Any difference, morphological or behavioral, between males and females of the same species. In many animals, the sex of an individual can be determined at a glance. For example, roosters have bright plumage, a large comb, and an elaborate tail, all of which are lacking in hens. Sexual dimorphism arises as a result of the different reproductive functions of the two sexes and is a consequence of both natural selection and sexual selection. Primary differences, such as the structure of the reproductive organs, are driven by natural selection and are key to the individual's function as a mother or father. Other differences such as the peacock's (Pavo cristatus) enormous tail are driven by sexual selection and increase the individual's success in acquiring mates.