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The relationship between rate of oxygen consumption, heart rate and thermal conductance of the dik-dik antelope (Rhynchotragus kirkii) at various ambient temperatures
0300-9629/82/090021-04$03.00/O 0 1982 Pergamon Press Ltd
Camp. Eiochrm Physiol. Vol. 73A. No. I. pp. 21 lo 24. 1982 Printed in Great Britain
THE RELATIONSHIP BETWEEN RATE OF OXYGEN CONSUMPTION, HEART RATE AND THERMAL CONDUCTANCE OF THE DIK-DIK ANTELOPE (RHYNCHOTRAGUS KZRKZI) AT VARIOUS AMBIENT TEMPERATURES J. M. Z. Comparative
Animal
KAMAU
and G. M. 0. MALOIY
Physiology Research Unit, Department University of Nairobi, Kenya
(Rrceiretl
19 Ocrohrr
of Animal
Physiology,
1981)
Abstract-l. The extent of cardiovascular adjustments to heat and cold were investigated between ambient temperatures of 5 and 45-C by measuring conductance and the rates of oxygen consumption and heart beats. 2. Minimum heart rate was observed at 25’C (I 14 + 9 beatsimin). In the heat at 45°C heart rate was observed to increase only slightly (127 k 12 beats/min) but in the cold -5°C heart rate nearly doubled that at 25°C. 3. Thermal conductance was on average 0.031 ml0, (g.hr.“C)-’ below 25°C but increased by more than 20 times at 40°C. 4. A positive correlation between heart rate and rate of oxygen consumption was demonstrated below 25 C and the relation may be of practical use
INTRODUCTION
the rate of oxygen consumption has stimulated research into the use of heart rate as a possible indirect measure of metabolic rate under conditions of varying temperature and energy demand. Some information is available so far only in a few mammals and birds. It was the aim of this experiment to show, not only the role of the cardiovascular system in thermoregulation, but also the possible relation between heart rate and the rate of oxygen consumption in the dik-dik antelopes. In domestic animals such data concerned with the relation between metabolic rate and heart rate exist for sheep (Brockway & McEwan, 1969; Webster, 1967) and in calves, both in the laboratory and out-ofdoor conditions (Holmes er al., 1967). Although cardiovascular responses under different environmental conditions have been investigated in many species of wild animals, in only a few studies has the relation between heart rate and metabolic rate been investigated. These studies include those in the white-tailed deer (Holter er al. 1976) and in the rodents (Morhadt & Morhadt, 1971). Substantial work has, however, been done in birds and this has been reviewed recently by Roberts & Deavers (1980). From the above studies it appears that provided the animal is at rest and not emotionally stressed, heart rate could be used as an indirect measure of metabolism. Telemetry may probably prove to be the best method for determining heart rate.
The cardiovascular system has many functions with regard to thermoregulation and energy metabolism. These include, for example, the transfer and distribution of heat from one part of the body to another, the supply of oxygen and nutrients to the metabolising tissues and removal of carbon dioxide and other waste metabolites from these tissues. The observed increase in the rate of oxygen consumption both in the cold and in hot environments, and the attendant changes in skin temperature of the trunk and extremities (Kamau J. M. Z. & Maloiy, G. M. 0. unpublished results) suggested involvement of the cardiovascular systeem in increasing oxygen supply to the tissues and changes in heat distribution. The demands for increasing oxygen supply to the tissues are met by varying either the heart rate, the rate of oxygen consumption per heart beat (called hereafter the oxygen pulse), stroke volume, or a combination of these (Bartholomew & Tucker, 1963). Heart rate and the rate of oxygen consumption were measured in these experiments, oxygen pulse was then calculated from heart rate and the rate of oxygen uptake. It was hoped that these parameters would suffice to indicate the adjustments made by the heart to changes in energy demand in the hot and cold environment. Simultaneous measurements of the rate of oxygen consumption rectal and ambient temperature during the measurements of heart rate in these experiments allowed calculation of conductance which is a good indicator of magnitude of heat exchanges between the animals and their environment. The relation between cardiovascular functions, especially heart rate and metabolic rate as measured by
MATERIALS AND METHODS Animals Four adult dik-diks (Rhgnchotrccgus kirkji) were used in this investigation. Their body weight ranged between 21
J. M.
22
Z. KAMAU
and G. M. 0. MALOIY
4.555.2
kg. The animals used had been in captivity for nearly two years at the time of these experiments. They were housed in single cages and fed ad libitum, on Grewia similis leaves and early weaning calf pellets. Water was available to them all the time. The animals were used to being handled and were also trained to conform to various experimental manipulations before the experiments were started. Heart rate and the rate of oxygen consumption were monitored simultaneously with the animals inside a 541 metabolic chamber, housed in a climatic room whose temperature was controlled to +l C. The experiments were conducted at 5 C intervals between 5‘C and 45 C. At each of these temperatures animals were given at least 2 hr to equilibrate to the set temperature in the climatic chamber. Before the beginning of each set of measurements the animals were weighed to the nearest 10th of a gram. Heart rate
A heart rate transmitter (Mackay, 1970) was fastened across the shoulders of the dik-diks using elastic bands, The leads of the transmitters were attached to the skin of each foreleg just below the elbow joint using heart clips, The dik-diks were then placed in the metabolic chamber. The signals from the heart rate transmitter were received by an FM receiver (Nacro-Biosystems). The sounds from the heart rate signals were counted for one minute (using a stop watch). Periodically four to five one minute samples were taken during a single experiment run which lasted for at least one hour. A mark was made on chart recording the rate of oxygen consumption during the sampling of heart rate. By determining the lag period of the oxygen analyses and the metabolic chamber (by passing nitrogen into the metabolic chamber with the animal inside and noting the time taken for the chart recorder for the oxygen consumption to respond), it was possible to calculate the rate of oxygen consumption during the heart rate sampling period. Rate ~~/‘ouyym consumption The rate of oxygen consumption was determined within the Climattc Room. The animals were placed in a 54 1. metabolic chamber. Room air was drawn through this chamber at a flow rate between 8tSlOOO l/hr. The fall in oxygen concentration of the room air as it passed through the chamber was determined using a paramagnetic oxygen analyser (Beckman F3). To do this, an aliquot of the air from the Chamber was dried using Silica gel, and then
0”
300
-
1
I
10
Ambient
I1
/
20
30
temperature
I,, 40
, *C
Fig. 2. The dashed line connects means (filled circles) and SEM (vertical bars) of oxygen pulse calculated at different ambient temperature (n = 4 animals). The solid line represents the regression analysis of oxygen pulse against ambient temperatures between 30 and 5 C. delivered to the analyser. The analyser was calibrated for I”, full scale deflection by varying pressure within the measuring ceil as recommended by the manufacturers, Flow through the Chamber was determined using a flowmeter regularly calibrated with a Vol-u-meter (Brooks). By assuming an RQ of 0.85 equation 4b by Withers (1977) was used to calculate oxygen consumption.
Rectal temperature was taken at the end of each experiment by inserting a previously calibrated thermister probe (YSI) 8 cm into the rectum. Tkernwl conductuncu (C) Thermal conductance was calculated from the ratio of the rate of oxygen consumption (V,) and the difference between the rectal temperature (Tree) (assumed to correspond to core temperature) and the ambient temperature (Ta). i.e. I,
.yo2
c=
TreeTu
RESULTS Heart rate
Figure
1 shows
the relation
between
heart
rate
(HR) and ambient temperature (Tu). Below Ta 25 C HR increased linearly with decreasing ambient tem-
perature. The linear regression lation is given by the equation HR beatsimin
1 0
I
1
10
1
/
20
Ambient
1 30
1
-t-
-1
/
,
40
temperature,*C
Fig. I. The dashed line connects means (filled circles) and SEM (vertical bars) of heart rates obtained from all four animals at different ambient temperature. The solid line represents the regression analysis for all individual animal heart rates at each controlled temperature from 25 to 5’C.
analysis
for this re-
= 186.8-2.89 TLI
r = 0.82. The minimum HR of 98 beats/min was recorded at a Tu of 25°C. The maximum HR of 216 beats/min was recorded at a Tu of 5°C. A slight increase in HR was noted at Tu’s between 25 and 30°C. Above Tu 30°C however, HR remained steady at approximately 125 beats/min.
Figure 2. Below a Tu of 30 C, oxygen pulse increased linearly with decreasing To. The equation for the regression analysis being: Oxygen pulse 1110,jbeat There
was a slight
= 415.5-3.38
increase
in oxygen
Tu, Y = 0.50
pulse
at Tu
Oxygen consumption and heart rate in the dik-dik antelope above 3O’C. The minimum oxygen pulse of 255 ~1 O,/beat was recorded at a Ta of 30°C while the maximum pulse of 448 &beat was recorded at a Tu of 5’C. Thertnul conductance Below Ta 25°C thermal conductance ranged from 0.027 + 0.005 and 0.035 + 0.004 ml O2 (g. hr. “C)- ‘. Between Ta 25 and 35 C thermal conductance increased steadily and was followed by a very steep rise as Ta increased to 40 C. Thermal conductance recorded at Tu of 40’ C was 0.55 + 0.07. nearly 20 times greater than that at a Ta of 25’C. The very large increase in conductance was due to the rapid closing of the difference between rectal temperature and air temperature. Heart rate rs oxygen consumption When the five minute simultaneous recording of heart rate and the rate of oxygen consumption for each animal at ambient temperatures between 25 and 5 C were pooled and analysed by method of least squares, the following linear regression equation emerged :
V,, (l.hr)-’
= 0.38 + 0.02 HR r = 0.68.
Thus, heart rate was positively related to metabolic rate. It is important to note here that the heart rates used to derive the above equation were taken at periodic intervals and included values for dik-diks that were standing or lying down in the metabolic chamber. DISCUSSION
Resting heart rate under the thermoneutral conditions is inversely related to body mass and can be predicted from the equation of Wang & Hudson (1971): Heart rate (HR) beats/min
= 816W-“.25
where W = body mass in grammes. According to this equation, the predicted heart rate for a 4800 g dik-dik would be 98 beats/min. This is in agreement with the minimum heart rate of 98 beatsjmin recorded in one animal at an ambient temperature of 25°C in this study. The value of 98 beatsimin was, however, only 85’4, of the average heart rate for the four animals at 25°C. In this investigation heart rate did not level off within the thermally neutral zone (24-35 C, unpublished results). A sharp increase in heart rate occurred between 25 and 30 C. Above 30 C ambient, no change was observed in heart rate despite the observed increase in the rate of oxygen consumption (Fig. 1). Lack of increase in heart rate in the heat may be attributed to a drop in peripheral resistance due to vasodilation, especially in respiratory muscles and nasopharygeal regions and opening of arterio-venous anastomoses as has been demonstrated in sheep (Hales, 1973). The capacity of the dik-dik to increase their heart rate in a cold environment is nearly as great as their capacity to increase the rate of oxygen consumption. At 5 C ambient, the heart rate was 220”,, greater than that at 25 C. It was observed that even under such
23
low temperature the animals were still capable of increasing their heart rates further, even if for a short time, after a slight disturbance by, for instance, loud noises or the sudden sighting of the experimenter. Metabolic rate (as measured here in ml O2 per unit time) is the product of heart rate, stroke volume and utilization (0,) coefficient (Maynard, 1960). The product of stroke volume and utilization coefficient has been termed the oxygen pulse (Bartholomew & rate = heart 1963). Thus metabolic Tucker, rate x oxygen pulse. In the dik-dik oxygen pulse increased both above and below a Tu of 30 C. The reason for the increase may have been due to an increase in oxygen utilization rather than an increase in stroke volume as has been suggested elsewhere (Maynard, 1960). A Tu of about 30°C corresponds to the beginning of thermally induced polypnoe in these animals (unpublished observation). An increase in oxygen pulse gives more support to the observed increase in the rate of oxygen consumption during panting contrary to earlier findings (Hoppe et al., 1975: Maskrey & Hoppe, 1979) that during panting in the dik-dik the rate of oxygen consumption decreases. According to the Newtonian model of cooling, heat loss from an inactive mammal at a temperature below the thermoneutral zone is proportional to the temperature difference between the animal core (Th) and its environment (Tu). The slope of the relation is the proportionality constant C or conductance, provided the regression line intercepts the Tu line at Th (Herreid & Kessel, 1967). The cardiovascular system plays a major role in regulating the magnitude of heat flux to and from an animal. Conductance was therefore assessed simultaneously with oxygen consumption and heart rate in this investigation. The ranges of conductance obtained were in agreement with the predicted value for animals of 4.8 kg calculated according to the equation of Bradley & Deavers ( 1980) viz :C = 0.76 W-o.426 Where W = body mass in grammes, C = conductance in ml 0, (g.hr. ‘C)-I. For a 4.8 kg dik-dik the expected conductance would be approximately 0.02 1. For proper management of wild animals and their habitats one must inevitably establish the minimum energy needed for maintenance. To do this one must use either predictive equations, Kleiber (1961) or actually measure the rate of oxygen consumption of the animals. The technicalities involved in such experiments, and the fact that the animals are wild and difficult to handle, has probably imposed a serious constraint on wildlife research. There have been several attempts to relate oxygen consumption to heart rate in both wild and domesticated animals (Brockway 8~ McEwan, 1969; Webster. 1967; Holmes et al., 1967; Holter et al.. 1976: Morhadt & Morhadt, 1979). Often, in these studies, oxygen consumption and heart rate were shown to give strong positive correlations, although sometimes with large individual variations. There are indications from human studies that, to be of value, the relation between heart rate and heart production need to be determined over a 24 hr period due to the possible diurnal variations in heat production and activity (Dauncey & James, 1979). In the present study, a strong positive corre-
J. M. Z. KAMAU and G. M. 0. MALOIY
24
lation has been shown to exist between the rate of oxygen consumption and heart rate under a wide range of controlled environmental temperatures. In view of the differences in the levels of energy demand diurnally during exercise, it is debatable whether the results obtained here could be extended to predict the metabolic rate in a field type situation. Nevertheless, heart rate could well provide a means of estimating metabolic rate in these wild ruminants. In conclusion, the cardiac response, conductance and oxygen pulse in the dik-dik antelope are designed to more than cope with demands of energy and heat conservation under a wide range of ambient temperatures. The strong positive correlation shown here between rate of oxygen consumption and heart rate may provide a simpler means of determining metabolic rate in resting dik-diks in the laboratory or field.
DAUNCEY M. J. & JAMES W. P. T. (1979). Assessment heart rate method for determining energy expenditure man, using a whole-body calorimeter. Br. .I. Nutr.
of in 42,
t-13.
HERREID C. F. & KESSEL B. (1967). The thermal conductance in birds and mammals. Camp. Biochem. Phljsiol. 21,405-414. HALES J. R. S. (1973) Effect of exposure to hot environments on the regional distribution of blood flow and on cardiorespiratory function in sheep. Pjugers Arch. 344, 133-148. HOLMES C. W., STEPHENS D. B. & TONER J. N. (1976) Heart rate as a possible indicator of the energy metabolism of calves kept out of doors. Licestock Prod. SC. 3, 33-341. HOLTER J. B., URBAN W. E.. HAYES H. H. & SILVER H. (1976) Predicting heart rate from telemetered heart rate in white-tailed deer. J. Wildl. Mgmf. 40, 626-629. HOPPE P. P.. JOHANSEN, K., MALOIY G. M. 0. & MUSEWE V. (1975) Thermal panting reduces oxygen uptake in the dik-dik. Acfa Physiol. Stand. 95(2), 9A. KLEIBER M. (1961) The Fire of Life, 454 pp. Wiley, New York.
Ackrlowledyemerlt~This
assistance England. telemetry
study received partial financial from the Leverhulme Trust Funds, London, Dr. E. Muller kindly allowed us the use of his set-up. REFERENCES
BARTHOLOMEW G. A. & TUCKER V. A. (1963) Control of changes in body temperature metabolism and circulation by Agamid lizard, Amphibo/uru.s harhartus. Ph~siol. Zoo/. 36, 199-218. BRADLEY R. S. & DEAVERS D. R. (1980). A re-examination of relationship between thermal conductance and body weight in mammals. Comp. Biochem. Physiol. 65A, 465476. BROCKWAY J. M. & MCEWAN E. H. (1969) Oxygen uptake and cardiac performance in sheep. J. Ph!xiol. Land. 202, 661-669.
MACKAY R. S. (1970) Biomedical Telemetry. Wiley New York. MASKREY M. & HOPPE P. P. (1979). Themoregulation and oxygen consumption in the Kirk’s dik-dik (Madogua kirkii) at ambient temperatures between lG45”C. Comp. Biochem.
Physiol.
62A,
827-830.
MAYNARD D. M. (1960) Heart rate and body size in the spiny lobster. Physiol. Zoo/. 33, 241-251. MORHADT E. & MORHADT S. S. (197 1) Correlation between heart rate and oxygen consumption in rodents. Am. J. Physiol. 221. 158&1586. WANG L. C. H. & HUDWN J. W. (1971) Temperature regulation in normothermic and hibernating Eastern Chipmunk, Tumias striatus. Comp. Biochem. PhJ,siol. 38A, 59-90. WEBSTER A. J. F. (1967) Continuous measurement of heart rate as an indicator of energy expenditure in sheep. J. Nurr. 21, 769-785.
Camp. Eiochrm Physiol. Vol. 73A. No. I. pp. 21 lo 24. 1982 Printed in Great Britain
THE RELATIONSHIP BETWEEN RATE OF OXYGEN CONSUMPTION, HEART RATE AND THERMAL CONDUCTANCE OF THE DIK-DIK ANTELOPE (RHYNCHOTRAGUS KZRKZI) AT VARIOUS AMBIENT TEMPERATURES J. M. Z. Comparative
Animal
KAMAU
and G. M. 0. MALOIY
Physiology Research Unit, Department University of Nairobi, Kenya
(Rrceiretl
19 Ocrohrr
of Animal
Physiology,
1981)
Abstract-l. The extent of cardiovascular adjustments to heat and cold were investigated between ambient temperatures of 5 and 45-C by measuring conductance and the rates of oxygen consumption and heart beats. 2. Minimum heart rate was observed at 25’C (I 14 + 9 beatsimin). In the heat at 45°C heart rate was observed to increase only slightly (127 k 12 beats/min) but in the cold -5°C heart rate nearly doubled that at 25°C. 3. Thermal conductance was on average 0.031 ml0, (g.hr.“C)-’ below 25°C but increased by more than 20 times at 40°C. 4. A positive correlation between heart rate and rate of oxygen consumption was demonstrated below 25 C and the relation may be of practical use
INTRODUCTION
the rate of oxygen consumption has stimulated research into the use of heart rate as a possible indirect measure of metabolic rate under conditions of varying temperature and energy demand. Some information is available so far only in a few mammals and birds. It was the aim of this experiment to show, not only the role of the cardiovascular system in thermoregulation, but also the possible relation between heart rate and the rate of oxygen consumption in the dik-dik antelopes. In domestic animals such data concerned with the relation between metabolic rate and heart rate exist for sheep (Brockway & McEwan, 1969; Webster, 1967) and in calves, both in the laboratory and out-ofdoor conditions (Holmes er al., 1967). Although cardiovascular responses under different environmental conditions have been investigated in many species of wild animals, in only a few studies has the relation between heart rate and metabolic rate been investigated. These studies include those in the white-tailed deer (Holter er al. 1976) and in the rodents (Morhadt & Morhadt, 1971). Substantial work has, however, been done in birds and this has been reviewed recently by Roberts & Deavers (1980). From the above studies it appears that provided the animal is at rest and not emotionally stressed, heart rate could be used as an indirect measure of metabolism. Telemetry may probably prove to be the best method for determining heart rate.
The cardiovascular system has many functions with regard to thermoregulation and energy metabolism. These include, for example, the transfer and distribution of heat from one part of the body to another, the supply of oxygen and nutrients to the metabolising tissues and removal of carbon dioxide and other waste metabolites from these tissues. The observed increase in the rate of oxygen consumption both in the cold and in hot environments, and the attendant changes in skin temperature of the trunk and extremities (Kamau J. M. Z. & Maloiy, G. M. 0. unpublished results) suggested involvement of the cardiovascular systeem in increasing oxygen supply to the tissues and changes in heat distribution. The demands for increasing oxygen supply to the tissues are met by varying either the heart rate, the rate of oxygen consumption per heart beat (called hereafter the oxygen pulse), stroke volume, or a combination of these (Bartholomew & Tucker, 1963). Heart rate and the rate of oxygen consumption were measured in these experiments, oxygen pulse was then calculated from heart rate and the rate of oxygen uptake. It was hoped that these parameters would suffice to indicate the adjustments made by the heart to changes in energy demand in the hot and cold environment. Simultaneous measurements of the rate of oxygen consumption rectal and ambient temperature during the measurements of heart rate in these experiments allowed calculation of conductance which is a good indicator of magnitude of heat exchanges between the animals and their environment. The relation between cardiovascular functions, especially heart rate and metabolic rate as measured by
MATERIALS AND METHODS Animals Four adult dik-diks (Rhgnchotrccgus kirkji) were used in this investigation. Their body weight ranged between 21
J. M.
22
Z. KAMAU
and G. M. 0. MALOIY
4.555.2
kg. The animals used had been in captivity for nearly two years at the time of these experiments. They were housed in single cages and fed ad libitum, on Grewia similis leaves and early weaning calf pellets. Water was available to them all the time. The animals were used to being handled and were also trained to conform to various experimental manipulations before the experiments were started. Heart rate and the rate of oxygen consumption were monitored simultaneously with the animals inside a 541 metabolic chamber, housed in a climatic room whose temperature was controlled to +l C. The experiments were conducted at 5 C intervals between 5‘C and 45 C. At each of these temperatures animals were given at least 2 hr to equilibrate to the set temperature in the climatic chamber. Before the beginning of each set of measurements the animals were weighed to the nearest 10th of a gram. Heart rate
A heart rate transmitter (Mackay, 1970) was fastened across the shoulders of the dik-diks using elastic bands, The leads of the transmitters were attached to the skin of each foreleg just below the elbow joint using heart clips, The dik-diks were then placed in the metabolic chamber. The signals from the heart rate transmitter were received by an FM receiver (Nacro-Biosystems). The sounds from the heart rate signals were counted for one minute (using a stop watch). Periodically four to five one minute samples were taken during a single experiment run which lasted for at least one hour. A mark was made on chart recording the rate of oxygen consumption during the sampling of heart rate. By determining the lag period of the oxygen analyses and the metabolic chamber (by passing nitrogen into the metabolic chamber with the animal inside and noting the time taken for the chart recorder for the oxygen consumption to respond), it was possible to calculate the rate of oxygen consumption during the heart rate sampling period. Rate ~~/‘ouyym consumption The rate of oxygen consumption was determined within the Climattc Room. The animals were placed in a 54 1. metabolic chamber. Room air was drawn through this chamber at a flow rate between 8tSlOOO l/hr. The fall in oxygen concentration of the room air as it passed through the chamber was determined using a paramagnetic oxygen analyser (Beckman F3). To do this, an aliquot of the air from the Chamber was dried using Silica gel, and then
0”
300
-
1
I
10
Ambient
I1
/
20
30
temperature
I,, 40
, *C
Fig. 2. The dashed line connects means (filled circles) and SEM (vertical bars) of oxygen pulse calculated at different ambient temperature (n = 4 animals). The solid line represents the regression analysis of oxygen pulse against ambient temperatures between 30 and 5 C. delivered to the analyser. The analyser was calibrated for I”, full scale deflection by varying pressure within the measuring ceil as recommended by the manufacturers, Flow through the Chamber was determined using a flowmeter regularly calibrated with a Vol-u-meter (Brooks). By assuming an RQ of 0.85 equation 4b by Withers (1977) was used to calculate oxygen consumption.
Rectal temperature was taken at the end of each experiment by inserting a previously calibrated thermister probe (YSI) 8 cm into the rectum. Tkernwl conductuncu (C) Thermal conductance was calculated from the ratio of the rate of oxygen consumption (V,) and the difference between the rectal temperature (Tree) (assumed to correspond to core temperature) and the ambient temperature (Ta). i.e. I,
.yo2
c=
TreeTu
RESULTS Heart rate
Figure
1 shows
the relation
between
heart
rate
(HR) and ambient temperature (Tu). Below Ta 25 C HR increased linearly with decreasing ambient tem-
perature. The linear regression lation is given by the equation HR beatsimin
1 0
I
1
10
1
/
20
Ambient
1 30
1
-t-
-1
/
,
40
temperature,*C
Fig. I. The dashed line connects means (filled circles) and SEM (vertical bars) of heart rates obtained from all four animals at different ambient temperature. The solid line represents the regression analysis for all individual animal heart rates at each controlled temperature from 25 to 5’C.
analysis
for this re-
= 186.8-2.89 TLI
r = 0.82. The minimum HR of 98 beats/min was recorded at a Tu of 25°C. The maximum HR of 216 beats/min was recorded at a Tu of 5°C. A slight increase in HR was noted at Tu’s between 25 and 30°C. Above Tu 30°C however, HR remained steady at approximately 125 beats/min.
Figure 2. Below a Tu of 30 C, oxygen pulse increased linearly with decreasing To. The equation for the regression analysis being: Oxygen pulse 1110,jbeat There
was a slight
= 415.5-3.38
increase
in oxygen
Tu, Y = 0.50
pulse
at Tu
Oxygen consumption and heart rate in the dik-dik antelope above 3O’C. The minimum oxygen pulse of 255 ~1 O,/beat was recorded at a Ta of 30°C while the maximum pulse of 448 &beat was recorded at a Tu of 5’C. Thertnul conductance Below Ta 25°C thermal conductance ranged from 0.027 + 0.005 and 0.035 + 0.004 ml O2 (g. hr. “C)- ‘. Between Ta 25 and 35 C thermal conductance increased steadily and was followed by a very steep rise as Ta increased to 40 C. Thermal conductance recorded at Tu of 40’ C was 0.55 + 0.07. nearly 20 times greater than that at a Ta of 25’C. The very large increase in conductance was due to the rapid closing of the difference between rectal temperature and air temperature. Heart rate rs oxygen consumption When the five minute simultaneous recording of heart rate and the rate of oxygen consumption for each animal at ambient temperatures between 25 and 5 C were pooled and analysed by method of least squares, the following linear regression equation emerged :
V,, (l.hr)-’
= 0.38 + 0.02 HR r = 0.68.
Thus, heart rate was positively related to metabolic rate. It is important to note here that the heart rates used to derive the above equation were taken at periodic intervals and included values for dik-diks that were standing or lying down in the metabolic chamber. DISCUSSION
Resting heart rate under the thermoneutral conditions is inversely related to body mass and can be predicted from the equation of Wang & Hudson (1971): Heart rate (HR) beats/min
= 816W-“.25
where W = body mass in grammes. According to this equation, the predicted heart rate for a 4800 g dik-dik would be 98 beats/min. This is in agreement with the minimum heart rate of 98 beatsjmin recorded in one animal at an ambient temperature of 25°C in this study. The value of 98 beatsimin was, however, only 85’4, of the average heart rate for the four animals at 25°C. In this investigation heart rate did not level off within the thermally neutral zone (24-35 C, unpublished results). A sharp increase in heart rate occurred between 25 and 30 C. Above 30 C ambient, no change was observed in heart rate despite the observed increase in the rate of oxygen consumption (Fig. 1). Lack of increase in heart rate in the heat may be attributed to a drop in peripheral resistance due to vasodilation, especially in respiratory muscles and nasopharygeal regions and opening of arterio-venous anastomoses as has been demonstrated in sheep (Hales, 1973). The capacity of the dik-dik to increase their heart rate in a cold environment is nearly as great as their capacity to increase the rate of oxygen consumption. At 5 C ambient, the heart rate was 220”,, greater than that at 25 C. It was observed that even under such
23
low temperature the animals were still capable of increasing their heart rates further, even if for a short time, after a slight disturbance by, for instance, loud noises or the sudden sighting of the experimenter. Metabolic rate (as measured here in ml O2 per unit time) is the product of heart rate, stroke volume and utilization (0,) coefficient (Maynard, 1960). The product of stroke volume and utilization coefficient has been termed the oxygen pulse (Bartholomew & rate = heart 1963). Thus metabolic Tucker, rate x oxygen pulse. In the dik-dik oxygen pulse increased both above and below a Tu of 30 C. The reason for the increase may have been due to an increase in oxygen utilization rather than an increase in stroke volume as has been suggested elsewhere (Maynard, 1960). A Tu of about 30°C corresponds to the beginning of thermally induced polypnoe in these animals (unpublished observation). An increase in oxygen pulse gives more support to the observed increase in the rate of oxygen consumption during panting contrary to earlier findings (Hoppe et al., 1975: Maskrey & Hoppe, 1979) that during panting in the dik-dik the rate of oxygen consumption decreases. According to the Newtonian model of cooling, heat loss from an inactive mammal at a temperature below the thermoneutral zone is proportional to the temperature difference between the animal core (Th) and its environment (Tu). The slope of the relation is the proportionality constant C or conductance, provided the regression line intercepts the Tu line at Th (Herreid & Kessel, 1967). The cardiovascular system plays a major role in regulating the magnitude of heat flux to and from an animal. Conductance was therefore assessed simultaneously with oxygen consumption and heart rate in this investigation. The ranges of conductance obtained were in agreement with the predicted value for animals of 4.8 kg calculated according to the equation of Bradley & Deavers ( 1980) viz :C = 0.76 W-o.426 Where W = body mass in grammes, C = conductance in ml 0, (g.hr. ‘C)-I. For a 4.8 kg dik-dik the expected conductance would be approximately 0.02 1. For proper management of wild animals and their habitats one must inevitably establish the minimum energy needed for maintenance. To do this one must use either predictive equations, Kleiber (1961) or actually measure the rate of oxygen consumption of the animals. The technicalities involved in such experiments, and the fact that the animals are wild and difficult to handle, has probably imposed a serious constraint on wildlife research. There have been several attempts to relate oxygen consumption to heart rate in both wild and domesticated animals (Brockway 8~ McEwan, 1969; Webster. 1967; Holmes et al., 1967; Holter et al.. 1976: Morhadt & Morhadt, 1979). Often, in these studies, oxygen consumption and heart rate were shown to give strong positive correlations, although sometimes with large individual variations. There are indications from human studies that, to be of value, the relation between heart rate and heart production need to be determined over a 24 hr period due to the possible diurnal variations in heat production and activity (Dauncey & James, 1979). In the present study, a strong positive corre-
J. M. Z. KAMAU and G. M. 0. MALOIY
24
lation has been shown to exist between the rate of oxygen consumption and heart rate under a wide range of controlled environmental temperatures. In view of the differences in the levels of energy demand diurnally during exercise, it is debatable whether the results obtained here could be extended to predict the metabolic rate in a field type situation. Nevertheless, heart rate could well provide a means of estimating metabolic rate in these wild ruminants. In conclusion, the cardiac response, conductance and oxygen pulse in the dik-dik antelope are designed to more than cope with demands of energy and heat conservation under a wide range of ambient temperatures. The strong positive correlation shown here between rate of oxygen consumption and heart rate may provide a simpler means of determining metabolic rate in resting dik-diks in the laboratory or field.
DAUNCEY M. J. & JAMES W. P. T. (1979). Assessment heart rate method for determining energy expenditure man, using a whole-body calorimeter. Br. .I. Nutr.
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HERREID C. F. & KESSEL B. (1967). The thermal conductance in birds and mammals. Camp. Biochem. Phljsiol. 21,405-414. HALES J. R. S. (1973) Effect of exposure to hot environments on the regional distribution of blood flow and on cardiorespiratory function in sheep. Pjugers Arch. 344, 133-148. HOLMES C. W., STEPHENS D. B. & TONER J. N. (1976) Heart rate as a possible indicator of the energy metabolism of calves kept out of doors. Licestock Prod. SC. 3, 33-341. HOLTER J. B., URBAN W. E.. HAYES H. H. & SILVER H. (1976) Predicting heart rate from telemetered heart rate in white-tailed deer. J. Wildl. Mgmf. 40, 626-629. HOPPE P. P.. JOHANSEN, K., MALOIY G. M. 0. & MUSEWE V. (1975) Thermal panting reduces oxygen uptake in the dik-dik. Acfa Physiol. Stand. 95(2), 9A. KLEIBER M. (1961) The Fire of Life, 454 pp. Wiley, New York.
Ackrlowledyemerlt~This
assistance England. telemetry
study received partial financial from the Leverhulme Trust Funds, London, Dr. E. Muller kindly allowed us the use of his set-up. REFERENCES
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