Tolerance of Cold and Bergmann's Rule

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As size is well known to affect the energy relations between an endotherm and its environment, one would expect that size may also influence or be related to geographic distribution. Bergmann's Rule postulates that geographic races of small size are generally found in the warmer parts of a species' range and races of larger size in the cooler parts. This rule has been attacked by Scholander (1955, 1956) and Irving (1957) on the grounds that the differences in size involved are too small to provide significant heat conservation and that adaptation to cold depends mainly on improved insulation (Scholander et al., 1950a). The rule has been defended by Mayr (1956) and Hamilton (1961) on empirical grounds, and recently from a theoretical basis by LeFebvre and Raveling (1967). Good comparative data on the weights of different races of a species are difficult to find, but variations of the order of 10 to 100+ per cent exist between extreme northern and southern forms. If we assume that standard metabolism in the zone of thermal neutrality, or the energy requirements of the bird at complete rest and in a post absorptive condition, increases with weight (W) as W0724 (Lasiewski and Dawson, 1967) and surface area as W0.667, then a 50 per cent increase in weight would reduce the rate of heat loss per unit area body surface only 2.4 per cent. This advantage might well be offset by the 34 per cent increase in the birds' standard metabolism and hence energy requirements. The importance of size on a bird's tolerance of cold may be analyzed in a more significant manner by comparing the standard metabolism for species in the zone of thermal neutrality with their standard metabolism at 0?C. The lower limit of temperature tolerance for several small tropical resident and migrant passerine species and temperate zone permanent residents in the summer is 0?C or slightly below (Table 2). Winter residents in temperate regions and migrants to the Arctics, however, are able to tolerate much lower temperatures. Equations for the regression of standard metabolism (M = kcal/bird-day) on ambient temperature below the zone of thermal neutrality have been compiled for a number of species from data available in the literature (Table 1). These equations make possible the drawing of regression lines for standard metabolism in relation to weight (W = grams) both for the zone of thermal neutrality and for 0?C (Figure 1). The equations for these lines are as follows: