In a study published this week in The American Naturalist, scientists from South Africa and Brazil have provided the first empirical evidence that rival plants may engage in direct competition for space on pollinators, influencing which pollen makes it to the next flower.
The study suggests that plants have evolved strategies to manipulate pollen placement on pollinators, drawing parallels with sperm competition in animals.
This is particularly notable because, unlike animals, plants do not engage in direct contact when mating. Instead, pollen (which carries plant sperm) is transported by pollinators such as birds or insects, making competition for prime space on pollinators’ bodies crucial to reproductive success.
Professor Bruce Anderson, an evolutionary ecologist at Stellenbosch University and lead author of the study, said of the findings: “Flowers visited by hummingbirds deposit their pollen on the birds’ bills, but space is limited. Some flowers have evolved a catapult mechanism to shoot their pollen at the hummingbird’s bill, forcefully dislodging grains from rival plants and ensuring their own pollen has a better chance of successful transfer.”
This mechanism, known as explosive pollen placement, was previously known to exist, but the study provides the first quantitative evidence of its effectiveness.
The researchers used slow-motion video footage and quantum dot-labelled pollen to observe how Hypenea macrantha, a deep red flower native to Brazil, employs the catapult-like mechanism to remove rival pollen from a hummingbird’s bill and place its own pollen in the same spot.
The results revealed a significant boost in reproductive success for plants employing this technique.
Anderson pointed out that while male-male competition is well-documented in the animal kingdom, the study highlights similar competitive behaviour among plants: “We now know that plants can manipulate pollinators to compete with rival pollen grains, which adds a new dimension to how we understand plant reproduction.”
Professor Vinícius Brito, a botanist at the Federal University of Uberlândia in Brazil and co-author of the study, said that floral explosions were previously thought to mainly facilitate pollen placement or encourage pollinators to visit more flowers. However, their research suggests the mechanism also displaces rival pollen, enhancing male reproductive success.
“By displacing previously deposited pollen, flowers increase their chances of successfully fertilising ovules,” Brito said.
The study’s findings open up new avenues of research into plant-pollinator interactions and the evolutionary strategies plants have developed to improve reproductive success. It also highlights how little is known about the competitive dynamics between plants during pollination.
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