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. 
On that and the next day he went about his normal activities – working and driving his car. On the third post-operative day the patient experienced pain so acute that he found it difficult to walk. His testicle was swollen and bluish in colour. He consulted his GP who diagnosed an infection and prescribed antibiotics. The following day he was seen by the surgeon who advised the patient to continue taking the antibiotics. The patient's testicle remained swollen and acutely painful for two months, then began to shrink. Five months after the surgery it had reduced to about half the size of the left testicle and was hard, smooth and insensitive.