Thermoregulation and total body insulation in the neonatal foal

J. therm. Biol. Vol. 17, No. I, pp. 1-10, 1992

0306-4565/92 $5.00+0.00 Copyright © 1992 Pergamon Press plc

Printed in Great Britain. All rights reserved

T H E R M O R E G U L A T I O N AND TOTAL BODY INSULATION IN THE NEONATAL FOAL J. C. OUSEY, 1 A. J. MCARTHUR, 2 P. R. MURGATROYD, 3 J. H. STEWART I a n d P. D. ROSSDALEl t Beaufort Cottage Stables, High Street, Newmarket, Suffolk CB8 8JS, 2Department of Physiology and Environmental Science, University of Nottingham School of Agriculture, Sutton Bonington, Loughborough LEI2 5RD and 3Dunn Clinical Nutrition Centre, 100 Tennis Court Road, Cambridge CB2 IQL, England

(Received 26 January 1991; accepted in revised form 15 October 1991) Abstract--1. The metabolic rates of pony foals (Equus caballus), age 2-4 and 7-9 days, were measured at different air temperatures between 2 and 40°C in order to identify their lower critical temperature (Ttc) and the environmental demand for heat when the air temperature (Ta) was below this value. In addition, respiratory quotient, rectal temperature and heart rate were measured. 2. The mean values of Tic were about 22 and 19°C for the 2-4 and 7 9 day old foals, respectively. The wide variation (about 10°C) in Tic within each group was attributed to differences in thermal insulation. Below thermoneutrality, metabolic rate, heart rate and respiratory quotient increased with decreasing Ta,

but rectal temperature remained fairly constant at about 38.4°C. 3. The foals thermoregulated effectively under the environmental conditions studied. Key Word Index: Neonatal foal; thermoregulation; thermal insulation; lower critical temperature; metabolic rate

INTRODUCTION

The importance of maintaining neonates at environmental temperatures within their thermoneutral zone has been demonstrated for many species. For example, Silverman (1959) reported that the mortality rate of premature babies decreased significantly when they were maintained in a warm (31.8°C) environment; and Alexander (1962) found that mortality in lambs due to hypothermia was more likely to occur when they were exposed to wet and windy conditions than when kept dry in still air. The literature contains much information about the thermal requirements of infants and the neonates of domestic livestock (e.g. Sinclair, 1978; Poczopko, 1981). The lower critical temperature T1¢(°C) for these species, defined as the air temperature below which a homeotherm must increase its metabolic rate to meet the environmental demand for heat, ranges from about 36°C for infants (Hill and Rahimtulla, 1965) to about 9°C for calves (Webster, 1974). In contrast, there is a paucity of data describing thermoregulation in foals. In the U.K. most Thoroughbred foals are born at night between January and April, when daily mean temperatures outdoors are usually below 10°C. Although most foaling boxes have radiant heaters, the thermal environment into which foals are delivered is usually poorly controlled. The optimum range of environmental temperatures for the young foal, and its ability to thermoregulate at temperatures below the lower critical value, are not known. The aim of this study was to measure the metabolic rates of young foals over a wide range of air temperatures Ti(°C), to identify their thermoneutral zone (zone of minimal metabolism) and lower critical temperature, and to determine their capacity to increase metabolic

rate in response to air temperatures below the lower critical value. THEORY

For homeotherms, the relationships between the rates of heat production, evaporative and non-evaporative heat loss, and environmental temperature can be described by the "metabolic diagram" (Mount, 1979). The lower critical temperature, which defines the lower end of an animal's thermoneutral zone, depends on its thermoneutral rate of heat production Mt,(W m -2) and on its components of thermal insulation. This temperature can be determined from Tie = Tb-- [(rs + re + re)(M,n -- 2E,m)/pCp] -}-[rc+re]AEsm/pC p (1)

where Tb = rs = rc = re = pep =

body-core temperature (°C), thermal resistance of body tissue (s m - l ) , thermal resistance of coat (s m - 1), thermal resistance of environment (s m - l), volumetric specific heat of air (1290Jm -3 K-l), 2 = latent heat of vaporization of water (J g- 1), Esm = minimum rate of water loss per unit surface area by evaporation from skin ( g m -2 s - l ) , E~ = minimum rate of water loss per unit surface area by evaporation from respiratory system (gm-2s-I).

Equation (1) is similar to that given by Blaxter (1989) for the air temperature at which the heat emission from an animal with fully vasoconstricted skin, and with the skin and lungs losing minimal amounts of

2

J.C. OUSEVet al.

water vapour, is equal to its heat production in the thermoneutral zone (resting value). The equation can be derived by manipulating the heat flow equations, and assumes that conduction of heat to the ground and convective heat losses from the respiratory system are small enough to ignore. Here, the heat flux densities are expressed in watts per unit area of body surface, and thermal resistance is in units of s mrather than the traditional unit of insulation (K m2W t) often used by animal scientists. A thermal resistance of 100 s m ~is equivalent to an insulation of 0.078 K m 2W 1 (Monteith and Unsworth, 1990). Below thermoneutrality, the flux densities of latent heat from the skin surface (,~E,m) and respiratory system (2Erm) are minimal, with both usually below 1 0 W i n 2 and nearly all of the heat loss from the body is by non-evaporative routes (i.e. convection and thermal radiation exchange). However, the flux density of heat (total) from the body exceeds M,n. In these conditions, the rate of heat production per unit surface area M (W m 2) required for homeothermy can be calculated from M = pcp[Tb -- "F~,]/[r~+ r,: + r,,] +[(r~+r~))~E,m/(r~+r~+r~)]+)~Erm.

(2)

Equations (I) and (2) show that lower critical temperature and metabolism in the cold depend largely on the components of thermal resistance between an animal's body-core and its environment, The slope of the line relating M to 7~ below thermoneutrality is determined by the total thermal resistance of the body, and this line usually extrapolates to intercept the temperature axis at a value above body-core temperature (McArthur, 1991a). The total body resistance comprises three insulating layers acting in series. The first, the tissue resistance r~, acts between the body-core and skin surface. This quantity depends on the thickness of the skin, on the amount of subcutaneous fat and on peripheral blood flow (Blaxter, 1989). Values of r~ are usually considered to be at a maximum (vasoconstriction) when air temperature is at or below the lower critical value, unless air temperatures are less than 0 C when cold-induced vasodilation may occur. The second, the coat resistance r~, depends on the depth and structure of the hair layer, on the wind speed to which the animal is exposed and on the gradients of temperature and humidity within the coat (Cena and Clark, 1978). The third, the environmental resistance r~, governs the non-evaporative heat losses from the coat surface by convection to the surrounding air and by thermal radiation exchange with surrounding surfaces (e.g. walls, floor). The value of r~ depends mainly on the size of the animal and on the wind speed to which it is exposed (Monteith and Unsworth, 1990). We can define the total thermal resistance of the body r~ot (sm -1 ) by rto~ = r~ + r,. + r~.

(3)

The value of r~o~ for an animal can be determined from the slope of a straight line fitted to measurements of its rate of heat production at different environmental temperatures below thermoneutrality, in accordance with equation (2). When the rate of

heat production M is measured at a single air temperature below Tic, the resistance rto~can be evaluated from equation (2) provided that figures can be obtained for the two terms on the right-hand side involving the latent heat losses ).Esm and 2E~m. McArthur (1991a) showed that, to a reasonable approximation, equation (2) can be rearranged to give riot = p c p ( T b - T a ) / ( M - MT~, )

(4)

where Mro is the metabolic rate predicted by equation (2) when T, = Tb. For most species, this notional metabolic rate is about 10 W m -2. Similarly, equation (1) can be approximated by fl~-- Th -- r~o~(Mtn - - M x b ] / p c p . MATERIALS

AND

(5)

METHODS

Animals

Twenty-four spontaneously delivered, healthy pony foals were studied at ages 2 4 and 7-9 days. The mean body mass (±SD), mb (kg), of each group was 31.1 (+5.7) and 35.3 (.+4.9) kg, respectively. The surface area (m 2) of each foal was estimated as 0.1 mob '67, based on the formula of Meeh (1879). The mean ( + S D ) values of surface area for the two groups were 1.0 (-+0.1) and 1.1 (_+0.1) m 2, respectively. M e t a b o l i c chamber

The foals were studied individually in an insulated metabolic chamber which was 3.6 × 3.6 × 2.4 m in size and in which the air temperature was controlled thermostatically to within + I~C. Air temperature was monitored by two mercury-in-glass thermometers (accuracy _+0.1 C) situated close to the foal. Steady state measurements of metabolic rate were made at air temperatures between 2 and 40°C. The wind speed varied with position in the chamber, but close to the foal it was usually less than 0.5 m s D e t e r m i n a t i o n o f metabolic rate

Metabolic rate was determined by open-circuit indirect calorimetry, using a ventilated mask placed over the foal's muzzle. The steady flow rate of air through the mask was between 40 and 80 1. min ~ , according to the size of the foal, and was measured by a Rotameter flow meter (type 200/24). Samples of the inspired and ventilating air were collected into Douglas bags (101. volume) over periods of about 4 rain. These samples were analysed within 2 h of collection for oxygen and carbon dioxide content by means of Servomex (Model 137) and PK Morgan (model 801) gas analysers, respectively. The ventilation rate of the mask was corrected to give the dry flow rate at STP. The rates of oxygen consumption and carbon dioxide production were derived according to the assumption that nitrogen and other inert gases are conserved in the ventilating air (Dauncey et al., 1978). Respiratory quotient (RQ) and metabolic rate (Win -2) were calculated from these rates of gaseous exchange. The final values of oxygen consumption and carbon dioxide production are estimated to be within + 2% of the true values.

Thermoregulation Experimental protocol Within the two age groups, foals of a given age were studied at two air temperatures, differing by no more than 5°C. Each foal was separated from its dam and placed in a metabolism crate within the metabolic chamber, under constant supervision. It stood on a rubber mesh mat on the base of the crate which was raised off the floor of the metabolic chamber. The foal was fed every 1.25-2 h by bottle or stomach tube, using dam’s milk or milk replacer, and kept in the chamber for at least 3 h to ensure thermal equilibrium was established. Just before the next feed, it was placed in left lateral recumbency in the metabolism crate and restrained until it relaxed and (usually) fell asleep. The foal lay with its legs partially extended and this posture was maintained while respiratory gases were collected. Heart rate was monitored by hand; the number of heart beats was counted over a period of 15-30 s before and during the gas collection, to ensure that the foal was in a “resting” state. At the end of the gas collection, rectal temperature was measured with a small clinical mercury-in-glass thermometer. It was noted whether the foal was shivering or sweating by visual/tactile observations. The depth of the pelage was measured on a small number of foals (n = 9), using a graduated probe held normal to the skin surface. If the foal became excited during the collection of respiratory gases, the procedure was abandoned and repeated when the foal was calm. Statistical procedures

Linear regression analysis was performed and the correlation coefficient r was calculated as described by Armitage (1971). Mean values + 1 SD are given where appropriate. RESULTS

Figures l(a) and (b) show metabolic rate plotted against air temperature for foals age 2-4 and 7-9 days, respectively. These graphs indicate that the lower critical temperature for both groups of foals is about 20°C. Above 20°C the metabolic rate of each group was relatively constant, at about 70 and 85 W m-*, respectively. Sweating was observed when foals from both age groups were exposed to air temperatures above 38°C. Below about 20°C the metabolic rate of both groups increased progressively with decreasing air temperature and shivering was observed in all foals. However, some of the 24 day old foals shivered at air temperatures up to 26”C, and some 7-9 day old foals shivered at air temperatures up to 22°C. Pilo-erection enabled the foals to increase their coat depth in the cold, typically from about 0.3-l .4 cm when air temperature was reduced from a thermoneutral value to about 10°C. The highest metabolic rates were observed in the younger age recorded values being group, the maximum 171 W m-* for a 2-4 day old foal and 153 W m-* by a 7-9 day old foal at air temperatures of 6.5 and 2.7”C, respectively. The lines drawn on Fig. 1 are discussed below. Figures 2(a) and (b) show that the rectal temperatures of both groups remained almost constant over

3

in the neonatal foal

the range of air temperature studied, with mean (*SD) values of 38.3 (kO.3) and 38.4 (f0.3) “C for the 2-4 and 7-9 day old foals, respectively. Figures 3(a) and (b) show that the RQ for all the foals was between about 0.7 and 0.9. The slight negative correlation with air temperature was statistically significant (P ~0.001, r =0.62 and r =0.53 for 24 and 7-9 day old foals, respectively). The heart rate of both groups of foals increased when air temperature decreased below 20°C. Figures 4(a) and (b) show that the heart rate was positively correlated with metabolic rate (P -C0.001, r = 0.59 and 0.66 for 2-4 and 7-9 day old foals, respectively). The total body resistance r,,,, of each foal at air temperatures < 20°C was calculated from equation (4). Figures 5(a) and (b) shows these values of r,,, plotted against air temperature for the respective age groups. The mean (+ SD) values of rto, for each age group were 333 (+ 66) and 343 ( f 59) s m-‘, respectively. There were no consistent changes with age in the total body resistance values for individual foals. However, there were considerable differences in body resistance between individuals: the smallest values of rtoLwere about 250 s m-’ and the largest values nearly 500 s mm‘. Furthermore, Fig. 5 reveals evidence of a significant decrease in the total body resistance of some foals with decreasing air temperature below thermoneutrality. DISCUSSION

The results presented in Fig. 1 demonstrate that the metabolic rates of recumbent pony foals were fairly constant and minimal at air temperatures between about 20 and 35”C, consistent with the thermoneutral zone (zone of minimal metabolism) described by Mount (1979). The increase in metabolic rate with decreasing air temperature below this zone, observed in both age groups, is attributable to an increased non-evaporative heat loss from the body surface. At an air temperature of 5°C the metabolic rates were almost double the thermoneutral values. The large variation in metabolic rates between individual foals at a given temperature in the cold can be accounted for by differences in the total body resistance (Fig. 5) which are likely to be a consequence of differences in coat depth and/or in the amount of subcutaneous fat. These differences in thermal insulation will also affect the lower critical temperature of individual foals, as indicated by equation (1). The values of rtat presented in Fig. 5 were used in a simple iterative procedure to estimate the dependence of q, on the total body resistance of foals in each age group. At the start of this calculation, mean values of M,, were estimated from the data (Fig. 1) obtained when T, 2 20°C. A value of T,c for each group was then calculated by means of equation (5) using the mean total body resistance values of 333 s m-’ (foals 2-4 days) or 343 s m-l (foals 7-9 days). Using this value of T,c, the mean values of M1, were recalculated from the data in Fig. 1 and an improved value of T,, was then determined from equation (5). This procedure was repeated until the recalculated values of h4,, and Tk for each group were insignificantly different from the previous values. Table 1 presents these recalculated

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Fig. 1. Metabolic rate plotted against air temperature for pony foals, age (a) 2 4 days and (b) 7-9 days. The solid lines show the mean values of metabolic rate, and the metabolic rates in the cold for foals of high and low total body resistance r,o, . The dashed lines indicate the mean and range of values for lower critical temperature. values for the two age groups, the values of T~c being 22.1 and 18.6°C, respectively. The m i n i m u m and m a x i m u m values of T,c for each group were also evaluated from equation (5). This calculation was done using the values of Mr. given in Table 1 and the average of the three lowest and three highest values of total body resistance obtained

for each age group at air temperatures below the respective mean values of TLc (i.e. 255 and 4 6 5 s m -1 for foals age 2-4 days, and 260 and 435 s m ~ for foals age 7-9 days). Table 1 presents these m a x i m u m and m i n i m u m values of T~c for each age group. The solid lines on Figs l(a) and (b) show the mean values of metabolic rate for each group of

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foals at and below therrnoneutrality, and the metabolic rates in the cold for foals of high and low (constant) total body resistance. The dashed vertical lines indicate the mean value of T~ for each group, and the maximum and minimum values as calculated above according to differences in insulation. Table 1 indicates that there is a wide range (approximately 10°C) in values for T~ within each group of foals. The mean values of T~ for the 2--4 and 7-9 day old foals are above the mean daily air temperatures

usually encountered outdoors during the early part of the foaling season in the U.K. However, the value of Tic for a well insulated (r,o, = 500 s m - l ) foal could be as low as II°C compared with 26°C for a poorly insulated (rtot = 250 s m - 1) foal. Table 1 also indicates that the mean value of T~¢for the older pony foals was about 4°C lower than that for the younger foals, a consequence of the increase of about 12 W m-2 (16%) in resting metabolic rate with age. A gradual rise in resting metabolic rate (expressed per unit surface

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Fig. 3. Respiratory quotient plotted against air temperature for pony foals, age (a) 2-4 days and (b) 7-9 days. area) with age has been reported for other species, and is usually associated with increases in insulation (i.e. coat depth, fat deposition), in body size (reducing the surface area to mass ratio of the individual) and in plane of nutrition (Alexander, 1974; Mount, 1976; Dauncey and Ingram, 1986). The mass of the foals studied here increased at a rate of about 0.5 kg d -1 , in contrast to infants who lose mass during the first few days post partum. This increase in mass of the foals is consistent with an increase in mares' milk

production during the first few weeks of lactation (Oftedal et al., 1983). However, the results presented in Fig. 5 indicate that the total body insulation of the foals remained almost constant between days 2-4 and 7-9 post partum and, therefore, did not contribute significantly to the decrease in lower critical temperature during the first week post partum. The values of T~ presented in Table 1 apply only to dry foals which are recumbent, resting and in an environment where the wind speed is low and there

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M e t a b o l i c r a t e (W m- 2) Fig. 4. Heart rate plotted against the metabolic rate for pony foals, age (a) 2 4 days and (b) 7-9 days. is no sunlight. For comparison, Ousey et al. (1991) reported that newborn foals which were wet with amniotic fluid and shivering during the first 2 h post partum had metabolic rates which were above 200 W m -2, even when the air temperature was 20°C. Values of Tic for these wet foals are likely to have been much higher than those reported here for dry foals. Also, healthy young foals spend much of their time outdoors, standing or physically active. Ousey et al. (1991) observed that the metabolic rates of standing foals (dry) were about 50% above the rates when

recumbent, an increase which is likely to lower the value of T~¢. Other climatological variables (wind, solar radiation, etc.) affect the thermal status of animals (McArthur, 1991b) and, consequently, the critical air temperature for foals outdoors may differ considerably from the values given in Table 1. Furthermore, the thermoregulatory responses of the neonate can be altered by its clinical status. For example, Rossdale (1968) reported that the rectal temperatures of Thoroughbred foals suffering from prematurity, asphyxia and convulsions were different

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Air t e m p e r a t u r e (°C) Fig. 5. Total body resistance rtot plotted against air temperature (~<20°C) for pony foals, age (a) 2 4 days and (b) 7 9 days. The solid lines connect thermal resistance values for the same individual, determined at different air temperatures. from those of healthy foals during the first 48 h post partum. Clearly, health must also be considered when choosing the thermal environment for equine neonates. The warm limit (upper critical temperature) of the zone of minimal metabolism could not be estimated from the measurements reported here. The metabolic rates of the older foals were relatively constant up to about 40°C, the highest air temperature which could

be attained in the chamber. In the younger group, one foal increased its metabolic rate by 20% above the mean thermoneutral value when the air temperature reached 38.2°C (Fig. l(a)); the corresponding increase in rectal temperature to 39.0°C suggests that the upper critical temperature had been reached for this foal. Figure 5 indicates that the thermal insulation of individual foals can alter below thermoneutrality.

Thermoregulation in the neonatal foal Table 1. Mean body mass, surface area, resting metabolic rate, total body resistanceand range of lower critical temperature values for pony foals (age 2-4 and 7-9 days) Foal Mean body Mean surface Resting Mean value Mean value M i n i m u m Maximum age mass area metabolic rate Mtn of rioI of Tz value of Tic value of Tic (d) (kg) (m2) (W m 2) (s m- i ) (°C) (°C) (°C) 2--4 7-9

31.1 35.3

1.0 1.1

73 85

F o r some foals, the resistance rtot decreased by nearly 30% when air temperature was reduced below T~c. Pilo-erection a n d vasoconstriction enable a n animal to increase re a n d rs, respectively ( M o u n t , 1979). Pilo-erection in response to cold was observed in foals o f b o t h age groups a n d it is likely t h a t the foals h a d vasoconstricted when the air temperature was close to Tic. Blaxter et al. (1959) noted that the body tissue resistance of clipped sheep decreased w h e n they shivered, a n d M c A r t h u r (1987) presented similar evidence for cattle. Biaxter et al. (1959) suggested t h a t this decrease in r s below thermoneutrality m a y be caused by a n increase in blood supply to the shivering muscles, thus opening the capillary beds within the muscle, or to the release of heat from peripheral muscles. The decreases in the foals' insulation in the cold reported here were associated with strong or severe shivering a n d are consistent with these suggestions. However, a decrease in rtot below t h e r m o n e u t r a l i t y may be attributable in p a r t to a decrease in the external insulation (re + re) caused by the rapid body m o v e m e n t s during shivering. Table 2 c o m p a r e s m e a n values of the resting metabolic rate for the foals studied here with values for the n e w b o r n of o t h e r species. The Table also shows the corresponding values (where available) of Tic, the t h e r m o n e u t r a l range a n d values of total body resistance rtot (estimated from e q u a t i o n (4)) for these species. Values of rtot are based in part o n an animal's surface area, which is usually estimated from M e e h ' s formula as 0.094 m 0"67. The coefficient 0.094 is a n average for different species, and is likely to be too low for the foal which has longer legs t h a n the " a v e r a g e animal". A coefficient o f 0.10 was used in the present analysis, a value which is consistent with o u r o w n m e a s u r e m e n t s (unpublished) of the relationship between surface area a n d mass of the foal. F o r c o m p a r a t i v e purposes, Table 2 also presents values of

333 343

22.1 18.6

15.8 13.1

26.0 23.5

oxygen c o n s u m p t i o n ( t h e r m o n e u t r a l rate) expressed in ml kg t min -~. Alexander (1975) classified n e w b o r n animals on the basis of their t h e r m o r e g u l a t o r y capacity as altricial, precocious or i m m a t u r e neonates, the third of these groups (e.g. marsupials) having virtually no thermal control at birth. Altricial newborn, such as the human, rat a n d dog, tend to have p o o r thermal control and their lower critical temperatures are just a few degrees below their n o r m a l body-core temperature. Precocious newborn, which include the foal, are more developed at birth t h a n altricial n e w b o r n a n d can m a i n t a i n h o m e o t h e r m y over a wider range o f air temperatures; they have a wider zone of minimal metabolism a n d Tic can be as m u c h as 30°C below n o r m a l body-core t e m p e r a t u r e (e.g. calves). Like the lamb a n d calf, the foal can withstand a n air temperature as low as 2°C without a c o n c o m i t a n t fall in rectal temperature, by increasing metabolic heat p r o d u c t i o n to a rate which is a b o u t twice that in the thermoneutral zone. However, the period over which foals can m a i n t a i n this high level of metabolism is not known. In the present study, foals were exposed to each air temperature for between 4 and 5 h; one foal remained in 12.5°C for 8.5 h, during which time its metabolic rate remained consistently elevated at a b o u t 100 W m -2. The significant increase in R Q with decreasing air temperature, reported here for b o t h age groups, suggests t h a t the foals increased their utilization of muscle glycogen with shivering in the cold. In contrast, observations m a d e o n lambs a n d calves indicate that, in these species, there is a cold-induced decrease in R Q associated with non-shivering thermogenesis (NST) and oxidation of fatty acids (Alexander, 1961; Alexander a n d Williams, 1968; Vermorel et al., 1983). The presence of b r o w n adipose tissue a n d N S T has not been observed in foals; the results presented here suggest that shivering is a n i m p o r t a n t mechanism for

Table 2. The lower critical temperature (Tic), resting rate of oxygen consumption, resting metabolic rate and total body resistance of newborn animals Oxygen* consumption Metabolic* Thermalt Age Tz (ml kg- t rate resistancertot Species (d) (°C) rain-] ) (Wm -2) (S m -] ) Reference Comments Human 3-10 32.5 ~7.0 30.93/ 224 Hey and O'Connell 0970) Naked Human 3-10 28 ~7.0 30.8~ 448 Hey and O'Connell (1970) Clothed Human 3-10 25 ~7.0 30.93/ 598 Hey and O'Connell (1970) Cot-nursed Monkey < 1 35 9-10 28.5:~ 209 Dawes (1968) Pig < 1 34 11 40.63/ 194 Mount (1959) Dog 3 30 14.7 34.43/ 381 Crighton and Pownall (1974) Sheep < 1 25 -60-70 235 Alexander(1974) Varies with breed Cow <1 9 -100 430 Webster(1974) Varied with breed Horse 2-4 22 7.0 73 333 Presentstudy Varies between individuals *Resting values in thermoneutral range. tValues based on equation (4) in text (the value of MTb= 5 W m- 2 was used to calculate rtot for infants). 3/Values based on 1 I. 02 ---20.1 kJ.

l0

J.C. OUSEYet al.

heat production in foals exposed to cold. Three values o f RQ in Fig. 3 are below 0.7. These unusually low values could be the consequence of carbohydrate p r o d u c t i o n from fat (Kleiber, 1961). Alternatively, re-breathing o f some carbon dioxide in the ventilated mask could account for these low values of RQ but the flow rate through the mask was considered high enough to prevent this. Figure 4 shows a positive correlation between heart rate and metabolic rate for the foals. A linear relationship between these two quantities has been established for other species (Mount, 1979). However, Webster (1967) reported differences in the regression equations between heart rate and metabolic rate derived for individual sheep during cold exposure. The scatter o f points about the lines fitted in Fig. 4 is likely to be a consequence o f biological variability between individuals (data for up to 24 animals are shown on the same graph). In conclusion, we report that the lower critical temperature for healthy young pony foals is, on average, a b o u t 20°C. However, because o f differences in body insulation, T~ can differ considerably between individual foals. Values o f T~ range from about 26°C (low insulation) to about I Y C (high insulation). These figures apply only to foals which are dry, recumbent and exposed to still or slowly moving air (no sunlight). A change in posture to standing, or exposure to o u t d o o r conditions, may alter the values o f TI~ considerably. Like other precocious newborn, the foal can increase its metabolic rate markedly in response to cold. Our results indicate that healthy p o n y foals can thermoregulate effectively at air temperatures as low as 2°C for periods o f several hours. Acknowledgements--The Horserace Betting Levy Board and Darley Studfarm Management provided financial support for this project and facilities were made available by the Animal Health Trust, Newmarket, Cambridge University and Dunn Clinical Nutrition Centre, Cambridge. The authors thank Dr J. A. Clark for his comments on the manuscript and the numerous people who assisted with the study. REFERENCES

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