One possible advantage to lek formation (in Guianan cock-of-the-rock and other species) is severe selection and consequent rapid evolutionary advancement, all of which is possible due to the high expendability of males. Only a few males are needed to fertilize the next generation. The courtship behaviour is similarly theorized to have arisen from differences in division of labour between the two sexes. Females expend their energy on building nests and rearing young, while males spend most of their time and energy of finding mates and caring for their plumage.