Why do bumblebee workers vary in size?

Abstract

This thesis investigates size variation in the worker caste of social bumblebees (Bombus spp.) by examining various morphological and behavioural characteristics in relation to body size. I confirm size variation in the worker castes of 23 Bombus species and that foragers are, on average, larger than in-nest bees and that workers of pocket-making species vary more than pollen-storers. It is believed that larger bees are suited to foraging tasks and small bees to in-nest tasks, large bees being able to collect more forage per trip. I confirm that larger workers gather nectar more efficiently. Workers show behavioural plasticity: smaller workers occasionally forage and larger bees often perform in-nest tasks and are superior brood carers. By manipulating the worker size distributions in artificial nests, I show that a workforce of large bees is superior to one of the small workers in terms of colony growth. Larger workers are supposedly better adapted to colder conditions than smaller workers. I show that bumblebees from cold regions have longer thoracic setae than those from hot regions and that, within species, foraging workers of races from cooler regions are often found to be larger than those from warmer regions. I also show that, in southern  England, ambient temperature has no influence on nectar foraging efficiency or on the size of forager, but that pollen foraging trips are made in warmer conditions. Bumblebees obtain from flowers. I show that different plant species attract foragers of different mean size, that larger bees have longer tongues and handle deeper flowers more quickly and shallower flowers more slowly than smaller workers. That size variation enables a colony to efficiently harvest nectar and pollen from a variety of floral resources is the most encouraging adaptive explanation for size variation in bumblebee workers thus far.

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