Geographic Variation in Bumblebee Flight Morphology Suggests Aerodynamic Limitations on Upslope Range Shifts

Parsons, Zach
Geographic ranges of diverse species have shifted poleward and up mountain slopes in response to warming climate. Upslope shifts facilitate tracking of climatic niches over short geographic distances. However, flying organisms moving upslope encounter a novel challenge – reduced air density limiting force production of wings. Whether flying insects may overcome this limitation to upslope shifts through changing flight morphology is largely unknown. Recent studies reveal climate-driven northward and upslope range shifts for dozens of bumblebee (genus Bombus) species, which thrive in diverse habitats from sea level to over 5000 m elevation. We measured geographic variation in flight morphology of two Western North American bumblebees (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. We measured body mass of field-caught foragers before and after emptying the crop, and wing area by analyzing images of clipped wings of over 1500 individual bees. The alpine specialist, B. bifarius was smaller overall, with field mass increasing significantly with latitude and altitude. The larger species, B. vosnesenskii, showed no change in body size with latitude or altitude. Wing loading (body mass relative to wing area) changed little with latitude but decreased strongly with altitude, particularly for B. bifarius, a species more common in alpine areas. Relatively high wing loading and limited changes in wing loading with altitude for B. vosnesnskii may limit the ability of this species to move upslope to track climate change.
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