Ecological factors producing geographic variation in body size of the seed beetle Stator limbatus.
Stillwell, Craig*,1, Morse, Geoff2, Fox, Charles, 1 Department of Entomology, Lexington, KY, USA2 Department of Entomology, Davis, CA, USA
ABSTRACT- One of the most interesting geographic patterns observed in nature is that body size of animals usually increases (Bergmann,s rule) or decreases (converse Bergmann,s rule) with increasing latitude. These patterns are believed to be the result of some selective factor that varies consistently with latitude such as mean annual temperature. We examined geographic variation in body size of the seed beetle, Stator limbatus, over a broad geographic range (40o range in latitude) from northern South America to the southwestern USA. We measured several morphological traits of field-collected specimens from 96 sites. Like many other animals, S. limbatus exhibited a latitudinal cline in body size, being smallest in the tropics and increasing in size as you move away from the tropics. To determine what factors are responsible for producing the cline, we tested the relative influence of mean temperature, variation in temperature (measured as the difference between the hottest and coldest months of the year), and host species seed size. Interestingly, mean temperature explained only a small amount of variance in size whereas variation in temperature and host seed size explained more of the variance; beetles increased in size with increasing seed size and increasing variation in temperature. Our results suggest that both variation in temperature and host seed size interact to produce the observed latitudinal cline in S. limbatus. Thus, factors other than mean annual temperature may better explain latitudinal clines in animals.
Key words: Bergmann's rule, body size, temperature, geographic variation
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