When a man becomes sexually excited , the nerves surrounding his penis become active, causing the muscles around the arteries to relax and more blood to flow into the penis. The spongelike material then absorbs the additional blood, making the penis stiff and hard, or erect. This erection tightens the veins so the blood can’t leave the penis, enabling the penis to remain erect. After a man ejaculates or if his arousal fades, detumescence occurs, in which the brain sends a signal to allow the blood to leave the erect penis, and it returns to its flaccid state.
2) Irreversible adaptation to sperm competition . It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster ) for sperm competition and thus had to place its testes outside the body.  This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo , a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.
since 1916, so if it were shrinking penises and testicles, we’d probably have heard something more than rumors about it by now. (The FDA’s established acceptable daily intake [ADI] for tartrazine is milligrams per kilogram of body weight per day, which is about .01 ounces for a 132-pound person.) Many other common food products contain Yellow No. 5 as well, so more than just Mountain Dew drinkers would notice the effect if it were real. (Soft drinks don’t contain a significantly larger amount of Yellow No. 5 than other food