COMPENSATION IN RESTING METABOLISM FOR EXPERIMENTALLY INCREASED ACTIVITY

To study zebra finch allocation of energy to day and night at two diff erent workloads, we assessed the daily energy turnover from: (1) metab olizable energy of the food, and (2) doubly-labeled water. In both exp eriments we imposed two levels of activity on captive zebra finches (T aeniopygia guttata), by applying different computer-controlled workloa d schedules. A low workload required 20 hops, and a high workload 40 h ops to obtain 10 s access to food. In experiment 1, we further measure d nocturnal energy expenditure by overnight oxygen consumption. From e xperiment 2 we derived an estimate of the costs of hopping activity, f rom inter-individual association of daily amount of hopping and daily energy expenditure. Surprisingly, the daily energy budget was, on aver age, reduced slightly when birds were subjected to a high workload. Si nce hopping activity was 50% higher during the high workload than duri ng the low workload, the birds apparently compensated, even over-compe nsated, for the increased energetic demands of activity. Nocturnal ene rgy expenditure was indeed reduced for the high workload, which was la rgely due to a reduction in resting metabolic rate. Economizing on ene rgy was more than could have been accomplished by a reduction in mass alone, and we discuss the occurrence and potential mechanisms of physi ological compensation. The amount of energy saved during the night did account for part of the total amount of energy saved. We surmise that the strategy of energetic compensation observed during the night was extended into the inactive hours of the day.