Within the same geographic region, hummingbird clades co-evolved with nectar-bearing plant clades, affecting mechanisms of pollination. The same is true for the sword-billed hummingbird ("Ensifera ensifera"), one of the morphologically most extreme species, and one of its main food plant clades ("Passiflora" section "Tacsonia"). Hummingbirds exhibit sexual size dimorphism according to Rensch's rule, in which males are smaller than females in small species, and males are larger than females in large-bodied species. The extent of this sexual size difference varies among clades of hummingbirds. For example, the Mellisugini clade exhibits a large size dimorphism, with females being larger than males. Conversely, the Lophomithini clade displays very little size dimorphism; males and females are similar in size. Sexual dimorphisms in bill size and shape are also present between male and female hummingbirds, where in many clades, females have longer, more curved bills favored for accessing nectar from tall flowers. For males and females of the same size, females will tend to have larger bills. Sexual size and bill differences likely evolved due to constraints imposed by courtship because mating displays of male hummingbirds require complex aerial maneuvers. Males tend to be smaller than females, allowing conservation of energy to forage competitively and participate more frequently in courtship. Thus, sexual selection will favor smaller male hummingbirds. Female hummingbirds tend to be larger, requiring more energy, with longer beaks that allow for more effective reach into crevices of tall flowers for nectar. Thus females are better at foraging, acquiring flower nectar, and supporting the energy demands of their larger body size. Directional selection will thus favor the larger hummingbirds in terms of acquiring food. Another evolutionary cause of this sexual bill dimorphism is that the selective forces from competition for nectar between the sexes of each species are what drive the sexual dimorphism. Depending on which sex holds territory in the species, it is advantageous for the other sex to have a longer bill and be able to feed on a wide variety of flowers, decreasing intraspecific competition. For example, in species of hummingbirds where males have longer bills, males do not hold a specific territory and have a lek mating system. In species where males have shorter bills than females, males defend their resources and therefore females must have a longer bill in order to feed from a broader range of flowers.