In humans, testosterone plays a key role in the development of male reproductive tissues such as testicles and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. If feminizing hormone therapy is stopped, testosterone levels will typically rise, leading to a regain in muscle mass and strength. Testosterone’s effect on protein synthesis is one of the key reasons why it is so effective at increasing muscle strength and mass and why testosterone levels are so closely related to strength and athletic performance. While natural testosterone levels significantly influence muscle protein synthesis, the effects of boosters on size are nuanced and often overstated. Feminizing hormone therapy aims to reduce testosterone levels and increase estrogen levels. Higher pre-natal testosterone indicated by a low digit ratio as well as adult testosterone levels increased risk of fouls or aggression among male players in a soccer game. One study proposed that natural selection may have caused men to be more sensitive to situations in which their status is challenged, and that testosterone is the key factor that causes these situations to spark into aggression. Studies have found higher pre-natal testosterone or lower digit ratio to be correlated with higher aggression. The second theory is similar and known as "evolutionary neuroandrogenic (ENA) theory of male aggression". It is therefore the challenge of competition among males that facilitates aggression and violence. The relationship between sex steroids and SHBG in physiological and pathological conditions is complex, as various factors may influence the levels of plasma SHBG, affecting bioavailability of testosterone. This binding plays an important role in regulating the transport, tissue delivery, bioactivity, and metabolism of testosterone. The part of the total hormone concentration that is not bound to its respective specific carrier protein is the free part. Fairer offers from test subjects with higher testosterone in the original study increase the likeliness of the offer being accepted by the negotiating partner, therefore decreasing the probability of both participants leaving without any money. In humans, testosterone appears more to promote status-seeking and social dominance than simply increasing physical aggression. Thus the link between testosterone and aggression and violence is due to these being rewarded with social status. Rats who were given anabolic steroids that increase testosterone were also more physically aggressive to provocation as a result of "threat sensitivity". It does this by enhancing protein synthesis within the muscles allowing them to heal and grow back stronger. These cells will proliferate and then fuse with the existing muscle fibers, which contributes to the overall size and strength of the muscles. This occurs when testosterone binds to androgen receptors that are located in muscle cells. This is the process by which the body builds new proteins, primarily in muscle tissue. Can testosterone boosters improve my mood and energy levels? There has been speculation that these changes in testosterone result in the temporary reduction of differences in behavior between the sexes. Women's level of testosterone is higher when measured pre-intercourse vs. pre-cuddling, as well as post-intercourse vs. post-cuddling. This reaction engages penile reflexes (such as erection and ejaculation) that aid in sperm competition when more than one male is present in mating encounters, allowing for more production of successful sperm and a higher chance of reproduction.
