Milkweeds have developed horns on their sacs of pollen grains that appear to be used to fight off pollen from other plants of the same species, in a manner analogous to animals fighting it out for a chance to mate.
While the nature of the animal kingdom is red in tooth and claw, plants seem to lead an altogether more conflict-free lifestyle, parasites and herbivores aside. It's true getting in early with production, along with the volume of seed produced, can matter. Likewise anything that causes pollen to travel to the right place will help, but the contest is usually thought of more as a race than a battle.
However, it seems that when it comes to passing on one's genes things can be vicious, if in a rather more passive-aggressive manner than deer slamming their antlers against each other for the right to inseminate potential partners.
Milkweed produce sacs of pollen grains. These hook onto birds and insects to be deposited on other flowers. At least that is the idea. However, sometimes the carriers get a bit promiscuous in their flitting from plant to plant, collecting so many that the sacs get tangled up in each other. In such a circumstance it appears the outermost sac is best placed to pollinate a flower, while the first one to have been collected by the insect is at a severe disadvantage.
In the New Phytologist Dr Andrea A Cocucci of the Argentine Instituto Multidisciplinario de Biología Vegetal proposes that the horn-like structures on the sacs of the genus Oxypetalum that seem to serve no other purpose are there to prevent additional sacs connecting to the first onboard. If true, “The present work could be the first evidence of male physical struggles and of the acquisition of weapons related to these struggles, that are analogous to those known in animals,” Cocucci says.
To back up the idea, Cocucci first demonstrated that this tangling tends to reduce mating success (at least in two of the four species he studied), and then experimented with what happens if the horns are removed. He found that pollen without horns was more likely to end up serving as a carrier for rivals, and therefore less likely to inseminate a female flower.
Some species of Oxypetalum don't produce horns and Cocucci thinks this is because when insects are mostly visiting flowers from a single plant on a journey it is beneficial for sacs to be tangled together, “but as soon as interactions turn negative, the selection would favor the acquisition of horns.”
Of course in nature the development of any weapon generates a response, and milkweed species with horns also have clasping structures to enable sacs to get past this defense. The fact that milkweeds are hermaphrodites only adds to the irony of the male flowers building weapons and counter-weapons against each other to fertilize the females.
“Neither self-propulsion nor well-developed sensory perception are required for sexual selection to take place through intrasexual struggles,” Cocucci says. “Apparently, only physical contact is enough to influence the reproductive success of competitors and to promote the evolution of defensive and attack weaponry.”