Ambient Temperature (°C)

FIGURE 4.43 Compensating for the cold of winter. The graph shows the metabolic rates of wild rats (Rattus norvegicus) captured in Canada in the summer and in the winter and tested at different ambient temperatures. Winter-acclimatized rats show greater heat-producing capacity (cold-induced thermogenesis) at ambient temperatures below -10°C (14°F). Each data point corresponds to the mean (± SE) of approximately eight animals. (Source: Hart, J. S. & Heroux, O. (1963). Seasonal acclimatization in wild rats (Ratt)us norvegicus). Canadian Journal of Zoology 41: 711-716.)

greater thermogenic (i.e., heat-producing) capacity, as demonstrated by the higher metabolic rates at test temperatures of -20°C and below. At -40°C, the metabolic rate of winter-acclimatized rats is 40% higher than that of summer-acclimatized rats. Thus, the capacity for cold-induced thermogenesis oscillates with the seasons. The

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