Stamen dimorphism in bird-pollinated flowers – investigating alternative hypotheses on the evolution of heteranthery

Heteranthery, the presence of distinct stamen types within a flower, is
commonly explained as functional adaptation to alleviate the ‘pollen
dilemma’, defined as the dual and conflicting function of pollen as
pollinator food resource and male reproductive agent. A single primary
hypothesis, ‘division of labour’, has been central in studies on
heteranthery. This hypothesis postulates that one stamen type functions in
rewarding pollen-collecting pollinators and the other in reproduction,
thereby minimizing pollen loss. Only recently, alternative functions (i.e.
staggered pollen release), were proposed, but comparative and experimental
investigations are lagging behind. Here, we use 63 species of the tribe
Merianieae (Melastomataceae) to demonstrate that, against theory,
heteranthery occurs in flowers offering rewards other than pollen, such as
staminal food bodies or nectar. Although shifts in reward type released
species from the ‘pollen dilemma’, heteranthery has evolved repeatedly de
novo in food-body-rewarding, passerine-pollinated flowers. We use field
investigations to show that foraging passerines discriminated between
stamen types and removed large stamens more quickly than small stamens.
Passerines removed small stamens on separate visits towards the end of
flower anthesis. We propose that the staggered increase in nutritive
content of small stamens functions to increase chances for outcross-pollen
transfer.