Secondary sex characteristics are features that appear at sexual maturity in animals and during puberty in humans, especially the sexually dimorphic phenotypic traits that distinguish the two sexes of a species (male and female), but that, unlike the sex organs, are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which display fitness, giving an individual an advantage over its rivals in courtship and aggressive interactions. They are distinguished from the primary sex characteristics — the sex organs — which are directly necessary for sexual reproduction to occur.