it was already mentionedthat the verifiedempirically(cf. Bertalanffy1957). Only clinicallyimportantcase, the determinationof one furtherexamplewill be given. We can com- basal metabolismin man, applies the surface pare the values of the catabolic constantK which rulein the somewhatmodifiedformof the Dubois mammalian werecalculatedfromthe growthcurves,withthe standard formula.Correspondingly, pattern values of proteinturnoveras directlydetermined growthroughlyfollowsthe characteristic In a numberofcases, theconstants of the firstgrowthtypediscussed. by experiment. witha degreeofcorrespondence In detail, however, there are complications. have been verified and somewhatparadoxically,there sincethe theoreticalmodel Unfortunately whichis quite striking, and, on the other are relativelyfew good data suitable for this is admittedlyoversimplified hand,theerrorin thephysiologicaldeterminations typeof analysis. If the basal or restingmetabolismin the rat of proteinturnoveris considerable.For example, in 1938 the authorcalculatedthe turnoverrate of (Fig. 6) is measuredover the entirelife span, it the protein from the growth curve of man. The appears that as a crudeoverallapproximation value found was 1.165 g./kg. body weight/day. surfaceruleobtains,thatis, the overallallometric Eleven years later, Sprinson and Rittenberg regressionline has a slope of about 23. In more (1949) calculated protein turnoverfrom their detail,however,thereis a breakin thisline,such withN15,and founda value of 1.3g./ that the firstpart of the allometricline is steeper, experiments kg. body weight/day.Only a sound theorycould and the second part much flatter,than would correspondto a slope of 23. The breaktakesplace have permittedsuchprediction. at a body weightof about 100 g., that is, preGROWTH IN MAMMALS AND MAN cedingsexual maturation.As we have found in It is, however,obvious that the theoryrepre- otherinvestigations(Bertalanifyand Pirozynski, and that withfurther 1952, 1953), similar breaks appear in quite a sentsa firstapproximation, THE QUARTERLY REVIEW OF BIOLOGY 228 curveis different fromall others.Since our growth formulas to a large number of species, apply 1000 _ the shapes of theirgrowthcurves are the same, 800 _ and the same curve can be used to representthe 600 growthof various species, simplyby takingdifE 400 Wt1nYmuS *th r g. ferentscales fortimeand body size. Fig. 8 shows the growthcurves of a fish and a mouse. The latter, similar to that of the rat, also shows a 300 growthcycle in detailedanalysis,whichdoes not CP 100 much alter the picture.If, however,the growth curve of man is entered,it appears to be unique. 8e0 ib. verQor2 n0 The second part of the curve, beginningwith is t fn 5 50 puberty, follows the general pattern. The first 4 40 In infancyand part, however,is very different. I I I 11II 1II1II1 30 400~ 10 15 20 30 40 60 100 200 childhood,the curve is enormouslyprotracted. Body weighti'n g. A new growthcycle is added, as it were,to the or RELATivE GRowTHI typicalpatternof growth.Althoughthis change FIG. 6. DISCONTINUITIES IN THlE ALBiNo RAT is heraldedin thegrowthcyclesoflowermammals, !FAIIdiscontinuities appearat a bodyweightof ap- onlyin man does it lead to a singularshape of the 100g.,i.e.,beforepuberty. A correspond-growthcurve.This growthcurveofman,abnormal proximately of theentire is foundin thegrowth ingdiscontinuity animal(see Fig. 7). Onlyregression linesareshownin as it were,is of courseconnectedwithchangesin data and statisticalanalysisare the hormonalbalance. This is demonstratedby individual thefigure; and pathological cases, such as pubertaspraecox in givenin the originalpapers.AfterBertalanffy Piozynski(1952,1953)and Racine(1953). pituitarydysfunction, when pubertytakes place an at in othermammalsand early age, as it does number of physiologicalcharacteristics,at the in still apes. The singular growth curveofman is a same age and body size: in the allometricgrowth of the liver, the involutionof the thymus,the quantitative expression of the retardation of ofliverand thymus, and certainly human developmentwhich, according to Bolk tissuerespiration (1926), is one of the basic factorsin the evolution thereare others(Fig. 6). There is small wonder of man. At the same time, it is an important that these breaks and shiftsare found,as the factor forhis uniquenessin nature.Animalsrun cominginto play of the sex hormonesentails a theirdevelopmentalperiodspeedily,and through deep-reachingchange in the entire metabolic pattern. _ _ _ __ ___300 300 On the other hand, the growthcurve of the rat was analyzed long before the physiological E __E studiesjust mentioned(Bertalanify,1938, 1951a; c200 -200 _ _ _ _- _ a discussionof recentliteratureon rat growthis given in Bertalanffy,1957). The result was foundthat the growthof the rat followsthe first too R100 _______ growthtype, with a characteristicchange,however,in the values of the constants,again at the criticalpoint of around 100 g. body weight.If these growthcycles are taken into account, an 200 300 400 0 100 Time in days excellentfit of the empiricalgrowthcurves by FIG. 7. GROWTH OF THE ALBINO RAT meansof our formulascan be obtained(Fig. 7). So it wouldappear thatmammalsbelongto the Donaldson'sdata (d1). Length growth calculated according to equafirsttype, with the qualification,however,that weight growth----, (7). The calculation showstwogrowth cycles,with two growth cycles must be distinguished,the ation breakat approximately 100g. bodyweight. Donaldtransitionfromthefirstto thesecondcycletaking son'sdataaretodaynotconsidered tobe optimal, since dietshave increasedthegrowth in place at the timeof sexual maturation.This is to modernlaboratory the rat.The figure shows,however, the excellentnube consideredno morethan a firstapproximation. mericalfitthat can be obtainedwiththe theoretical However,thereis one organismwhose growth formulas. AfterBertalanffy (1941b). 1500 _ . \P QUANTITATIVE LAWS IN METABOLISM 80~~~~~~~~~~ - -______ - 3: (;0 ~ 0 _ _ 40 _ _ 20 20 5 o0 5 Mouse x lo years lO weeks Abramis bramao 5 ~~0 0 5 ~~ 0- 7V o0 37 SUMMARY Man* < c 40 z- ________x _ 229 and the searchCouncil,theNationalCancerInstitute, ResearchCouncilof Canada.A surveyof Humanities the problemof animalgrowthin generalis givenin 1951a,and 1957. Bertalanffy, 100 _80 AND GROWTH _____ years______ Time in days FIG.8. CO1PARISON O0FTHE GROWTH CURVES OF A FISH,MOUSE,AND MAN AfterBertalanffy (1951a). soon they reach puberty and the adult stage. Man, on the otherhand,is givena longperiodof youth,and is thusenabled to learnand to collect experience.Thus the characteristic humangrowth curve is a prerequisitefor that mental development and civilizationwhich so sharply distinguishesman fromall otherbeings. ACKNOWLEDGMENT Thisessayis baseduponworkcarriedthrough in the author'slaboratories at theUniversity ofViennaand theUniversity ofOttawa(Canada).Partofthiswork was aided by researchgrantsfromthe NationalRe- between connections Workaimedat establishing metabolismand growthis reviewed.In thevarious animal classes, three "metabolic types," i.e., formsof dependenceof metabolicrate on body size can be distinguished:proportionalityof metabolicrate to surfacearea, or to weight,or one intermediatebetween surface and weight proportionality.The various theoriesregarding the size dependenceof metabolismare discussed, with particularconsiderationof the relation of to tissue respirationto body size. Corresponding the "metabolic types" mentioned, there are growth by different "growthtypes" distinguished curvesof the speciesconcerned.A generaltheory ofanimalgrowth,advancedby theauthor,permits explanationof the connectionbetweenmetabolic typesand growthtypes.The theoryis illustrated by examplestaken frominvertebrateand vertebrate classes. Mammalian and human growth, whilegenerallyfollowingthe "firstgrowthtype," show breaksin the growthcurve connectedwith puberty.The data and conceptspresentedherald a new chapter of physiology,the comparative physiologyofgrowth. LIST OF LITERATURE E. F. 1949. Quantitative relationsin the physiologicalconstitutionsof mammals. Science, 109: 579-585. 1953. The BERTALANFFY, F. D., and C. P. LEBLOND. continuousrenewal of the two types of alveolar cells in the lung of the rat. Anat. Rec., 115: 515541. uber BERTALANEBY, L. VON. 1934. Untersuchungen die Gesetzlichkeitdes Wachstums. I. 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