Secondary sex characteristics are the physical characteristics of an organism that are dependent on its sex, but not directly part of its reproductive system. In humans, many of these characteristics start to appear during puberty. In animals, many start to appear at sexual maturity. Secondary sex characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species. In evolution, secondary sex characteristics are the product of sexual selection for traits that show fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions. Secondary sex characteristics include, for example, the manes of male lions, the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. These characteristics are believed to be produced by a positive feedback loop known as the Fisherian runaway produced by the secondary characteristic in one sex and the desire for that characteristic in the other sex. Male birds and fish of many species have brighter coloration or other external ornaments. Differences in size between sexes are also considered secondary sexual characteristics. In humans, visible secondary sex characteristics include enlarged breasts and widened hips of females, facial hair and Adam's apples on males, and pubic hair on both. Secondary Sex Characteristics vs. Primary Sex Characteristics The reproductive organs in male or female organisms are usually identifiable at birth and are ascribed as the Primary Somatic Sex Characteristics. In the male, this would be the penis, scrotum, and the ability to produce sperm that will help form a zygote. In the female, this would be the uterus, vagina, fallopian tubes, clitoris, cervix, and the ability to have offspring. The primary sex organs are different from the secondary sex organs because they produce gametes, which is a mature haploid germ cell male or female which will unite with another of the opposite sex during sexual reproduction to form a zygote. The secondary sex characteristics differ in that they will not be identifiable at birth, they will develop over time as the subject matures and becomes sexually active. Those characteristics are breast in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding. In the animal kingdom, an extraordinary diversity of structures exists that cannot be explained by natural selection (Darwin 1871). Evolutionary roots In The Descent of Man and Selection in Relation to Sex, Charles