ABSTRACT
Respiratory water loss and rectal temperature were measured in domestic fowl running for 10 min on a treadmill at speeds of 1·24−4·3 hm h-1 in air temperatures of 20 ± 2 °C or 32 ± 2 °C.
At given speeds the water loss at 32 ± 2 °C was approximately twice that at 20 ±2 °C and the end-exercise rectal temperature was 0·5−0·8 °C higher.
At 20 ± 2 °C, respiratory evaporation accounted for 10−12 % of the total metabolic energy used at all speeds. At 32 + 2 °C, the fractional respiratory heat loss fell from 26−5 % at 1·24 km h-1 to 17 % at 3·6 km h-1. The fraction of the total metabolic energy stored as body heat rose progressively with air temperature.
INTRODUCTION
Studies of thermoregulation in flying birds have shown that at air temperatures of 0−30 °C heat loss by respiratory evaporation accounts for 10−25 % of the total metabolic energy production (summary of data in Torre-Bueno, 1978) and the major heat loss appears to take place through the integument. Only one study has been made of thermoregulation in a running bird: Taylor et al. (1971) found that in exercising rhea most of the heat produced during 20 min periods at high work rates and in high air temperatures was stored by the body and later dissipated. Any attempt to compare the heat balances of running and flying birds based solely on the available evidence could be limited by the great size disparity between the rhea (18−25 kg) and those flying birds investigated (< 1 kg), since scale effects might partially mask underlying physiological responses. For instance, during relatively short (10−20 min) periods of exercise, when a stable temperature may not have been attained, the ratio of integumentary heat loss to heat storage must inevitably be influenced by the surface area/ mass characteristics of the animals concerned. We have carried out measurements of respiratory water loss and heat storage in exercising domestic fowl, a much smaller bird than the rhea, and find that despite scale differences, the pattern of exercise thermoregulation is essentially the same in both species.
METHODS
Experimental measurements
Experiments were carried out on four adult female domestic fowl (mean b.w. 2-08 kg) belonging to the same group of animals and subjected to the same exercise regime as that described by Brackenbury, Avery & Gleeson (1981). For the measurement of respiratory water loss the animals wore a loose-fitting plastic mask from which an airstream bearing the expired gases was drawn at a measured rate of 20−30 1 min-1 by means of a vacuum pump. The stream passed into a 5 1 vessel containing a Novasina electronic sensor (Humitec, Sussex, England) which measured the relative humidity and temperature of the gas. The sensor was connected via a cable to a linearizer circuit and meter outside the vessel and the relative humidity was displayed simultaneously on a Grass pen recorder. The sensor was regularly calibrated to ± 1 % R.H. against saturated salt solutions supplied by the manufacturer. In situ the sensor produced an 80 % response to a rectangular change in input within 55 s, at the maximum flow rate of 30 1 min-1. The water content of the gas was computed from the measured temperature and humidity values using standard thermodynamic charts. The rate of respiratory water loss was calculated from the total airflow rate multiplied by the difference between the water content of the ambient air and the air plus expired gas mixture drawn from the animal. The apparatus was tested by drawing through the mask weighed amounts of steam from a beaker of water heated to 60−70 °C and the mean recovery was better than 99·5 %. The mean R.H. of the ambient air during the experiments was 47·3 % at 20 ± 2 °C and 19·6 % at 32 + 2 °C.
Experimental procedure
At the start of the experiments the mask was suspended inside the treadmill chamber in order to establish the water content of the ambient air. The animal was then introduced into the chamber and, at 20 ± 2 °C, fitted with the mask and allowed to settle for 5−10 min before the exercise was begun. At 32 + 2 °C the animals were allowed to adjust to the chamber environment for at least an hour before fitting the mask. was recorded continuously during the pre-exercise and exercise periods and for a further 5−10 min after the exercise. In a separate series of experiments, the animals ran without the masks and rectal temperature was measured before and immediately after exercise. There was no visible difference in the performance of the animals with or without the mask.
RESULTS
The resting was 11·81 ± 0·36 S.E. mg kg-1 min-1 at 20 ± 2 °C and 36·3 + 2·6 mg kg-1 min-1 after 1 h exposure to temperatures of 32 ± 2 °C. During exercise at 20 + 2 °C, rose rapidly at the beginning of the run and reached a plateau in most cases after 3−6 min (Fig. 1). At 32 ± 2 °C, following its initial rapid rise, continued to increase at a much slower rate throughout the remainder of the exercise (Fig. 1). After exercise at 20 ±2 °C, returned progressively to normal at approximately the same rate as its rise at the beginning of the exercise, and there was. no evident panting in the birds. In contrast, after exercise at 32 ± 2 °C the birds usually panted vigorously, and this was accompanied by a reduction of of a new plateau 15−30% below its end-exercise value (Figs. 1, 2). The sharpness of the rises and falls of is possibly underestimated in Fig. 1 owing to the lag in the response of the sensor. The mean attained over the last 5 min of exercise rose progressively with running speed and was approximately twice as great at the higher air temperatures (Figs. 1, 2). The end-exercise body temperature also rose with running speed and at a given speed was 0·5−0·8 °C at the higher air temperatures (Fig. 2).
DISCUSSION
Respiratory water loss at rest and after exercise
The resting in fowl (11·81 mg kg-1 min-1) compares with values obtained at similar air temperatures in duck (11 mg kg-1 min-1; Bouverot & Hildwein, 1974) and domestic fowl (16 mg kg-1 min-1; Richards, 1976), both using gravimetric methods. The resting at 32 ± 2 °C (36-3 mg kg-1 min-1) is considerably less than that in the duck (47 mg kg-1 min-1, measured at 30 °C) and in Richards’ domestic fowl (50 mg kg-1 min-1). However, in both the latter cases the inspired air had been dried by passage across calcium chloride columns and the vapour pressure deficit between the respiratory surfaces and the air was greater than in the present experiments. At 32 ± 2 °C the pre-exercise birds of the present study were evidently not panting vigorously since the maximum sustained after exercise (82·54 ± 3·3 mg kg-1 min-1, Fig. 2) when respiratory rate rose to 200−250 min-1 (see Fig. 4, Brackenbury et al. 1981) was more than 2·3 times as great. This compares with a value of 72 mg kg-1 min-1 measured by Menuam & Richards (1975) in resting, maximally panting birds.
Heat partition during exercise
In Fig. 3, the measured is expressed in terms of the equivalent heat loss (cal kg-1 min-1) as a percentage of the total metabolic rate E, also given in cal kg-1 min-1 and estimated from data on the steady-state oxygen consumption of the same birds during exercise (Brackenbury, Avery & Gleeson, 1981). The calorific equivalent of oxygen was assumed to be 5 kcal l-1 (R.Q. = 0-95) and the heat of vaporization of water 0·58 kcal g-1. The rate of heat storage by the body Hs was calculated from the mean rate of increase of rectal temperature during exercise, taking the specific heat of the tissues as 0·82 cal g-1 (Tucker, 1968). This calculation is probably a slight overestimate since it is known that, at least in flying birds, there is a temperature gradient between the core and the subcutaneous tissues (Hart & Roy, 1967; Torre-Bueno, 1976). Also shown for comparison in Fig. 3 are the values for fractional respiratory heat loss and fractional heat storage arrived at by Taylor et al. (1971) in the exercising rhea. Evaporative heat loss in this case was based on measurements of water loss over 20 min periods of running, and this may help to explain the high values of fractional evaporative heat loss in the rhea compared to the chicken in which MR,H20 was still rising at the end of the 10 min runs (Fig. 1).
Fig. 3 shows that the general pattern of heat partition during exercise is similar in both the fowl and rhea : a progressive fall in the effectiveness of respiratory evaporatfli at higher work rates and a progressive increase in the fractional heat storage, both features being enhanced at the higher air temperatures. In the extreme case, Taylor et al. found that during 20 min runs at 10 km h-1 at an air temperature of 43 °C, 75 % of the total energy used by the rhea was stored as heat and subsequently liberated in the post-exercise period. These authors suggested that hyperthermia might play an adaptive role, reinforcing heat loss by conduction through the integument and to this extent saving on water loss through the respiratory system. Taylor et al. (1971) had reported a similar physiological strategy in the African hunting dog.
It is probable that during the course of exercise in both rhea and fowl the thermal conductance of the integument increases as a result of a rise in peripheral circulation. Bjirfold to 10-fold increases in skin conductance have been estimated in several flying birds, including budgerigar (Tucker, 1968), black duck (Hart & Berger, 1970), hummingbird (Berger & Hart, 1972) and pigeon (Butler, West & Jones, 1977). Hart & Roy (1967) measured a 6- to 7-fold rise in heat flow across the breast muscles of pigeons during flight. The thermal conductance of the integument is also influenced by behavioural changes: Hart & Roy (1967), Tucker (1968), Bernstein (1976) and Torre-Bueno (1976, 1978) have all drawn attention to the importance of the positioning of the legs, wing undersurfaces and other sparsely feathered areas in promoting convective heat loss during flight. Erection of body feathers and exposure of the axillary regions by wing drooping were sometimes observed in running fowl, and the bright colbur of the combs and wattles indicated an increased blood perfusion that probably led to significant heat losses from these surfaces.
Plying birds like running birds respond to an increase in air temperature by increasing respiratory evaporation. At 18−20 °C the fractional respiratory evaporative heat losses in budgerigar (Tucker, 1968), hummingbird (Berger & Hart, 1972), crow (Bernstein, 1976) and starling (Torre-Bueno, 1978) were approximately 0·11, 0·14, 0·07 and 0·11 respectively ; at 28−30 °C these values had increased by an average of 70% to 0·13, 0·25, 0·13 and 0·25 respectively. The corresponding increase over a similar temperature range in running fowl was 90−160% depending on running speed (Fig. 2). It is evident, however, that even at raised environmental temperatures respiratory evaporation is only of secondary importance compared to the skin as a means of heat loss during flight and running.
ACKNOWLEDGEMENTS
This work was supported by the Science and Agricultural Research Councils.