The buck in the milkweed: Evidence of male-male interference among pollinaria on pollinators

Abstract

Direct physical confrontation among conspecifics for access to mates is a form of sexual selection well known among animals, but not thought to take place in plants. Consequently, no structures are known that can be considered weapons evolved under such confrontation. Pollinaria of milkweeds may physically compete for access to attachment points on the pollinators? body, and occasionally pollinaria may link onto pre-existing pollinaria on a pollinator resulting in concatenation. We tested the hypotheses that concatenation may result in interference between proximal and distal pollinaria and that features of non-concatenating pollinaria might be attributed to prevention of concatenation, by analyzing pollen donation efficiency, experimental manipulation of the phenotype, and the phylogenetic patterns of co-occurrence of traits.It is shown that concatenation was able to diminish the reproductive performance of proximal pollinaria, that horns on pollinaria prevented concatenation, and horn acquisition was correlated with a loss of concatenation. The experimental removal of horns in species that did not concatenate reverted to concatenation. The present could be the first evidence of male body-body struggles and of the acquisition of weapons related to these struggles in the fashion that are known in animals.