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Effect of body weight, ambient temperature and huddling on oxygen consumption and body temperature of young mice
Comp. Biochem. Physiol., 1975, Vol. 51A, pp. 79 to 82. Pergamon Press. Printed in Great Britain
EFFECT OF BODY WEIGHT, AMBIENT TEMPERATURE AND HUDDLING ON OXYGEN CONSUMPTION AND BODY TEMPERATURE OF YOUNG MICE MARGARETW. STANIER A.R.C. Institute of Animal Physiology, Babraham,
Cambridge, England
(Received 18 January 1974)
Abstract-I. At ambient temperatures of 30 and 36°C oxygen consumption of newborn mice was linearly related to body weight. 2. Even l-day-old mice responded to an ambient temperature cooler than that of the nest by increasing oxygen consumption. 3. Mice weighing 5 g maintained a body temperature of about 34°C at an ambient temperature of 3O”C, but mice weighing 1.5 g could not do so. 4. At 3O”C, mice of 3-5 g body weight observed in groups of two or four animals had a lower oxygen consumption than a litter-mate observed alone.
INTRODUCTION AN EARLIER investigation
into the metabolic rate of mice of different strains selected for large or small adult body weight had suggested that in the newborn period oxygen consumption (used as a measure of metabolic rate) bore a relation to body weight which was very different from that commonly accepted for adult mammals in which metabolic rate varies with the three-quarter power of the body weight (Kleiber, 1961). For the newborn mice oxygen consumption was proportional to body weight to the mean power of 1.2. The lirst purpose of the present work was to define more precisely the relation of oxygen consumption to body weight in newborn mice of mixed genetic origin, and to determine any effect of ambient temperature. The second purpose was to find whether there was any effect on oxygen consumption of grouping animals together, and the third was to find whether, in the newborn period, the increased metabolic rate observed on cooling the ambient air was sufficient to enable the animal to maintain a constant deep-body temperature. MATERIALS AND
METHODS
Animals and experimental conditions Mice were mated according to a breeding pattern devised to maintain randomized genes. Thirty litters of their progeny, either first or second litters of the parents, were used for measurement of oxygen consumption. The mice, aged between 1 and 11 days, were studied at three ranges of body weight: betweenl-5 and2 g, again between 3 and 3.5 g and again between 4.5 and 5 g. For measurement of oxygen consumption, the mice under experiment were removed from the nest for 1-2 hr, and placed in a metabolic chamber in a temperature-controlled room
held constant to within 0+5”C. The animals were placed in the chamber about 10 min before the start of measurements. In order to obtain a conveniently measurable oxygen consumption within 2 hr, groups of two, three or four litter-mates were placed together in the same cage. The value for oxygen consumption obtained for each group was divided by the appropriate figure to calculate mean oxygen consumption per animal (for the larger animals it was possible to use a single individual for measurement, and this was done in the final series of experiments). Measurements were carried out at 30°C and at 36°C in the morning and afternoon respectively of the same day, on a group of litter-mates of a given body weight though not necessarily the same individuals of the litter. The temperature of 36°C was close to that of the nest, and to the temperature at which previous workers have observed minimal oxygen consumption (Lagerspetz. 1966). The temperature of 30°C was chosen as being below the critical temperature of (adult) hairless mice (Mount, 1971) and in the range at which previous workers have observed
maximal oxygen consumption in newborn mice. Mice of 4.5-5 a weight were also studied at 15-25°C.
ThreeSeparate series of meas urements of oxygen consumption were carried out, at intervals of about 2 months, as litters became available. The effect of group size on oxygen consumption was observed in the third series, by making measurements on one, two, four and six litter-mates placed in separate cages of the apparatus over the same period. Oxygen const4mption The apparatus for recording oxygen consumption consisted of a row of cylindrical spirometers, each of about 15Oml capacity, built in a tank 4Ocm long, 10.5 cm wide and 18 cm deep. Each spirometer bell, tied to a counterpoise by a string led over a pulley-wheel, floated on water contained in a tank. The counterpoise bore a pointer touching a tied vertical scale made from 79
MARGARETW. STANJER
80
millimetre graph paper, and the position of the pointer could be read on the scale at intervals during a period of observation as the bell fell. The scale of each spirometer was calibrated by injecting into the spirometer known volumes of air from a syringe. Each spirometer was filled at the beginning of the period of observation with oxygen from a large reserve spirometer tank, kept in the temperature-controlled room. It was then connected through silicone-rubber and copper tubing with a metabolic chamber in the form of an inverted l-l. beaker. This contained a cage which held the group of mice, so making a closed-circuit system. The cage, a cylinder of wire mesh with a wire mesh disc as its floor, was fixed above a dish of soda asbestos (Carbasorb, 3-6 mesh, B.D.H. Ltd.) to absorb carbon dioxide. The dish and cage were placed in a tray containing liquid paralhn, and covered by the inverted beaker. The liquid parafhn formed a seal and the copper tubing from the spirometer entered through the lip of the beaker. Up to five groups of mice could thus be observed simultaneously. The sixth spirometer, filled with oxygen and connected with an empty chamber, was used as a control to correct for changes of volume resulting from small changes in ambient temperature and pressure during the period of observation. Body temperature For measurement of deep-body temperature a copperconstantan thermojunction (tip dia. O-6 mm) was inserted about 10 mm into the rectum of a mouse which lay in a cage in the temperature-controlled room. The temperature of the room was changed in a stepwise fashion from 36 to 30°C and back again, during a period of observation of about 2 hr. The thermocouple was sufficiently flexible to permit the animals to crawl, but in fact they were at rest during most of the 2-hr period. A reference thermojunction for the thermocouple system was held at 33°C in
a water-bath in the next room. At 5-min intervals, readings were taken of the temperature of the rectal thermocouple, and of that of the ambient air close to the cage. For observations at 2O”C,on mice placed alone or in a group of litter-mates, the same procedure was followed, but the reference thennojunction was held at 31 or 26°C. RESULTS
The mean oxygen consumption per mouse, in ml/hr, for all series of observations, is plotted against the body weight in Fig. 1. This shows (a) that the consumption at 36°C was less than at 30°C and (b) that at 36°C the oxygen consumption was linearly related to body weight. At 30°C although Table 1. Oxygen consumption
of newborn
the increase in oxygen consumption with weight appeared to be less in the range 3.5-5 g than in the range 15-3 g, there was no statistical evidence of any departure from linearity over the whole weight range.
I
I
I
3
1.5 2
0
Body weight
I
35
1
5
(grams)
Fig. 1. Relation of oxygen consumption to body weight in infant mice, Mus musculus, at ambient temperatures of 30°C (0) and 36°C (0). Mean values; bars show 2
standard errors.
Table 1 shows the oxygen consumption per g body weight at the two temperatures, for each range of body weight. The values at 36°C are similar to those recorded by Fitzgerald (1953) for mice at 35°C; and at 36 and 30°C are similar to the results of Lagerspetz (1966) at 35 and 30°C. The effect of group size on oxygen consumption is shown in Fig. 2. For all mice at 36”C, and in the smallest mice at 30°C group size had no significant effect on mean oxygen consumption per animal. In the larger mice at 3O”C, although there was considerable variation between litters, for any one litter the oxygen consumption per animal was significantly lower for measurements made in a group than for measurements on a solitary individual (for difference P =
In the largest animals (body weight 4.5-5 g) observations of the effect of group size on oxygen consumption per animal were carried out at some
mice at 36 and 30°C in ml/hr per g body weight Mean+ S.E.M. (No. of observations in parentheses)
Body weight range (1.5-2-O g) Body weight range (3-O-3.5 g) Body weight range (4.5-5.0 g)
I
4-5
36°C
30°C
2.3OkO.22 (14) 2.76F0.06 (15) 2.8450.12 (18)
41250.27 (14) 3.77kO.18 (18) 3.05 + 0.17 (21)
Oxygen consumption in young mice
81
lower temperatures. The results, shown in Fig. 3, revealed that in a group of mice at 20°C the rate of oxygen consumption per animal was in the same range as that of a solitary mouse at 30°C.
??I2
.a
A7
3228y 24 a f
20g 16-
3
12-
E 8P s 4 @
e
5
I
I
I
8
I
I
16
L x 12 0
Fig. 4. Difference between deep-body temperature of infant mice and low ambient temperature, approximately 3O”C, for mice of various body weights. The figure beside each point is the age of the mouse in days. Circles : different mice; triangles: same mouse during growth.
8
Fig. 2. Relation of oxygen consumption in ml/mouse per hr to size of group in which measurement was made. The symbols represent different litters of mice, and identical symbols are joined by lines for ease of inspection. Observations were carried out at 30°C (upper graph) and 36°C (lower graph).
-L~o.o-~o_o_,o_o
,*:
30
15 2u 25 30 Amblent
36
15 20 tempewtureCC)
25
30
Fig. 3. Oxygen consumption (ml/animal per hr), in mice weighing 45-5 g measured (A) alone or (B) in groups of four litter-mates, at various ambient temperatures. Points show mean values, bars show 2 standard errors. Figure 4 shows the deep body temperature, in degrees C above 30°C ambient, maintained by mice of various weights. All the mice maintained their deep body temperature above that of the surrounding air. But at about 4 g body weight there was a marked increase in the temperature difference (above the low ambient temperature) which could be maintained, a point further illustrated in Fig. 5, in which the actual experimental results on a 2.6 g and a 4.4 g mouse are compared. The effect of grouping on deep-body temperature is illustrated in Fig. 6, which shows change of deepbody temperature of a 5-g mouse at 20°C. For the
1 I II I I 102030405060708090100 Tlmdmln)
0
36
I
I
I
I
Fig. 6. Variation in deep-body temperature of one of a changes of ambient temperature from 36 to 30°C and back. O-O, Mouse temperature; O-O, ambient temperature. A, Mouse aged 2 days, weight 2.6 g. B, Mouse aged 6 days, weight 4.4 g.
II 0
I
I
10 20’ 30
1
I
40
I
50
I
I
60 70 80 Time (mm)
I
I
90
I
I
I
100 110 lx)
I
I
130 140
Fig. 6. Variation in deep-body temperature of one of a group of four mice, 7 days old, 5 g weight, at ambient temperature of 20°C. At point X, its three litter-mates were removed, and at point Y they were replaced.
MARGARET W.
82
first period observations, the mouse under observation was huddled with three litter-mates. Then the litter-mates were removed, and its body temperature fell at the rate of about 4”C/hr. When body temperature reached a fairly constant low level, the littermates were replaced. At once the animal’s body temperature started to rise, and regained its original level within about 30 min.
DISCUSSION The results shown in Fig. 1 indicate that oxygen consumption in the newborn mouse bears a closer relation to body weight than to metabolic body size, expressed as body weight to the three-quarter power. This supports observations by Kleiber (1961) on several mammalian species and by Hill & Rahimtulla (1965) on human, that in the newborn the exponent for metabolic body size approximates to unity. Under the conditions of these experiments, even on the first day of life, when only 1.5 g in weight, the newborn mouse responds to a lowered ambient temperature by increasing, not decreasing, its oxygen consumption. This result is in accordance with that of Lagerspetz (1966) who found that during a fall of ambient temperature from 35 to 3O”C, young mice increased both oxygen consumption and motor activity. It is also in accordance with observations by Taylor (1960) on young rats, which, even at the age of 4 hr, responded to a cool ambient temperature by increased oxygen consumption. It is, however, at variance with the earlier observations of Fitzgerald (1953) who found a fall in oxygen consumption of young mice as ambient temperature was lowered from 35 to 30°C. The increased oxygen consumption at the lowered ambient temperature implies an additional heat production. The question as to whether this thermogenesis is sufficient to maintain a constant deep-body temperature is answered by the results shown in Figs. 3 and 4. The deep-body temperature fluctuates with the ambient temperature, but remains above it in the range 36-30°C. The older mice (5 g) could hold their body temperature some 4°C above an ambient temperature of 3O”C, whereas the smallest mice maintained a body temperature only about 1°C above 30°C ambient. The increased ability to cope
STANIER
with the lowered ambient temperature coincided with the first appearance of a slight covering of fur and so was probably related to the thermal insulatory effect of an unstirred layer of air over the skin. The response of metabolic rate to grouping was similar to that of the newborn pig (Mount, 1960). The effect of group size on oxygen consumption is confirmatory evidence that raised ambient temperature decreases rather than increases metabolic rate. In the grouped mice at 30°C the huddling, and consequential raising of the equivalent ambient temperature, brought down oxygen consumption per animal, as compared with that of a litter-mate in solitude. On Fitzgerald’s hypothesis, the huddling of the grouped animals would have had the opposite effect on oxygen consumption. The huddling experiment illustrated in Fig. 6 shows that the effect on the young animal’s body temperature of temporary removal of the parents from the nest, in which the young remain huddled together, is much less severe than the effect of one animal’s falling out of the nest. At an ambient temperature of 20°C the body temperature of the young would fall to only 30°C in the one case, but down to 24°C in the other. REFERENCES
FITZGERALDL. S. (1953) The oxygen consumption of neonatal mice. J. exp. Zool. 124,415-425. K. A. (1965) Heat balance and HELLJ.R.& -LA the metabolic rate of newborn babies in relation to environmental temperature; and the effect of age and weight on basal metabolic rate. J. Physiol., Land. 180, 239-265.
M. (1961) The Fire of Life. Wiley, New York. LAGERSPETZ K.Y.H. (1966) Temperature relations of oxygen consumption and motor activity in newborn mice. Ann. Med. exp. Fem. 44,71-73. MOUNTL. E. (1960) . , The influence of huddlina and body size on the metabolic rate of the young pig. J. a&. KLEIBER
Sci., Can&. 55,101~105.
TAYLORP. M. (1960) Oxygen consumption of newborn rats. J. Physiol., Land. 154,153-168.
Key Word Index-Oxygen consumption; young mice; metabolic body size; temperature regulation; huddling; environmental temperature.
EFFECT OF BODY WEIGHT, AMBIENT TEMPERATURE AND HUDDLING ON OXYGEN CONSUMPTION AND BODY TEMPERATURE OF YOUNG MICE MARGARETW. STANIER A.R.C. Institute of Animal Physiology, Babraham,
Cambridge, England
(Received 18 January 1974)
Abstract-I. At ambient temperatures of 30 and 36°C oxygen consumption of newborn mice was linearly related to body weight. 2. Even l-day-old mice responded to an ambient temperature cooler than that of the nest by increasing oxygen consumption. 3. Mice weighing 5 g maintained a body temperature of about 34°C at an ambient temperature of 3O”C, but mice weighing 1.5 g could not do so. 4. At 3O”C, mice of 3-5 g body weight observed in groups of two or four animals had a lower oxygen consumption than a litter-mate observed alone.
INTRODUCTION AN EARLIER investigation
into the metabolic rate of mice of different strains selected for large or small adult body weight had suggested that in the newborn period oxygen consumption (used as a measure of metabolic rate) bore a relation to body weight which was very different from that commonly accepted for adult mammals in which metabolic rate varies with the three-quarter power of the body weight (Kleiber, 1961). For the newborn mice oxygen consumption was proportional to body weight to the mean power of 1.2. The lirst purpose of the present work was to define more precisely the relation of oxygen consumption to body weight in newborn mice of mixed genetic origin, and to determine any effect of ambient temperature. The second purpose was to find whether there was any effect on oxygen consumption of grouping animals together, and the third was to find whether, in the newborn period, the increased metabolic rate observed on cooling the ambient air was sufficient to enable the animal to maintain a constant deep-body temperature. MATERIALS AND
METHODS
Animals and experimental conditions Mice were mated according to a breeding pattern devised to maintain randomized genes. Thirty litters of their progeny, either first or second litters of the parents, were used for measurement of oxygen consumption. The mice, aged between 1 and 11 days, were studied at three ranges of body weight: betweenl-5 and2 g, again between 3 and 3.5 g and again between 4.5 and 5 g. For measurement of oxygen consumption, the mice under experiment were removed from the nest for 1-2 hr, and placed in a metabolic chamber in a temperature-controlled room
held constant to within 0+5”C. The animals were placed in the chamber about 10 min before the start of measurements. In order to obtain a conveniently measurable oxygen consumption within 2 hr, groups of two, three or four litter-mates were placed together in the same cage. The value for oxygen consumption obtained for each group was divided by the appropriate figure to calculate mean oxygen consumption per animal (for the larger animals it was possible to use a single individual for measurement, and this was done in the final series of experiments). Measurements were carried out at 30°C and at 36°C in the morning and afternoon respectively of the same day, on a group of litter-mates of a given body weight though not necessarily the same individuals of the litter. The temperature of 36°C was close to that of the nest, and to the temperature at which previous workers have observed minimal oxygen consumption (Lagerspetz. 1966). The temperature of 30°C was chosen as being below the critical temperature of (adult) hairless mice (Mount, 1971) and in the range at which previous workers have observed
maximal oxygen consumption in newborn mice. Mice of 4.5-5 a weight were also studied at 15-25°C.
ThreeSeparate series of meas urements of oxygen consumption were carried out, at intervals of about 2 months, as litters became available. The effect of group size on oxygen consumption was observed in the third series, by making measurements on one, two, four and six litter-mates placed in separate cages of the apparatus over the same period. Oxygen const4mption The apparatus for recording oxygen consumption consisted of a row of cylindrical spirometers, each of about 15Oml capacity, built in a tank 4Ocm long, 10.5 cm wide and 18 cm deep. Each spirometer bell, tied to a counterpoise by a string led over a pulley-wheel, floated on water contained in a tank. The counterpoise bore a pointer touching a tied vertical scale made from 79
MARGARETW. STANJER
80
millimetre graph paper, and the position of the pointer could be read on the scale at intervals during a period of observation as the bell fell. The scale of each spirometer was calibrated by injecting into the spirometer known volumes of air from a syringe. Each spirometer was filled at the beginning of the period of observation with oxygen from a large reserve spirometer tank, kept in the temperature-controlled room. It was then connected through silicone-rubber and copper tubing with a metabolic chamber in the form of an inverted l-l. beaker. This contained a cage which held the group of mice, so making a closed-circuit system. The cage, a cylinder of wire mesh with a wire mesh disc as its floor, was fixed above a dish of soda asbestos (Carbasorb, 3-6 mesh, B.D.H. Ltd.) to absorb carbon dioxide. The dish and cage were placed in a tray containing liquid paralhn, and covered by the inverted beaker. The liquid parafhn formed a seal and the copper tubing from the spirometer entered through the lip of the beaker. Up to five groups of mice could thus be observed simultaneously. The sixth spirometer, filled with oxygen and connected with an empty chamber, was used as a control to correct for changes of volume resulting from small changes in ambient temperature and pressure during the period of observation. Body temperature For measurement of deep-body temperature a copperconstantan thermojunction (tip dia. O-6 mm) was inserted about 10 mm into the rectum of a mouse which lay in a cage in the temperature-controlled room. The temperature of the room was changed in a stepwise fashion from 36 to 30°C and back again, during a period of observation of about 2 hr. The thermocouple was sufficiently flexible to permit the animals to crawl, but in fact they were at rest during most of the 2-hr period. A reference thermojunction for the thermocouple system was held at 33°C in
a water-bath in the next room. At 5-min intervals, readings were taken of the temperature of the rectal thermocouple, and of that of the ambient air close to the cage. For observations at 2O”C,on mice placed alone or in a group of litter-mates, the same procedure was followed, but the reference thennojunction was held at 31 or 26°C. RESULTS
The mean oxygen consumption per mouse, in ml/hr, for all series of observations, is plotted against the body weight in Fig. 1. This shows (a) that the consumption at 36°C was less than at 30°C and (b) that at 36°C the oxygen consumption was linearly related to body weight. At 30°C although Table 1. Oxygen consumption
of newborn
the increase in oxygen consumption with weight appeared to be less in the range 3.5-5 g than in the range 15-3 g, there was no statistical evidence of any departure from linearity over the whole weight range.
I
I
I
3
1.5 2
0
Body weight
I
35
1
5
(grams)
Fig. 1. Relation of oxygen consumption to body weight in infant mice, Mus musculus, at ambient temperatures of 30°C (0) and 36°C (0). Mean values; bars show 2
standard errors.
Table 1 shows the oxygen consumption per g body weight at the two temperatures, for each range of body weight. The values at 36°C are similar to those recorded by Fitzgerald (1953) for mice at 35°C; and at 36 and 30°C are similar to the results of Lagerspetz (1966) at 35 and 30°C. The effect of group size on oxygen consumption is shown in Fig. 2. For all mice at 36”C, and in the smallest mice at 30°C group size had no significant effect on mean oxygen consumption per animal. In the larger mice at 3O”C, although there was considerable variation between litters, for any one litter the oxygen consumption per animal was significantly lower for measurements made in a group than for measurements on a solitary individual (for difference P =
In the largest animals (body weight 4.5-5 g) observations of the effect of group size on oxygen consumption per animal were carried out at some
mice at 36 and 30°C in ml/hr per g body weight Mean+ S.E.M. (No. of observations in parentheses)
Body weight range (1.5-2-O g) Body weight range (3-O-3.5 g) Body weight range (4.5-5.0 g)
I
4-5
36°C
30°C
2.3OkO.22 (14) 2.76F0.06 (15) 2.8450.12 (18)
41250.27 (14) 3.77kO.18 (18) 3.05 + 0.17 (21)
Oxygen consumption in young mice
81
lower temperatures. The results, shown in Fig. 3, revealed that in a group of mice at 20°C the rate of oxygen consumption per animal was in the same range as that of a solitary mouse at 30°C.
??I2
.a
A7
3228y 24 a f
20g 16-
3
12-
E 8P s 4 @
e
5
I
I
I
8
I
I
16
L x 12 0
Fig. 4. Difference between deep-body temperature of infant mice and low ambient temperature, approximately 3O”C, for mice of various body weights. The figure beside each point is the age of the mouse in days. Circles : different mice; triangles: same mouse during growth.
8
Fig. 2. Relation of oxygen consumption in ml/mouse per hr to size of group in which measurement was made. The symbols represent different litters of mice, and identical symbols are joined by lines for ease of inspection. Observations were carried out at 30°C (upper graph) and 36°C (lower graph).
-L~o.o-~o_o_,o_o
,*:
30
15 2u 25 30 Amblent
36
15 20 tempewtureCC)
25
30
Fig. 3. Oxygen consumption (ml/animal per hr), in mice weighing 45-5 g measured (A) alone or (B) in groups of four litter-mates, at various ambient temperatures. Points show mean values, bars show 2 standard errors. Figure 4 shows the deep body temperature, in degrees C above 30°C ambient, maintained by mice of various weights. All the mice maintained their deep body temperature above that of the surrounding air. But at about 4 g body weight there was a marked increase in the temperature difference (above the low ambient temperature) which could be maintained, a point further illustrated in Fig. 5, in which the actual experimental results on a 2.6 g and a 4.4 g mouse are compared. The effect of grouping on deep-body temperature is illustrated in Fig. 6, which shows change of deepbody temperature of a 5-g mouse at 20°C. For the
1 I II I I 102030405060708090100 Tlmdmln)
0
36
I
I
I
I
Fig. 6. Variation in deep-body temperature of one of a changes of ambient temperature from 36 to 30°C and back. O-O, Mouse temperature; O-O, ambient temperature. A, Mouse aged 2 days, weight 2.6 g. B, Mouse aged 6 days, weight 4.4 g.
II 0
I
I
10 20’ 30
1
I
40
I
50
I
I
60 70 80 Time (mm)
I
I
90
I
I
I
100 110 lx)
I
I
130 140
Fig. 6. Variation in deep-body temperature of one of a group of four mice, 7 days old, 5 g weight, at ambient temperature of 20°C. At point X, its three litter-mates were removed, and at point Y they were replaced.
MARGARET W.
82
first period observations, the mouse under observation was huddled with three litter-mates. Then the litter-mates were removed, and its body temperature fell at the rate of about 4”C/hr. When body temperature reached a fairly constant low level, the littermates were replaced. At once the animal’s body temperature started to rise, and regained its original level within about 30 min.
DISCUSSION The results shown in Fig. 1 indicate that oxygen consumption in the newborn mouse bears a closer relation to body weight than to metabolic body size, expressed as body weight to the three-quarter power. This supports observations by Kleiber (1961) on several mammalian species and by Hill & Rahimtulla (1965) on human, that in the newborn the exponent for metabolic body size approximates to unity. Under the conditions of these experiments, even on the first day of life, when only 1.5 g in weight, the newborn mouse responds to a lowered ambient temperature by increasing, not decreasing, its oxygen consumption. This result is in accordance with that of Lagerspetz (1966) who found that during a fall of ambient temperature from 35 to 3O”C, young mice increased both oxygen consumption and motor activity. It is also in accordance with observations by Taylor (1960) on young rats, which, even at the age of 4 hr, responded to a cool ambient temperature by increased oxygen consumption. It is, however, at variance with the earlier observations of Fitzgerald (1953) who found a fall in oxygen consumption of young mice as ambient temperature was lowered from 35 to 30°C. The increased oxygen consumption at the lowered ambient temperature implies an additional heat production. The question as to whether this thermogenesis is sufficient to maintain a constant deep-body temperature is answered by the results shown in Figs. 3 and 4. The deep-body temperature fluctuates with the ambient temperature, but remains above it in the range 36-30°C. The older mice (5 g) could hold their body temperature some 4°C above an ambient temperature of 3O”C, whereas the smallest mice maintained a body temperature only about 1°C above 30°C ambient. The increased ability to cope
STANIER
with the lowered ambient temperature coincided with the first appearance of a slight covering of fur and so was probably related to the thermal insulatory effect of an unstirred layer of air over the skin. The response of metabolic rate to grouping was similar to that of the newborn pig (Mount, 1960). The effect of group size on oxygen consumption is confirmatory evidence that raised ambient temperature decreases rather than increases metabolic rate. In the grouped mice at 30°C the huddling, and consequential raising of the equivalent ambient temperature, brought down oxygen consumption per animal, as compared with that of a litter-mate in solitude. On Fitzgerald’s hypothesis, the huddling of the grouped animals would have had the opposite effect on oxygen consumption. The huddling experiment illustrated in Fig. 6 shows that the effect on the young animal’s body temperature of temporary removal of the parents from the nest, in which the young remain huddled together, is much less severe than the effect of one animal’s falling out of the nest. At an ambient temperature of 20°C the body temperature of the young would fall to only 30°C in the one case, but down to 24°C in the other. REFERENCES
FITZGERALDL. S. (1953) The oxygen consumption of neonatal mice. J. exp. Zool. 124,415-425. K. A. (1965) Heat balance and HELLJ.R.& -LA the metabolic rate of newborn babies in relation to environmental temperature; and the effect of age and weight on basal metabolic rate. J. Physiol., Land. 180, 239-265.
M. (1961) The Fire of Life. Wiley, New York. LAGERSPETZ K.Y.H. (1966) Temperature relations of oxygen consumption and motor activity in newborn mice. Ann. Med. exp. Fem. 44,71-73. MOUNTL. E. (1960) . , The influence of huddlina and body size on the metabolic rate of the young pig. J. a&. KLEIBER
Sci., Can&. 55,101~105.
TAYLORP. M. (1960) Oxygen consumption of newborn rats. J. Physiol., Land. 154,153-168.
Key Word Index-Oxygen consumption; young mice; metabolic body size; temperature regulation; huddling; environmental temperature.