Continuous measurement of heart rate as an indicator of the energy expenditure of sheep.

I. The relationship between the heart rate and the energy expenditure of four sheep, S, S,, S, and S1, was studied under conditions known to increase energy metabolism. 2. A close relationship between heart rate and energy expenditure was shown to exist in three of the four sheep tested when energy expenditure was increased by cold exposure and by increased levels of food intake. 3. The source and magnitude of the errors associated with this relationship were studied by analysis of the variation occurring in the O2 pulse (ml O,/heart beat) of the sheep in response to factors other than those directly related to changes in heart rate. In two sheep, S, and S,, variations in Oa pulse were random. The errors associated with the estimation, from heart rate measurements, of the energy expenditure of these sheep were k 6-8 % and 5 8.1 % respectively. In the other two sheep, S, and S, 0, pulse tended to vary significantly during the experiments, in a way that was not directly related to changes in heart rate. The errors of the relationship in these sheep were * 8.6 % and f 13.8 % respectively. 4. Indirect evidence was obtained to suggest that continuous measurements of heart rate continued to reflect with reasonable accuracy the energy expenditure of the three sheep, S6, S, and S8 when they were standing and lying and when they were eating. 5 . Direct measurements were made of the 0, consumption and heart rate of sheep S , and S, during the act of eating. The energy expenditure of sheep S, estimated from its 0, consumption was close to that estimated from measurements of its heart rate. Large errors were noted between these two estimates in sheep S,. 6. It was concluded that sheep S, and S6 would be suitable for long-term experiments designed to predict energy expenditure from heart rate measurements made in the field. Sheep S, was considered to be of limited value for short-term experiments. Sheep S1 was considered to be unsuitable.