The two testes are located in the scrotum, which hangs between the legs behind the penis. Most of the testes is devoted to sperm production, but the remaining cells, called Leydig cells, produce testosterone. Testosterone caries out two very important endocrine tasks in males: it facilitates sexual maturation, and it enables sperm to mature to a reproductively-competent form. Healthy men remain capable of fertilizing an egg throughout their post-pubertal life. However, testosterone levels do show a gradual decline after about the age of 40 with a total drop of around 20% by age 80.
Testosterone also has the important endocrine function of providing sexual desire in both men and women. Although reproduction can occur without maximal desire, this added incentive increases the likelihood of reproduction. Human sexual behavior is also influenced by several other factors including thoughts and beliefs.
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