Projectile pollen helps this flower edge out reproductive competition

Some flowers may be using their pollinators as sexual battlegrounds.

Red, Brazilian flowers called Hypenea macrantha use projectile blasts of pollen to knock rival pollen off of hummingbirds’ beaks and replace it with their own, researchers report in a study to appear in the American Naturalist. If a male flower can send a hummingbird away with more of its own pollen and less of its competitors’, it increases its chances of siring seeds in the next female flower the bird visits.

H. macrantha flowers have both male and female reproductive organs. To avoid mating with themselves, individual flowers go through a male phase and then a female phase. They rely on hummingbirds to transfer pollen between flowers, bribing the birds with rewards of sweet nectar (SN: 3/2/15). When a hummingbird visits a flower in the male phase, its beak triggers a catapult-like mechanism that flings all the pollen from a petal-lidded compartment in a single burst. Afterward, the flower becomes female.

To see if the projectile pollen blew away the competition, evolutionary ecologist Bruce Anderson and colleagues simulated hummingbird visits by poking a hummingbird skull into flowers. They marked pollen with tiny fluorescent particles, then applied the fluorescent pollen to the part of the beak where pollen tends to accumulate. Next, they inserted the beak with its load of fluorescent pollen into a new set of male and female flowers and tracked where the marked and unmarked pollen particles ended up.

The hummingbird beaks lost twice as much fluorescent pollen when poked into explosive males versus when stuck inside inert, already-exploded flowers. Moreover, the more fluorescent pollen an explosion removed, the more successful that explosion was at depositing the flower’s own pollen onto the beak. High-speed video showed that pollen grains from exploding flowers functioned like missiles to knock existing pollen away.

“It’s almost like there’s a division of labor for pollen. Some of it is meant for mating, and some of it’s meant for fighting,” says Anderson, of Stellenbosch University in South Africa. More research is needed, he says, to find out whether the pollen blasts result in more offspring for male flowers.

The animal world is full of males trying to get rid of rivals’ sperm and replace it with their own (SN: 4/9/14). For example, many animal penises have elaborate shapes for scooping sperm out of females’ reproductive tracts. Even the capped human penis shape may have evolved to remove other men’s sperm, as evolutionary psychologist Rebecca Burch and colleagues have shown. This is the first experimental evidence for a similar sperm-removal strategy in plants.

“Plants are not just stationary objects,” says Burch, of the State University of New York at Oswego. “They engage in communication, competition and now active reproductive sabotage of other plants.”