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Bergmann's Rule and Climatic Adaptation in Woodrats (Neotoma)

James H. Brown and Anthony K. Lee
Evolution
Vol. 23, No. 2 (Jun., 1969), pp. 329-338
DOI: 10.2307/2406795
Stable URL: http://www.jstor.org/stable/2406795
Page Count: 10
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Bergmann's Rule and Climatic Adaptation in Woodrats (Neotoma)
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Abstract

We present data on the thermoregulatory capacities of representatives of 10 populations of the genus Neotoma from diverse climates in western North America. Body size was inversely correlated with environmental temperature, indicating that the populations follow Bergmann's Rule. In general this was true of intraspecific, as well as intrageneric variation. Among the woodrat populations, thermal conductance and lethal ambient temperature were positively correlated with environmental temperature and inversely correlated with body size. Our data suggest that body size has played an important role in adaptation to environmental temperature because it has influenced rates of heat loss. Larger woodrats have a selective advantage in cold climates apparently because their smaller surface-to-mass ratio and greater insulation permit them to conserve metabolic heat. For the opposite reasons, smaller animals are favored in deserts where heat dissipation is of relatively greater importance. Our findings appear to refute Scholander's (1955) argument that variations in body size among closely related populations have not resulted from adaptation to environmental temperature. We suggest that a physiological basis for variation in body size folllowing Bergmann's Rule is most likely to be found in those groups of homeotherms which (1) are composed of populations occurring in contrasting thermal environments, and which have had sufficient time and isolation to differentiate genetically in response to local conditions; (2) contain individuals that weigh less than one kilogram and rarely employ evaporative cooling to maintain body temperature below ambient; (3) are not subjective factors which outweigh the thermoregulatory consequences of body size.

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