Can alloethism in workers of the bumblebee, Bombus terrestris, be explained in terms of foraging efficiency?

Abstract

Bumblebee workers vary greatly in size, unlike workers of most other social bees. This variability has not been adequately explained. In many social insects, size variation is adaptive, with different-sized workers performing different tasks (alloethism). Here we established whether workers of the bumblebee, Bombus terrestris (L.) (Hymenoptera; Apidae), exhibit alloethism. We quantified the size of workers engaging in foraging compared to those that remain in the nest, and confirmed that it is the larger bees that tend to forage (X±SE thorax widths 4.34±0.01 mm for nest bees and 4.93±0.02 mm for foragers). We then investigated whether large bees are better suited to foraging because they are able to transport heavier loads of food back to the nest. Both pollen and nectar loads of returning foragers were measured, demonstrating that larger bees do return with a heavier mass of forage. Foraging trip times were inversely related to bee size when collecting nectar, but were unrelated to bee size for bees collecting pollen. Overall, large bees brought back more nectar per unit time than small bees, but the rate of pollen collection appeared to be unrelated to size. The smallest foragers had a nectar foraging rate close to zero, presumably explaining why foragers tend to be large. Why might larger bees be better at foraging? Various explanations are considered: larger bees are able to forage in cooler conditions, may be able to forage over larger distances, and are perhaps also less vulnerable to predation. Conversely, small workers are presumably cheaper to produce and may be more nimble at within-nest tasks. Further research is needed to assess these possibilities.