The testicles (singular: testis) are the internal sex organ of most sexually reproducing organisms. Sperm cells are produced in the testicles, which then form male sperm in humans together with the seminal fluid of the prostate.
In men, the testicles are also responsible for the production of the male sex hormone testosterone. A loss or serious illness of the testicles also results in a complete or partial loss of primary male characteristics (higher voice, reduced body hair, reduced sex drive, etc.) due to the lack of hormone production.
In contrast to women, the gonads in a man's testicles do not suddenly stop functioning when he reaches a certain age. In principle and in theory, a man can therefore remain fertile and sexually active throughout his life and into old age. Infertility (infertility - not to be confused with impotence or erectile dysfunction) that occurs with age varies from person to person and the exact causes have not yet been fully clarified.
