When they feed on nectar from the Hypenia macrantha flower, an herb that grows on riverbanks in the Brazilian Cerrado (wooded savanna), hummingbirds are surprised by an eruption of pollen grains, the male reproductive particles that stick to their beaks. Transferred to a flower of the same species during the female phase, the grains can then fertilize the ovules. If, however, the bird visits another flower in the male phase, a new explosion may remove part of the pollen that was already there, increasing the chances of reproduction for the last plant visited, according to an article published in October in The American Naturalist.
“This is a rare example of male competition among plant life,” observes botanist Vinícius Brito, of the Federal University of Uberlândia (UFU), one of the article’s authors. He says the pollen is disseminated by a floral mechanism similar to a catapult, and dislodges pollen grains already in place as if they were billiard balls. To verify the efficiency of the system, the researchers colored pollen grains with quantum dots—nanoparticles that appear fluorescent under ultraviolet light, enabling their location after being transported.
Next they used a hummingbird skull to place the colored pollen on the beak, and counted the number of grains retained using a microscope. Introduced into a flower, whose pollen had not been colored, the beak activated the catapult and the researchers counted the number of colored grains still adhering to it. According to Brito, the explosion removed an average of 40% of pollen from the beak. In one flower, 75% was removed.
The speed of the eruption, measured by video, was 2.62 meters per second (m/s), close to 9.5 kilometers per hour (km/h)—the equivalent to a person running at a moderate pace. This makes the ejection of H. macrantha one of the fastest among plants. The champion’s record, however, is impressive: the white mulberry (Morus alba) casts its grains at a dramatic 170 m/s, equivalent to 612 km/h—almost double the speed of a Formula 1 race car. Brito says that pollen explosion is a relatively common mechanism in plants, but its effects on male competition are little known to date.
Daniela Calaça / UFUHummingbird skull with pollen deposits (left) and grains marked with quantum dots (right)Daniela Calaça / UFU
“This is the first demonstration that a flower can remove pollen from another and replace it with its own,” points out botanist Arthur Domingos de Melo, of the Federal University of Sergipe, who did not participate in the study. He says that botanists suspected that the pollen grains would compete for space on the bodies of pollinators, but it was difficult to test the hypothesis of a direct physical impact. “The authors found a great model for the experiments.”
Male world
“The possibility of tracking pollen with the quantum dots may reveal unknown mechanisms of the male function of plants,” suggests South African botanist Bruce Anderson, of his birth country’s Stellenbosch University, in an video-call interview for Pesquisa FAPESP. Anderson is the lead author of the article and responsible for adapting the quantum dot technology to color the pollen.
“For a century, plant reproduction studies prioritized female structures such as fruits and seeds,” recalls Anderson. As well as being more visible, it is easy to identify the origin, as they develop while adhering to the flower, whereas pollens transferred by birds, insects, or the wind, are difficult to track.
Anderson studied reproductive competition in plants storing their pollen in small pouches that stick to insect legs, such as orchids, and says that as the space on the pollinator is reduced, these sacs can stick onto the reproductive grains of other flowers, which are then put out of use. “It’s a difficult plant to study because the male structure is very small,” observes Anderson, who came to Brazil in search of a plant with bigger structures. When he touched the H. macrantha flower for the first time during a visit with Brito to Rio Preto State Park in Minas Gerais, he immediately imagined that the explosion could be an example of male competition.
Some 94% of plants have hermaphrodite flowers, many having both male and female phases. When the bird triggers the H. macrantha catapult, the flower, in the male phase, embarks upon a two-day transition to the female phase, and is then able to receive pollen from other flowers. According to Brito, as the herb has 10 flowers, some of which in the male phase, the bird may visit more than one flower in the same plant, which intensifies the competitive force. “With each flower, more pollen sticks to the beak, replacing that of previously visited plants,” he adds.
Among animals, there are many different ways to compete for sperm. Dragonflies, for example, have hooks in their genitals that remove sperm left by rivals and insert theirs into the female reproductive tract.
Another strategy is the so-called mating plug, applied by males of different groups such as bees, reptiles, and primates. After copulation, the male’s secretion coagulates and closes the female’s reproductive tract. Even if the female is able to remove this plug, the male’s sperm gains time to fertilize the ovulum.
“We tend to think that plants reproduce passively, but they wage battles similar to those of animals,” says Anderson. “Brazil has an expressive number of species that can help us understand pollen competition among plants,” emphasizes the researcher, who intends to return to Brazil for further studies soon.
The story above was published with the title “Reproductive billiards” in issue 347 of january/2025.
Scientitic article
ANDERSON, B. Pollen wars: Explosive pollination removes pollen deposited from previously visited flowers. The American Naturalist. vol. 204, no. 6. oct. 10, 2024.
