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Primary Sexual Characteristics
Primary sexual characteristics are those that describe the gonads (sex organs) of the individual as being either male or female. The gonads are represented by ovaries (in females) or testes (in males). In mammals, sex is determined through the combination of X and Y chromosomes. If an egg is fertilized with a sperm carrying the X chromosome, the gonads will develop into ovaries and the offspring will be female; if the egg is fertilized with a sperm carrying the Y chromosome, the gonads will develop into testes and the offspring will be male. Some reptile species, including most turtles and all crocodilians, utilize temperature-dependent sex determination to control the sex ratios (number of males to females) of their offspring. In these species, eggs incubated within a low temperature range typically produce one sex and eggs incubated within a higher temperature range produce the other.
Secondary Sexual Characteristics
Hormones secreted through the gonads determine the development of classically male or female traits. These secondary sexual characteristics are not used in reproduction, but are apparent in most sexually dimorphic species -- species having two forms that are determined by their sex. Secondary sexual characteristics include human female breasts, male human facial hair, the mane on a male lion, and the bright, flashy plumage of many male birds and fishes.
Female Choice
The persistence of male ornamentation in animal populations is thought to be driven by female mate choice and/or male-male competition. The choosy female theory purports that females choose males with bigger, brighter and better ornamentation to increase her own genetic contribution to the population through one of two mechanisms. In the sexy sons hypothesis, the female chooses the flashy male because his ornamentation will be passed to her sons, thereby giving her sons a greater chance of reproducing and perpetuating her genes. The good genes hypothesis supposes that the female chooses the flashier male because his ornamentation may represent increased disease resistance or other fitness benefits that may be passed to her offspring.
Male-Male Competition
Some secondary sexual characteristics give a dominant male an advantage, such as the ability to overcome his opponents in physical battle, which may win that male the right to mate with a female, thus increasing his genetic contribution to the population. Because this dominant male will be able to mate with more females than less-dominant males, presumably due to his superior ornamentation (tusks, antlers, bulk), then the genes for that superior ornamentation will become prevalent in the population; in other words, that ornamentation will be naturally selected.