It's not uncommon for males of various species to physically battle it out in an effort to win over females. But plants, because they can't move or fully sense their environment, don't partake in this reproductive process. Well, at least that's what we thought.
As an indelible component of sexual selection, male vertebrates often engage in physical struggles (female-on-female competition also exists). During the rutting season, for example, male deer, elk, and moose fight with each other using their horns. Bears and tigers claw it out, while giraffes duke it out with their necks. It even happens at the microscopic scale via sperm wars.
But plants? Apparently yes. Recent observations of the South American milkweed genus Oxypetalum made by Andrea Cocucci from the Instituto Multidisciplinario de Biologia Vegetal of Argentina reveal that physical competition does in fact exist between plants.
During the reproductive stage, milkweeds hook sacs of pollen grains, called pollinia, to the bodies of birds and insects, which are subsequently deposited into another flower to complete the pollination process. But occasionally the pollinia from different parents gets all tangled up with each other. This happens on account of the limited number of attachment points on the pollinator.
It's here, says Cocucci, where the milkweeds do battle — one that's waged with horns.
The pollinia sacs are equipped with horn-like structures that appear to have no obvious biological use. Cocucci theorizes that the horns are used to prevent the sacs from being hooked together with pollinia from rival parent plants.
Remarkably, it's the first evidence of male-male physical struggle in plants through the acquisition of biologically evolved weapons.
As Cocucci concludes in his study:
Here we further suggest and provide evidence that plants may have evolved weapons of the sort known in animals as means of response to these struggles. Because the horns on the pollinaria of milkweeds are engaged in male physical struggles, they represent the botanical equivalents of the horns of many male vertebrates, such as bucks and insects.
Read the entire study at New Phytologist: "The buck in the milkweed: evidence of male–male interference among pollinaria on pollinators."