Meeting Abstract
Body size clines and their underlying drivers have long fascinated ecologists. After finding consistent increases in body size with latitude among endothermic species, Bergmann posited that the pattern was driven by heat conservation (larger organisms have relatively less surface area). This ecological rule has since been extended to non-endotherms and to intraspecific comparisons. Insects show both increases and decreases in size across latitude and altitude, with multiple explanations proposed for these contrasting patterns. As ectotherms, heat loss may not be the primary selective mechanism; instead, the temperature size rule, seasonal constraints on development time, and reproductive and energetic benefits of large body size may more strongly determine geographic variation in morphology. Heterothermic bumble bees (genus Bombus) are broadly distributed across latitude and altitude and develop in temperature-regulated nests, making them a unique intermediate test case for insect size clines. We measured geographic variation in body and wing morphology of two bumble bee species (Bombus vosnesenskii and Bombus bifarius) collected across multiple years and seasons from 36 to 48 °N latitude and from sea level to over 2900 m in elevation (>1700 individuals at 115 sites). B. vosnesenskii is broadly distributed but less common at higher elevations whereas B. bifarius is found only at mid to high elevations. Comparisons of inter- and intraspecific variation in morphology across latitude and altitude suggest that facultative endothermy may diminish the importance of heat conservation in driving size clines, that reduced air density at altitude may impose aerodynamic constraints on body size, and that climatic niche may alter clinal morphological variation.
