Metabolic heat production of parents and offspring

Camp. Biochem. Phytiol., 1972, Vol. 42A, pp. 953 zo 965. Pergamon Press. Printed in Great Btitain

METABOLIC

HEAT PRODUCTION OFFSPRING

OF PARENTS AND

N. J. DAWSON” Department of Physiology, University of New England, Armidale, N.S.W. (Received

2351, Australia

29 November 1971)

Abstract-l. Aspects of the physiological response of homeotherms to cold are reviewed. 2. A respirometer for small mammals is described. 3. The metabolic heat production of parents and offspring of the house mouse (Mus muscuZus)and of inbred mice, was determined. 4. The physiological and ecological implications of the results obtained are discussed.

INTRODUCTION

MAINTENANCE of the constancy of deep body temperature has been recognized as a phenomenon of mammalian homeostasis for many years; work on the physiological systems that maintain this constancy through control of heat production and heat loss, and on the accompanying neural control mechanisms, has given rise to a large literature which includes reviews by Bazett (1949), Hardy (1953, 1961, 1965), Strom (1960), von Euler (1961), Benzinger et al. (1963), Brengelmann & Brown (1965), Hammel (1965, 1968), Bligh (1966), Bard (1968) and Benzinger (1969). Studies on behavioural mechanisms subserving thermoregulation have also complimented these approaches (Robinson & Lee, 1946; Stinson & Fisher, 1953 ; Weiss & Laties, 1961 are representative contributions). The mechanisms alluded to above operate rapidly for the control of heat production and heat loss, and may conveniently be distinguished as “short-term” adjustments. If the studies mentioned above, and commonly referred to as work on “temperature regulation”, are taken to refer to relatively short-term adjustments, then it is possible to distinguish other studies of what may be categorized as “mediumterm” and “long-term” adjustments; these are properly included also under the heading “temperature regulation”. It has been observed that homeotherms exhibit a number of physiological responses to exposure to a constant low temperature, or to a cold climate, and these responses take place over a somewhat longer period of time than the “short-term” adjustments. These responses may be called “medium-term” adjustments, and are described largely under the headings “acclimation” and “acclimatization” by Hart (1957). From his review of the relevant literature, Hart (1957) concluded that, in general, animals acclimated * Present address: Department of Physiology, Auckland, Private Bag, Auckland, New Zealand. 953

School of Medicine,

University

of

9.54

N. J. DAWSON

to a constant low temperature exhibited increased metabolic rate, reduced overall insulation and lower lethal temperature limits, whereas animals acclimatized to an equivalent thermal load by exposure to a cold outdoor situation exhibited little or no change in metabolic rate, increased physical insulation and lower lethal temperature limits. These observations fit very well the model of the animal as a heat engine proposed by Hart (1957). “Long-term” adjustments to low temperatures, by definition, may be observed in species that have lived for many generations in a cold climate such as the Arctic. For the present discussion the classical work of Scholander et al. (1950a, b, c) will be cited. These workers found that, in general, the critical temperatures of tropical mammals were much higher than those of arctic mammals; and the slopes of the metabolism/temperature curves were much less steep in the arctic than in the tropical animals, which indicated greatly increased body insulation in the former. There were no significant differences in the rates of metabolic heat production between the two groups. Because natural selection presumably has acted on heritable characteristics over many generations to produce animals adapted to cold or hot climates, it may be supposed that the physiological characteristics of the arctic animals studied by the above workers are the result of genetic selection; such a pattern of response has been termed “adaptation” by Hart (1957). The author concluded that when temperature regulation is viewed in a broad way, as outlined briefly in the preceding paragraphs, one of the major questions left unanswered is that relating to the reason why natural selection appears to have “chosen” the insulative rather than the metabolic answer to cold stress. This conclusion led to the experimental work described by Dawson (1970), namely, a series of experiments all related to the final one where the possibility of a phenotypic correlation ‘between the metabolic rates of parents and their offspring was investigated with a view to suggesting an answer to the question posed above. It could be hypothesized that the metabolic response to cold climate would be disadvantageous because of the energy cost of such a response, and that the insulative avenue would be preferable from the point of view of food conservation. On the other hand, insulation might be more strongly heritable than metabolic rate, and therefore more subject to selection pressure. When the author initiated the work described in this paper there was no information in the literature available to him that allowed speculation on an answer to the above questions, but subsequently a valuable and relevant paper appeared by Pennycuik (1967). It was decided to measure the metabolic rate of a group of mice, and subsequently on their offspring, and to determine if a significant correlation existed between the two. Because no suitable respirometer was available for the work, one was designed, developed and constructed. This respirometer is described in some detail below. MATERIALS

AND METHODS

A?hdS Approximately twenty adult male and twenty adult female outbred agouti house mice (Mus musculus) and their offspring were used. In addition, seventeen adult male and female

FIG. 1. The elements of the metabolism chamber. Top to bottom: copper base, showing inlet ring for circulating oxygen; Perspex lid (on dark background) and knurled retaining bolts; gauze platform; gauze container for calcium chloride, and carbon dioxide absorbent.

METABOLIC

HEAT

PRODUCTION

OF PARENTS

AND

OFFSPRING

955

inbred mice of three strains (C3H, CBA, 101) were used. The animals were housed in an air-conditioned mouse colony at 22°C with a 14 hr light-10 hr darkness regime, and had ad lib. access to water and pelleted ration (Drug Houses of Australia Ltd.). The respirometer The respirometer was an automatic manometric type with four test chambers. The principle of the instrument is that as the animal consumes oxygen, and while carbon dioxide is removed by an absorbent (Sodasorb), the pressure at constant temperature is reduced; this causes displacement of the mercury in a slope manometer. Contacts are operated by the mercury in the manometer in such a way that the pressure is restored by the introduction of oxygen through a solenoid valve. The number of operations of the solenoid during a given time is counted; with a knowledge of the volume change necessary to operate the solenoid, the oxygen consumption per unit time may be calculated. Each part of the respirometer will now be described. Water-bath The water-bath is constructed of 1 cm thick Perspex sheet fastened with screws and Acrifix 92, with outside dimensions of 111.5 x 57 x 17 cm. Temperature control This is by means of a Tecam unit. The unit contains an impeller pump for mixing, which is connected to a tube system in the bath to ensure even mixing. The adequacy of mixing was tested by watching the movement of dye introduced into the water. No detectable variation in temperature during a switching cycle could be detected by a thermometer which read to O*l”C and which was placed in the vicinity of the metabolism chambers, Metabolism

chambers

These were constructed of Perspex for the lid and 18-gauge copper for the base. The Perspex is 3 mm thick, except for the sealing surfaces which are 6 mm thick (see Fig. 1). An airtight seal is ensured by the use of petroleum jelly between the contact surfaces of the lid and the chamber; the lid is held firmly by six knurled bolts (0 BA). The chamber contains an inlet around the bottom for circulating oxygen, with an outlet in the lid. In addition there is an inlet for oxygen from the solenoid valve and an outlet to lead to the manometer. There is a gauze platform on which the animal is placed, and beneath this a gauze container for calcium chloride and Sodasorb (Fig. 1). The chambers are kept submerged in the water-bath by means of lead weights placed on top of the lids. The overall dimensions of the chambers are 14 cm in dia. and 5.5 cm in height. Slope manometers These are constructed of 7 mm o.d. glass tubing. The contacts of Pt/Rh alloy are placed as follows: one in the upright arm; one half-way along the sloping arm; and an adjustable one inserted through a small rubber bung in the end of the sloping arm; these are numbered 1, 2 and 3, respectively, in Fig. 2. The manometers are rigidly mounted on a Perspex holder, which allows the overall slope to be adjusted. The solder on the contacts is covered with Insulex. The mercury is cleaned by-distillation before introduction. Oxygen circulation This is achieved by the use of diaphragm pumps (Inter Mini Pet) used for aerating aquaria, which give a circulation of approximately 60 cm*/min. The pumps are modified as detailed in Fig. 3. A 5 mm o.d. Perspex tube is glued to the inlet side, and a contact adhesive (Cargrip) is introduced in places indicated to ensure that the whole pump is airtight, which is not the case when used in the form supplied by the manufacturers,

N.J.

956

DAWSON A&B

Relay- STC 250.AEO K/O XG15-35V

C Solenoid valve - Dantosr

EVJO

Counter - STC 62 No.ZOOA 59-W-3 Diode - Mullard

0A605 OK3

Diode - Mullard

OA5 2RD

Transformer Manometer

- Ferguson contacts

TS24/60A

- PtlRh

FIG. 2. Electronic circuit to operate relays. Description in text.

Metal

washer

Plastic

CI

-1

washer

Rubber

diaphragm

0

Rubber

washer

E

Valve

housing

_I

o-

“Cargrip”

A-a/

’ -------------Petroleum

<

jetty

%

/--------“Acrifir

92”

i I \

Perspex Petroteum

m

m

“Cargrip”

FIG. 3. Details of modifications to aquarium pump.

tube (0.5 cm1 jelly

METABOLIC

HEAT

PRODUCTION

OF PARENTS

AND

OFFSPRING

957

The oxygen is circulated through the containers labelled I, K, L, M and H in Fig. 4. These are glass specimen tubes 2.5 cm dia. and 10 cm deep. The tubing used for connections is all of polyethylene. Oxygen supply This is from a Douglas bag filled from a cylinder prior to a run. The bag is connected to the chamber through a“Danfoss” solenoid valve Type EVJ4 designed for gases. Positive pressure is obtained by use of a wooden weight placed on the Douglas bag (Fig. 4).

FIG. 4. Diagram of one operative unit of the respirometer. The parts are as follows: A, wooden weight on Douglas bag; B, Douglas bag; C, oxygen cylinder; D, solenoid valve; E, metabolism chamber; F, inlet for circulating oxygen; G, mercury slope manometer; H, calcium chloride container, I, water bubbler; J, modified aquarium pump; K, potassium hydroxide bubbler; L, trap; M, concentrated sulphuric acid bubbler; N, bleed-valve and connection to calibrating burette. Also incorporated in the chain, but not shown, was a trap containing activated charcoal; and in the metabolism chamber a container for calcium chloride and carbon dioxide absorbent. Electronic circuitry The necessary circuitry and components are detailed in Fig. 2. Current supply to the manometer contacts is from a solid state full wave rectifier connected to a 240-25 V transformer. The mercury in the manometer progresses from contact 1 to 2 to 3 as the pressure is reduced. As soon as contact is made between 2 and 3 a circuit is completed which energizes relays A and B, thereby opening the solenoid valve C. As pressure is restored the contact between 2 and 3 is broken, but the solenoid valve remains energized because contact 3 is still closed, and it is not until contact between 1 and 2 is broken that this contact opens and the valve closes. Thus, the separation of contacts 2 and 3 governs the pressure drop necessary to operate the relay and the size of the aliquot of oxygen for operation. The diodes E are necessary to prevent too rapid collapse of the field in the relay coils on break, which would cause sparking between the mercury and the manometer contacts.

0*077*** 0.045 * 0.056** 0.060 * * oG40***

MIDP

MFP MFP MMP MMP

MIDP

AVOF

MFO MM0 MFO MM0 AVOF

o-057*** 0.033 * 0*041* * 0*044** 0*031***

MFP MFP MMP MMP

MFO MM0 MFO MM0

0.002

0.004

0.156 0.168 0.222 0.192

0.003

- 0*004

0.003 -0.107 0.106 0.130 -0.079

0.152 0.164 0.209 0.180

-0.100 0.098 0.128 - 0.089

0.011 0.011 0.012 0.011 0.016 0.016 0.017 0.015 0*004

0.078 0.068 0.151 0.146

0.065 - 0.060 - 0.025 -0.011

0.268 0.184 0.424 0408

1*760*** 2*102**** 2.034** 1*868***

MFP MMP MIDP MIDP

AVOF AVOF AVOF AVOF

0.246 0.250

-0.124 0.051

0.714 0.671

2.422 * * 1 *SOS*

0.138 0.151 0.175 0.148

- 0.077 0.073 0.118 - 0.106

0.456 0.489 0.473 0.410

2.325*** 1*450* 1*726** l-906***

MFP MFP MMP MMP MIDP MIDP

MFO MM0 MFO MM0 MFO MM0

S.E. of b

b

S.E. of a

a

X

Y

0.1247

-0.1693 0.1727 0.1497 -0.1139

-0.2247

0.2987 -0.3186 -0.0637 -0.0170 -0.1620 O-1640 0.1558 -0.1368

-0.1252 0.0551

-0.1387 0.1343 0.1712 -0.1956

r

25

18 15 17 15

25

9 9 9 25 18 15 17 15

18 16

18 15 17 15

N

0.014

0.067 0.054 0.065 0.053

0.030

0.049 0*040 0.048 0.039

1.965 1.965 I.965 1.836

2.076 1.640

2.075 I.673 2.017 1.640

Mean Y

0.020

0.026 0.021 0.025 0.021

0.014

0.019 0.016 0.019 0.016

0.301 0.301 0.301 0.536

0.796 0.653

0.796 0.661 0.780 0.676

S.D. of Y

0.371

0.094 0.087 0.073 0.073

0.200

0.071 0.066 0.056 0.056

1.015

0.830 0.040 0.034 0.029 0.031

0.024

1.384 1.599 0.752

0.800 0.695

2.786 2.590 3.149 2.283 2.716 2.686

1.432 1.203 1.134 1.246

S.D. of X

3.241 3.022 2468 2.502

Mean OfX

In Tables l-4 statistical significance is indicated by asterisks as follows: * 0.01
kcal/BW”‘B6 per hr

kcal/BW0’76 per hr

kcaI/lOO g per hr

Expression of metabolic rate

Resting metabolic rate of offspring and parents

TABLE ~-REGRESSION COEFFICIENTS

2

Female parents Male parents Female offspring Male offspring

Animals

0.212 0.851** - 0.071 0.300

a

0.423 0.246 0.797 0.420

SE. of a 0.022 - 0.007 0.022 0.008

b 0.016 0.008 0.023 0.011

S.E. of b 0.2727 -0.1724 0.2340 0.1713

r 24 24 18 16

N 0.768 0.652 0.693 0.570

P

of Y 0.326 0.213 0.278 0.227

SD.

Relationship of resting metabolic rate (kcal/hr) to body weight (g)

TABLE ~-REGRESSION COEFFICIENTS

24.65 27.90 33.33 35.08

x

3.94 5.14 2.83 5.04

S.D. of X

N. J. DAWSON

960 CdibrCZti0?l

This is achieved by attaching a water-filled burette (previously calibrated with mercury) to the bleed valve N in Fig. 4. Water is allowed to run from the burette until an operation of the solenoid is heard; the volume reading is noted, and more water is run out until another operation is heard, and the volume noted; the volume for a third operation is noted, and this volume difference is routinely taken as the aliquot volume. Suitable damping is achieved by constricting the supply tubes from the Douglas bag. The size of aliquot is adjusted by movement of contact 3 and by altering the overall slope of the manometer. The volume of the aliquot varied within approximately 8.5 per cent of the mean (average of ten calibrations taken randomly). Measurement

of metabolic rate

Before a measurement, the mice were brought from the mouse colony to the laboratory. As soon after as possible they were placed in the chambers; at the same time it was ensured that there was a large excess of calcium chloride and Sodasorb in the gauze containers mentioned above and in containers H of Fig. 4. The animals were allowed to equilibrate for at least 10 min; the readings of the counters were noted on a standard data sheet, and the bleed valves closed. After 1 hr the bleed valves were opened, and the counters again read. The animals were removed from the chambers, and the calibration of the manometers checked. The volume of oxygen consumed was reduced to standard temperature and pressure, and the equivalent heat production calculated on the assumption of a respiratory quotient of O-85. Thus, the heat production figures in this paper may be divided by 4.862 (Diem, 1962) to obtain oxygen consumption in litres at standard temperature and pressure. All measurements were carried out at 29°C.

RESULTS

Tables l-4 summarize the results, with metabolic heat production (Table 1) on three difference bases.

expressed

TABLE ~-RESTING METABOLICRATE (kcal/lOO g per hr) OF INBREDMICE Strain

N

Mean

S.D.

C3H CBA 101

7 5 5

1.012 1.398 1.735

1.070 0.413 1.409

F 2r,6 for difference between strains = 0.469. TABLE

~--DUPLICATE MEASUREMENTS OF METABOLICHEATPRODUCTION (kcal/lOO g per hr) First measurement

Second measurement

N

Mean

S.D.

Mean

S.D.

8

2.736

0,813

2.417

1.011

Student’s t-test for the difference between means = 0.966.

METABOLIC

HEAT

PRODUCTION

OF PARENTS

AND

OFFSPRING

961

DISCUSSION In developing and constructing the respirometer a number of points were noted which it may be beneficial to relate briefly here. (1) The very adequate temperature control was a concomitant of the large size of the water-bath, which highly damped the fluctuations in temperature. (2) The construction of the metabolism chambers partly of copper was based on the observation that temperature equilibration could not adequately be achieved in an all-Perspex container, due to the excellent insulative properties of Perspex. (3) The glass tubing used for the slope manometers appeared to be the optimum size after several sizes had been tried. (4) Adequate cleaning of the mercury was essential to prevent sticking to the sides of the manometers, and could only be achieved by distillation. (5) The covering of soldered joints with Insulex was necessary to prevent amalgamation of the mercury and solder with the resultant formation of a scum on the mercury meniscus. (6) After the modification of the aquarium pumps outlined above it was necessary to check them very carefully in operation for leaks. (7) Rubber pressure tubing, even when new, often was found to contain holes, and consequently was unsatisfactory; polyethylene tubing was the only available material that was satisfactory. (8) The Douglas bag reservoir for oxygen proved very satisfactory; for the lengths of run involved in this work the permeability of the bag to gas would not have been a source of error (Shephard, 1955). (9) The diodes labelled E in Fig. 4 are essential to prevent sparking and consequent dirtying of the mercury. (10) The method of calibration used was analogous to that used by Morrison (1951) on a more sophisticated respirometer, except that the animal was absent from the chamber during calibration; because the volume of the mouse was small compared to the volume of the chamber this was not considered an important source of error. That is, the volume of the chamber and associated apparatus was approximately 300 cm3; if a volume of 35 cm3 is assumed for a 35-g mouse, the animal has a volume equal to 11.7 per cent of the total volume. Because the animals were not widely divergent in body weights the between-animal error due to differences in volume would have been negligible. It is perhaps true that a more accurate respirometer than that constructed by the writer and described here would be desirable, but unfo~unately such an instrument was not available for this work. It was found by Benedict & Fox (1933) that the thermo-neutral temperature of normal mice was about 28.5”C; the work of Herrington (1941) suggests that this temperature is about 3O”C, whereas that of Pennycuik (1967) suggests that it is about 32°C. It is therefore felt that the temperature of 29°C used in the present work was well within the thermo-neutral range, a necessary condition if only because of the lability of the deep body temperature of the house mouse (Sumner, 1913). It will be noted that duplicate deter~natio~ of oxygen consumption were not carried out. This was deliberate, in order to avoid the complication of habituation. However, Table 4 indicates no significant difference between duplicate measurements when they were carried out.

962

N. J. DAWSON

Because of problems of a statistical, anatomical and physiological nature already discussed extensively by the author (Dawson, 1970) and recognized by other workers including Tanner (1949), Gopalan et al. (1955), Chiu & Hsieh (1960) and Joy et al. (1967) the question arose as to the best basis for expressing metabolic heat production. Criticisms may be made of all the modes of expression used in this paper. Basal metabolic rate in relation to the surface law has been discussed extensively by Brody (1945) and Kleiber (1961, 1965), as well as by other authors. The basic observation is that in homeotherms basal heat production per unit body weight decreases rapidly as weight increases. Therefore, because the relationship between basal metabolic rate and body weight is non-linear, simple body weight is not a suitable reference base for metabolism. A more suitable reference base should then be sought. Brody’s final suggestion is that IV’, where b is determined experimentally, be taken as the reference base for metabolism. However, for the reasons outlined by Dawson (1970), this procedure would be unsatisfactory if the value of the constant a in the allometric equation were such that the regression did not pass through the origin. Brody (1945) did not appear to be aware of this need, because he wrote (pp. 363-365): “The ratio of metabolism, X to metabolically efective body size, IV’, or to Xb, should be approximately constant. . . .” After reviewing the literature of his day, and from his own experimental work, Brody (1945) concluded that for mature animals of different species the value of the exponent b was approximately O-7 for metabolic rate and also for other parameters such as endogenous urinary nitrogen and sulphur excretion. Work reviewed by Falconer (1963) suggested that if metabolic rate were measured on forty mated pairs and on two offspring from each litter, some idea might be obtained of the heritability for this parameter. Unfortunately, the matings in this work proved not to be very fertile, and the small numbers shown in the Results section were all that were available in the time for the work. Thus, the work reported in this paper cannot be approached from the standpoint of quantitative genetics. However, the fact remains that no significant differences were found between inbred strains of mice, a conclusion at variance with Pennycuik (1967), who used equally small numbers but more strains, and that no significant correlation could be found between the metabolic rates of parents and offspring. It may not be unreasonable to conclude that if the heritability of metabolic rate were high, it would be evident in significant correlations even with the small numbers available for analysis in the present work. Pennycuik (1967) found that when mice were reared at 34”C, second and third generation animals had higher rates of oxygen consumption than those reared at Zl”C, but by the sixth generation their oxygen consumption was lower. This suggests that metabolic rate may be subject to selection pressure. There is, of course, much evidence that physiological characters are heritable in both a mendelian and a quantitative fashion, just as anatomical characters are. See, for example, Fuller (1951), Schlesinger & Mordkoff (1963), Slee (1964), Schlager (1965), Chai (1966),

METABOLIC HEAT PRODUCTION OF PARRNTSAND OFFSPRING

963

Evans (1966), Oki et al. (1966), Gluecksohn-Waelsch et al. (1967), Lyon et al. (1967), Schlager & Weibust (1967) and Blackmore (1969). The lack of correlation between parents and offspring in this work could be due to the small numbers, although improvement in correlation cannot be noticed in Table 1 when numbers are increased to the maximum possible. Differences in age would not be a source of error because all animals were adult, a necessary condition because oxygen consumption of mice increases up to about 25 days of age, then drops to a plateau at about 35 days (Williams et al., 1966). As pointed out by Dawson et al. (1969) and Dawson (1970), variations in body composition are a source of error in expressing metabolic rate, and there is no guarantee that the population of mice used in this work was homogenous with respect to gross body composition. Lack of significant correlation between metabolic rate and body weight was also observed in this work (Table 2). This is probably due to the relatively narrow range of body weights of the animals used, a cause to which Berman & Snapir (1965) attributed a similar result in the domestic fowl. It will have been noted that all measurements of resting metabolic rate were made in the thermoneutral zone, although this investigation arose out of questions relating to cold adaptation. Facilities were not available to regulate the waterbath of the respirometer at temperatures below the ambient temperature, therefore resting metabolic rates could not be measured at low temperatures. However, for the purposes of establishing a parent-offspring correlation of metabolic rate, measurement of the resting metabolic rate at the thermoneutral temperature is probably as satisfactory as that at some other temperature. It would be logical, however, now to investigate the possibility of a parent-ffspring correlation with respect to summit metabolism as an indication of capacity to respond to cold, and to carry out these measurements on larger numbers of mice than it was possible to use in the present investigation. Acknowledgements-The author thanks the following persons and organizations: Dr. A. Berman, Messrs. D. A. Sharp and M. E. D. Webster for valuable advice; Mr. J. D. Cullen for constructing the metabolism chambers and water-bath; the Australian Cattle and Beef Research Committee (now the Australian Meat Research Committee) for a Senior Postgraduate Studentship; Imperial Chemical Industries of Australia and New Zealand Ltd. for a Research Fellowship; and the University of New England for financial support. This paper reports part of the results of an investigation carried out for a doctoral thesis in the University of New England (Dawson, 1970). REFERENCES BARD P. (1968) Body temperature regulation. In Medical Physiology (Edited by MOUNTCASTLR V. B.), pp. 553490. C. V. Mosbey, Saint Louis. BAZETTH.C.(~~~~)T~~ regulationofbodytemperatures. In Physiology ofHeatRegulation and the Science ofClothing (Edited by NEWFJURGH L. H.), pp. 109-192. W. B. Saunders, Philadelphia. BENEDICT F. G. & Fox E. L. (1933) Der Energieumsatz normaler und haarloser Mause bei verschiedener Umgebungstemperatur. Pjliigers Arch. ges. Physiol. 231, 455-482.

964

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BENZINCERT. H. (1969) Heat regulation: homeostasis of central temperature in man. Physiol. Rev. 49, 671-759. BENZINGERT. H., KITZINGER C. & PRATT A. W. (1963) The human thermostat. In Temperatureits Measurement and Control in Science and Industry (Edited by HERZFELD C. M.), Vol. 3, pp. 637-665. Reinhold, New York. BERMANA. & SNAPIRN. (1965) The relation of fasting and resting metabolic rates to heat tolerance in the domestic fowl. Br. Pot&. Sci. 6, 207-216. BLACKMORED. W. (1969) Differences in pattern of oxygen consumption by newborn sheep of two breeds. J. A&z. Sci. 28, 650-652. BLICH J. (1966) The thermosensitivity of the hypothalamus and thermoregulation in mammals. Biol. Rev. 41, 317-367. BRENGELMANNG. & BROWN A. E. (1965) Temperature regulation. In Physiology and Biophysics (Edited by RUCH T. C. & PATTONH. D.), pp. 1050-1069. W. B. Saunders, Philadelphia. BRODYS. (1945) Bioenergetics and Growth. Reinhold, New York. CHAI C. K. (1966) Selection for leucocyte counts in mice. Genet. Res., Camb. 8, 125-142. CHIU C. C. & HSIEH A. C. L. (1960) A comparative study of four means of expressing the metabolic rate of rats. r. Physiol., Lond. 150, 694-706. DAWSONN. J. (1970) Factors affecting metabolic heat production. Thesis for the degree of Doctor of Philosophy, University of New England, Armidale, N.S.W., Australia. DAWSONN. J., FREDLINED. K. & STEPHENSONS. K. (1969) Physiological implications of genetic differences in body composition. Aust. J. exp. Biol. med. Sci. 47, P-7. DIEM K. (1962) (Editor) Documenta Geigy Scientific Tables. J. R. Geigy S.A., Basle. VONEULER C. (1961) Physiology and pharmacology of temperature regulation. Pharmac. Rev. 13, 361-398. EVANSJ. V. (1966) Red cell electrolytes and haemoglobin. XI Int. Congr. Huemat., Sydney, pp. 1-16. FALCONERD. S. (1963) Quantitative inheritance. In Methodology in Mammaliun Genetics (Edited by BURDETTEW. J.), pp. 193-216. Holden-Day, San Francisco. FULLER J. L. (1951) Genetic variability in some physiological constants in dogs. Am. J. Physiol. 166, 22-24. GLUECKSOHN-WAELSCH S., GREENGARD P., QUINN G. P. & TEICHER L. S. (1967) Genetic variations of an oxidase in mammals. r. biol. Chem. 242, 1271-1273. GOPALANC., SRIKANTIAS. G. & VENKATACHALEM P. S. (1955) Body composition and basal metabolism of normal subjects. J. appE. Physiol. 8, 142-144. HAMMEL H. T. (1965) Neurons and temperature regulation. In Physiological Controls and Regulations (Edited by YAMAMOTOW. S. & BROBECKJ. R.), pp. 71-97. W. B. Saunders, Philadelphia. HAMMEL H. T. (1968) Regulation of internal body temperature. A. Rev. Physiol. 30, 641-710. HARDY J. D. (1953) Control of heat loss and heat production in physiologic temperature regulation. Harvey Lect. 49, 242-270. HARDY J. D. (1961) Physiology of temperature regulation. Physiol. Rev. 41, 521-606. HARDY J. D. (1965) The “set-point” concept in physiological temperature regulation. In Physiological Controls and Regulations (Edited by YAMAMOTOW. S. & BROBECK J. R.), pp. 98-116. W. B. Saunders, Philadelphia. HART J. S. (1957) Climatic and temperature-induced changes in the energetics of homeotherms. Revue can. Biol. 16, 133-174. HERRINGTONL. P. (1941) Heat production and thermal conductance in small laboratory In Temperatureits Measurement and Control in animals at various temperatures. Science and Industry (Edited by FAIRCHILDC. O., HARDYJ. D., SOSMANR. B. 8z WENSEL H. T.), pp. 446-452. Reinhold, New York.

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metabolic rate; mouse; Mus