There is very little information available on the metabolic rate of the newborn pig in relation to its environment. Knowledge of this sort would be of considerable interest, as experience shows that the new-born pig is highly susceptible to chilling, which leads to coma and death, and because the animal has very little hair and subcutaneous fat and may therefore be expected to have a correspondingly low insulation against heat loss. The question arises as to the animal's response to cooling of the environment: whether the newborn pig is poikilothermous like the new-born rat (Fairfield, 1948) and mouse (Fitzgerald, 1953), or whether it responds in a manner typical of the adult homoeotherm, that is by increasing its heat production when the environmental temperature falls below the critical temperature for the animal. The results presented here show that the new-born pig is not poikilothermous and that over the environmental temperature range 4-38°C it exhibits a vigorous metabolic response to cooling of the environment. Further, the critical temperature is not definite, but may be in the region of 34-35°C.Some of these results have been the subject of preliminary communications (Mount, 1958a-c). METHODSObservations were made on a total of thirty-six new-born pigs of the Large White breed and eight of the Landrace breed, aged from 1 hr to 7 days and with weights ranging from 0-86 to 2-72 kg. Animals were removed singly from the sow and the rest of the litter in the farrowing pen, and weighed, and the rectal temperature was recorded by a clinical thermometer inserted to a depth of 3-5 cm for 2 min. The pig was then placed, without any other treatment, and without food or water, in the chamber of a closed-circuit metabolism apparatus. The animal was in the dark, and remained quiet for the duration of the experiment; only very rarely was there any agitation.Two chambers were used, both of the design shown diagrammatically in Fig. 1. The outer cover was made of sheet iron in one (volume, 34 1.), copper in the other (volume, 22 1.). Wet-and dry-bulb thermometers and an air stirrer were mounted in the top of the cover, which could be lowered over the housing for the pig, making a gas-tight liquid-paraffin seal in a base tray. The pig rested on 16-gauge galvanized iron wire in the form of a I in. (10 mm) mesh, with a similar

I . Measurements of energy and nitrogen balances were made on thirty-eight individually housed pigs (initial body-weights 21-38 kg) at environmental temperatures of 10, 15, 20, 25 and 30" with four levels of feeding at each temperature. Values for energy retention (ER), protein (P) and fat ( F ) deposition and body weight gain (AW) were calculated at each temperature at metabolizable energy (ME) intakes equivalent to once (M; 40 kJ/kg0.7s per d), twice (zM), three (3M) and four (4M) times the thermoneutral maintenance energy requirement.2. ER at each plane of nutrition increased with temperature to maximal values between approximately 20 and 25'; ER was negative at four of the five environmental temperatures at M.3. P increased significantly with increase in M E intake but was dependent on environmental temperatures only at intakes of M and 2M. The increase in P per unit increment in ME intake decreased from 0.16 at 10' to 0.12 at 30". The net efficiency of protein utilization also decreased with increase in environmental temperature from 0.54 at 10' to 0.39 at 30". 4. F increased significantly with increase in M E intake, but was more temperature-dependent than P, increasing to maximum values estimated to be between 20 and 25' at each level of intake; F at 30' was less than that at 2 5 O . The increase in F per unit increment in ME intake decreased from 063 at 10' to 0.51 at 30". .The optimum temperature for AW was dependent upon ME intake, varying from above 30" at M to less than 20' at 4M. The reduction in AW per 1' at 15" was also dependent upon the level of intake decreasing from 1.63 g/kg0.75 per d at M to -0.09 at 4M. 6. For a 35 kg pig the reduction in P, as a result of a I' decrease in temperature at 15" at an intake corresponding to 2.5M, a a s equivalent to a 4 g/d reduction in food intake; the corresponding equivalent for Fwas 28 g/d.In the mature animal, variations in food intake produce changes in body composition that are largely confined to the fat content, whereas in the growing animal the consumption of food is associated with increments in both protein and fat formation. In recent years there has been an increase in investigation into energy retention in the growing animal, with the associated importance of such investigation for meat production, for example in the pig (Kotarbinska, 1969; Fuller & Boyne, 1971 ;Verstegen, Close, Start & Mount, 1973; Thorbek, 1975) and for growth and development in man (Waterlow, Hill & Spady, 1976).Energy retention (ER) in the body takes place at a rate that is dependent on the interaction between the level of intake of metabolizable energy (ME) on the one hand, and the animal's need for maintenance and thermoregulation on the other. A considerable thermal demand by the environment leads to a reduction in the ME that the animal has available for growth when it is on a given level of food intake (Blaxter, 1977).The present work on energy retention in the growing pig has accordingly involved several planes of nutrition and levels of environmental temperature. T...

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I. Eight groups each of four castrated male pigs, 25-30 kg initial body-weight, were kept for periods of 3 weeks in a calorimeter equipped as a pig pen and maintained at either 8" or zoo. At each temperature two feeding levels (g food/kg body-weight per d) were used, 45 and 52 at So, and 39 and 45 at 20'. Metabolizable energy, heat loss and nitrogen balance were measured.2. Heat loss was higher at 8" than at zoo and was independent of plane of nutrition, whereas at 20' the higher heat loss occurred at the higher plane of nutrition. Energy retention depended on both temperature and feeding level, and was highest at the 52 g feeding level at 8".3. N retention was not influenced by environmental temperature but varied with plane of nutrition (correlation coefficient = 094), the increase being 9.98 ( & 0.8) mg N per g food increase. The correlation coefficient between N retention and body-weight gain was also 0'94; body. weight gain was correlated with N retention rather than with fat deposition. Fat gain was reduced at the lower feeding levels and at the lower environmental temperature at the feeding level of 45 g/kg.4. The partial efficiency of energy retention at zoo was 66.5%. From this efficiency the maintenance requirement (at zero energy retention) at zoo was calculated to be 418 kJ/kgo*'5. At 8" the partial efficiency of energy retention was 99.4%.Calorimetric measurements have shown that, in addition to effects associated with body size and thermal insulation, the rate of an animal's heat loss is determined principally by two factors, the plane of nutrition and the environmental temperature. The environmental temperature also determines which of these two factors is primary : in the zone of thermal neutrality, the plane of nutrition is the chief determinant, with the higher heat loss occurring at the higher level of feeding, whereas under cooler conditions heat loss is dependent on the environmental temperature and the plane of nutrition has little effect. This relation has been demonstrated for heat production measured by indirect calorimetry in individual clipped sheep (Graham, Wainman, Blaxter & Armstrong, 1959) and in groups of pigs (Verstegen, 1971), and for heat loss measured by direct calorimetry in groups of pigs (Close, Mount & Start, 1971).In the direct calorimetric experiments on pigs each group was exposed to two consecutive planes of nutrition while the environmental temperature was held constant. In earlier work (Holmes & Mount, 1967), each group had been exposed to two consecutive environmental temperatures while the plane of nutrition was held constant.In both instances at the lowest environmental temperatures used (7 and 9O respectively) at a given level of food intake the rate of heat loss was increased significantly and the rate of energy retention (metabolizable energy intake less heat loss) reduced correspondingly.In both sets of experiments on pigs there was the possibility that the results obtained