Sex Hormones

Sex hormones are certain types of chemical substances that prepare an animal's body for reproduction. They are secreted by a gland or an organ. Sex hormones determine both male and female sexual characteristics and can also influence a person's behavior.

Hormones are chemical messengers in both animals and plants. In animals, they are produced by glands. Hormones travel through the blood to target tissues where the hormones act as chemical regulators. Hormones influence reproduction, growth, and overall bodily balance, among other things. Sex hormones are important to all animals, but are especially so to vertebrates (animals with a backbone). In humans, the sex hormones—androgens, estrogens, and progestins—are essential for those body processes that are related to reproduction.

Although scientists have only begun to understand sex hormones in the last fifty years, their effects have long been known and recognized. Farmers have known for millennia that castration (or removing a male animal's testes; the sperm-making organs) not only makes these animals more manageable but improves the quality of their meat. In Renaissance times (fourteenth to sixteenth century), boys in the church choir were sometimes castrated to keep their beautiful high voices from changing.

THE PITUITARY GLAND

In humans, the glands that produce hormones involved with sex and reproduction are the pituitary gland, the gonads, and the adrenal glands. The pituitary gland is a small organ below or at the base of the brain. It is about the size of a pea, yet it has a powerful effect on several other glands in the body. The pituitary gland releases eight different hormones. These stimulate other parts or organs in the body, including the gonads. For example, the pituitary gland's follicle-stimulating hormone (FSH) promotes the production of sex cells in males (sperm) and the maturing of sex cells in women (ovaries). The prolactin produced by the pituitary gland gets the female body ready to produce milk when needed.

THE GONADS

The gonads consist of the testes in males and the ovaries in females. These usually come in pairs. The ovaries produce egg cells or ova as well as the hormones known as the estrogens and the progestins. These hormones are usually produced in regular cycles and control female sexual development, thus triggering female secondary sexual characteristics like breasts. The hormones also prepare the body once pregnancy occurs and help ensure that it can sustain the developing fetus (the human embryo).

THE ADRENAL

The main estrogens are estradiol, estrone, and estriol. The main progestin is called progesterone. There are also several kinds of androgens (or male sex hormones) produced by the gonads, but the primary one is testosterone. This hormone stimulates hair growth and the lowering of the voices when a male goes through puberty. The adrenal glands also come in pairs in humans, and their name comes from being located so close to the kidneys. Only the outer part, called the cortex, secretes sex hormones, since it is made up of tissue that is similar to that found in the ovaries and testes.

THE IMPORTANCE OF SEX HORMONES

Sex hormones are important throughout life. They come into play at times other than sexual maturity. These hormones are working from the time of early development to influence the developing fetus. For example, testosterone begins to work before a baby is born by stimulating the growth of male genitals. Like all sex hormones, it also influences the development of other organs that are not directly related to reproduction. Later in life, the production of female and male sex hormones is gradually reduced with aging. Major changes can occur as a woman no longer produces eggs that can be fertilized.

Sex hormones also influence behavior. This is most noticeable in animals. Since sexual reproduction is mostly a joint, or cooperative, effort in which each partner supplies half of the needed ingredients (egg and sperm), many animals go through cycles during which they are receptive to mating. With the exception of humans and a few other mammals, most animals go through only one or two estrous cycles (called "heat") during which their hormones motivate them to reproduce.

Sex hormones also influence animals to do all the necessary things to care for their young. These include nest-building, nursing, and feeding. In males especially, sex hormones account for much of the aggressive behavior displayed during the mating season. Studies of rhesus monkeys have shown that those with the highest levels of testosterone in their blood are the ones doing all of the threatening, chasing, and fighting. These monkeys are dominant, or usually near the top of their social order. If the goal of all living things is to reproduce, then sex hormones are the key to all life. These hormones guide the reproductive process, and affect bodily changes as well as behavior.