Foraging Strategies of 'Amegilla anomola' at Flowers of 'Melastoma affine': no Evidence for Separate Feeding and Pollinating Anthers

Abstract

Heteranthery in flowers with poricidal anthers has often been explained by the division of labour hypothesis, i.e. feeding and pollinating anthers. We examined whether or not there was any evidence to support the division of labour hypothesis in 'Melastoma affine', a pioneer shrub with large nectarless flowers that bear ten dimorphic stamens with poricidal anthers. 'Melastoma affine' requires the services of bees capable of buzz pollination in order to set fruit. 'Amegilla anomola' (Anthophoridae) is an important pollinator of Melastoma affine (Melastomataceae) in the rainforest margins of North Queensland, Australia. We examined individual foraging behaviour at flowers and determined that most bees were strict specialists during any one foraging bout, mostly buzzing only short or only long stamens with the latter being the preferred stamen choice. A significant association between stamen-type preference and stigma contact was observed. Bees visiting short stamens contacted stigmas more often than bees that only buzzed long stamens. The number of times an anther was buzzed on a single visit in order to extract pollen was directly associated with stamen type with long stamens being buzzed more often than short stamens. Sub-sampling of pollen in anthers showed that long stamens contained more pollen than short stamens. To examine the impact of this differential foraging behaviour on fruit-set we removed stamens from virgin flowers. When 'A. anomola' visited flowers in which all of the long stamens were removed there was no significant reduction in fruit-set. When all of the short stamens were removed fruit-set was significantly lower than in the control flowers (no stamens removed). In spite of their preference for longer stamens, the bees are still important pollinators of this species because in many individual foraging bouts bees collect pollen from shorter stamens during which they usually contact the stigma. We did not find evidence to support the division of labour hypothesis in 'Melastoma affine'. The relative importance of short and long anthers for outcrossing and the benefits of dimorphic stamens to plant fitness in situations where the suite of pollinators varies over time is discussed.