Scaling of nectar foraging in orchid bees

Morphology influences the rate at which foraging bees visit nectar flowers, the quantity of nectar they must consume to fuel their activities, and, consequently, the profitability of flower species. Because feeding time is a major determinant of visitation rate, I used a biomechanical model to examine how energy intake rate (E) varies with sucrose concentration, body mass (M), and proboscis length in orchid bees ( Apidae: Euglossini). Under geometric scaling, the optimal sugar concentration (S-max) should be largely independent of body size, and E proportional to M-1.0. In a comparative study of 30 orchid bee species ranging from 50 to 800 mg, S-max fell between 35% and 40% w/w, but E proportional to M-0.54, significantly less than model predictions. Proboscis length and radius scale geometrically with body mass, but proboscis length exhibits substantial size-independent variation, particularly in small bees. One cost of a long proboscis is a reduction in both E and S-max in accordance with the scaling model. This finding highlights a difference between the lapping mechanism used by bumblebees and the suction mechanism used by orchid bees. A field study confirms that orchid bees harvest nectars with between 34% and 42% sucrose, independent of body size.