The basal metabolism (BM) of an organism over a certain time period may be described as the total amount of heat in kilocalories (kcal) that the organism produces during this period, assuming that the organism is at rest and not subject to stress. The basal metabolic rate (BMR) is the rate in kcal per hour at which the organism produces heat. The BMR of an animal such as a desert rat fluctuates in response to changes in temperature and other environmental factors. The BMR generally follows a diurnal cycle, rising at night during low temperatures and decreasing during the warmer daytime temperatures. Find the BM for 1 day if BMR (t) = .4 + .2 cos (πt/12) kcal per hour (t = 0 corresponds to 3 a m.). (See Fig. 11.)

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kcal per hour Y y = 4 + 2 cos 12 t (hours) 0 12 24 Figure 11 Diurnal cycle of the basal metabolic rate.