BORRELL, Brendan J.; Univ. of California, Berkeley: Optimality and Allometry of Nectar Foraging in the Orchid Bees

Numerous insects fuel their daily activities by consuming floral nectars, and they imbibe these viscous fluids through a slender, tubular proboscis. Whereas the diverse shapes and sizes of their mouthparts represent evolutionary outcomes of historical contingency and ecological adaptation, the function of these structures is governed by a few biophysical principles. Theoretical models employing these principles suggest that optimal nectar sugar concentrations (NSC) for suction feeders should fall between 30-40% sucrose and be independent of body size. In order to test these predictions, I studied the scaling of nectar intake rate in 25 species of Central American orchid bees (Apidae: Euglossini) ranging in body mass from 50 mg to 800 mg. Regression slopes of species means for nectar intake rate at 35% and 55% sucrose are not significantly different from those predicted by isometric scaling models (0.55 and 0.44, respectively), suggesting that optimal NSC is independent of body size. A field study at three lowland and three montane sites in Costa Rica supports these findings. Orchid bees harvest nectars between 34-42% sugar, independent of body size. Within this narrow range, interspecific differences in NSC may be related to factors such as habitat use or floral access.