Stillwell R Craig, Morse Geoffrey E, Fox Charles W
Department of Entomology, University of Kentucky, Lexington, Kentucky 40546, USA.
Am Nat. 2007 Sep;170(3):358-69. doi: 10.1086/520118. Epub 2007 Jul 24.
Body size of many animals varies with latitude: body size is either larger at higher latitudes (Bergmann's rule) or smaller at higher latitudes (converse Bergmann's rule). However, the causes underlying these patterns are poorly understood. Also, studies rarely explore how sexual size dimorphism varies with latitude. Here we investigate geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Stator limbatus, collected from 95 locations along a 38 degrees range in latitude. We examine 14 variables to test whether clines in environmental factors are adequate to explain geographic patterns of body size. We found that body size and sexual size dimorphism of S. limbatus varied considerably with latitude; beetles were smaller but more dimorphic at lower latitudes. Body size was not correlated with a gradient in mean temperature, contrary to the commonly accepted hypothesis that clines are produced by latitudinal gradients in temperature. Instead, we found that three factors were adequate to explain the cline in body size: clinal variation in host plant seed size, moisture (humidity), and seasonality (variance in humidity, precipitation, and temperature). We also found that the cline in sexual size dimorphism was partially explainable by a gradient in moisture, though moisture alone was not sufficient to explain the cline. Other ecological or environmental variables must necessarily contribute to differences in selection on male versus female body size. The main implications of our study are that the sexes differ in the magnitude of clinal variation in body size, creating latitudinal variation in sexual size dimorphism, and that clines in body size of seed beetles are likely influenced by variation in host seed size, water availability, and seasonality.
在高纬度地区体型要么更大（伯格曼法则），要么更小（反向伯格曼法则）。然而，这些模式背后的原因却鲜为人知。此外，研究很少探讨两性体型差异如何随纬度变化。在此，我们调查了以种子为食的甲虫Limatus limbatus的体型和两性体型差异的地理变异情况，该甲虫是从沿纬度38度范围的95个地点采集的。我们研究了14个变量，以测试环境因素的渐变是否足以解释体型的地理模式。我们发现，Limatus limbatus的体型和两性体型差异随纬度有很大变化；在低纬度地区甲虫体型更小，但两性差异更大。体型与平均温度梯度无关，这与普遍接受的观点——渐变是由温度的纬度梯度产生的——相反。相反，我们发现有三个因素足以解释体型的渐变：寄主植物种子大小的渐变、湿度（水分）和季节性（湿度、降水量和温度的变化）。我们还发现，两性体型差异的渐变部分可由湿度梯度来解释，不过仅湿度不足以解释该渐变。其他生态或环境变量必定对雄性与雌性体型选择的差异有所影响。我们研究的主要意义在于，两性在体型渐变程度上存在差异，从而导致两性体型差异的纬度变化，而且种子甲虫的体型渐变可能受寄主种子大小、水分可利用性和季节性变化的影响。
