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.
During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass. This condition is called " sarcopenia ", and may be distinct from atrophy in its pathophysiology. While the exact cause of sarcopenia is unknown, it may be induced by a combination of a gradual failure in the " satellite cells " which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival. 
The skin of the penis is an ideal environment for bacteria to multiply, but the amount of bacteria depends on whether the man is circumcised or not. A study in the journal American Society for Microbiology found there were fewer kinds of bacteria on the penises of a group of men who got circumcised, compared to the samples taken when they were uncircumcised. The Centers for Disease Control and Prevention notes several studies found circumcision changes the bacteria ecology of the penis, which may explain why snipping the foreskin reduces the risk of HIV infection. The theory behind this is that anaerobic bacteria — bacteria that do not live or grow in the presence of oxygen — could cause the immune system to react in a way that makes cells more vulnerable to HIV infection in uncircumcised penises.