The choice that is not a function of testosterone is that they stimulate mammary gland development. Testosterone is the male sexual hormone that regulates sexual development, muscle mass, and red blood cell production. Testosterone is responsible for the development of male sexual characteristics. It is a type of androgen produced mainly by the testicles in the Leyding cells. The hormone regulates a number of functions alongside sperm production in males; include; bone mass, fat distribution, muscle size and strength and the production of red blood cell.