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FIGURE 4.41 Gain and lose weight. Many animals gain weight in the summer and lose weight in the winter. This graph shows the mean variation in body mass (± SE) of five European hamsters (Cricetus cricetus) maintained in an outdoor enclosure in Germany for 3 years. (Source: Wollnik, F. & Schmidt, B. (1995). Seasonal and daily rhythms of body temperature in the European hamster (Cricetus cricetus) under semi-natural conditions. Journal of Comparative Physiology B 165: 171-182.)

Jul Jan

Months

FIGURE 4.41 Gain and lose weight. Many animals gain weight in the summer and lose weight in the winter. This graph shows the mean variation in body mass (± SE) of five European hamsters (Cricetus cricetus) maintained in an outdoor enclosure in Germany for 3 years. (Source: Wollnik, F. & Schmidt, B. (1995). Seasonal and daily rhythms of body temperature in the European hamster (Cricetus cricetus) under semi-natural conditions. Journal of Comparative Physiology B 165: 171-182.)

The most obvious feature of winter is the cold weather, which transforms aqueous precipitation from rain to snow. The process of thermoregulation is not discussed in detail until Chapter 10; however, it should be obvious that warmblooded animals (mainly birds and mammals) must be able to produce more body heat during the winter than in the summer, so that they can counteract the environmental cold (Figure 4.42). At any time of the year, birds and mammals increase their metabolic rate when exposed to lower temperatures, but their ability to do so is enhanced by winter acclimatization.275-277 Consider Figure 4.43, which shows the mean metabolic rates of wild rats (Rattus norvegicus) captured in Canada during the summer (mean ambient temperature: 20°C) and the winter (mean ambient temperature: -6°C) and tested at various environmental temperatures. Note that the metabolic rates of the two groups of rats are very similar when the animals are tested between 0 and 20°C, but that winter-acclimatized rats have

FIGURE 4.42 A cold wolf. As the temperature of the environment falls in the winter, even well-insulated animals must increase their metabolic rate to maintain their body temperature at the normal level. (Source: Yellowstone National Park Wildlife Graphics, U.S. National Park Service.)

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Turbo Metabolism

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