Secondary sex characteristic

[5] 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.

In mammals, these characteristics may include increased mammary tissue in females and greater muscle mass in males.

The 1975 handicap principle extends this idea, stating that a peacock's tail, for instance, displays fitness by being a useless impediment that is very hard to fake.

[9] An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism.

Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration, and other features that have no immediate purpose for survival or combat.

The general structure and shape of the body and face, as well as sex hormone levels, are similar in preadolescent boys and girls.

As puberty begins and sex hormone levels rise, differences appear, though some changes are similar in males and females.

[23] Testosterone directly increases size and mass of muscles, vocal cords, and bones, deepening of the voice, and changing the shape of the face and skeleton.