Fur versus feathers: Pollen delivery by bats and hummingbirds, and consequences for pollen production

Background/Question/Methods - One floral characteristic associated with bat pollination (chiropterophily) is copious pollen production, a pattern that we confirmed in a local comparison of hummingbird- and bat-adapted flowers from a cloud-forest site in Ecuador.  Previous authors have suggested that wasteful transfer of pollen by bats accounted for the pattern.  We suggest that bats are actually efficient pollinators, and can transfer larger amounts of pollen than other pollinators.  This leads to intense male-male competition between bat-flowers, which selects for increases in pollen production.  To test this hypothesis, we measured pollen removal and delivery for bats and hummingbirds in flight cage experiments with artificial and real flowers.Results/Conclusions - Results support our hypothesis: pollen delivery to stigmas by birds rapidly saturates as a function of pollen removal from anthers, while the function for bats in equivalent circumstances saturates much more slowly.  Bat flowers therefore experience little diminishing of fitness returns on high pollen production, and increases in pollen production will be more linearly related to realized male reproductive success.   Our results also provide further evidence against previous hypotheses, given that the bats transferred pollen more efficiently than the hummingbirds.  We speculate that furred surfaces take up and hold more pollen grains than feathered surfaces, which seem to readily shed excess grains.