Returning to the question of whether the human scrotum is half-empty or half-full, the very existence of the external human scrotum suggests sperm competition in human evolution. Gorillas and gibbons, like most other mammals that don’t engage in sperm competition, generally aren’t equipped for it.
A scrotum is like a spare refrigerator in the garage just for beer. If you’ve got a spare beer fridge, you’re probably the type who expects a party to break out at any moment. You want to be prepared. A scrotum fulfills the same function. By keeping the testicles a few degrees cooler than they would be inside the body, a scrotum allows chilled spermatozoa to accumulate and remain viable longer, available if needed.
Anyone who’s been kicked in the beer fridge can tell you this is a potentially costly arrangement. The increased vulnerability of having testes out there in the wind inviting attack or accident rather than tucked away safely inside the body is hard to overstate—especially if you’re crumpled in the fetal position, unable to breathe. Given the unrelenting logic of evolutionary cost/benefit analyses, we can be quite certain this is not an adaptation without good reason.4 Why carry the tools if you don’t have the job?
There is compelling evidence pointing to a dramatic reduction in human sperm production and testicular volume in recent times. Researchers have documented worrisome decreases in average sperm count as well as reduced vitality of the sperm that do survive. One researcher suggests that average sperm counts in Danish men have plummeted from 113 x 106 in 1940 to about half that in 1990 (66 x 106).5 The list of potential causes for the collapse is long, ranging from estrogen-like compounds in soybeans and the milk of pregnant cows to pesticides, fertilizers, growth hormones in cattle, and chemicals used in plastics. Recent research suggests the widely prescribed antidepressant paroxetine (sold as Seroxat or Paxil) may damage DNA in sperm cells.6 The Human Reproduction study at the University of Rochester found that men whose mothers had eaten beef more than seven times per week while pregnant were three times more likely to be classified as subfertile (fewer than twenty million sperm per milliliter of seminal fluid). Among these sons of beef eaters, the rate of subfertility was 17.7 percent, as opposed to 5.7 percent among men whose mothers ate beef less often.
Humans seem to have much more sperm-producing tissue than any monogamous or polygynous primate would need. Men produce only about one-third to one-eighth as much sperm per gram of spermato-genic tissue as eight other mammals tested.7 Researchers have noted similar surplus capacities in other aspects of human sperm and semen-production physiology.8
The correlation between infrequent ejaculation and various health problems offers further evidence that present-day men are not using their reproductive equipment to its fullest potential. A team of Australian researchers, for example, found that men who had ejaculated more than five times per week between the ages of twenty and fifty were one-third less likely to develop prostate cancer later in life.9 Along with the fructose, potassium, zinc, and other benign components of semen, trace amounts of carcinogens are often present, so researchers hypothesize that the reduction in cancer rates may be due to the frequent flushing of the ducts.
A different team from Sydney University reported in late 2007 that daily ejaculation dramatically reduced DNA damage to men’s sperm cells, thereby
Frequent orgasm is associated with better cardiac health as well. A study conducted at the University of Bristol and Queen’s University of Belfast found that men who have three or more orgasms per week are 50 percent less likely to die from coronary heart disease.11
