Handicap theory posits that if a female had to choose between two suitors who had beaten out all other competitors, but one had done so with a hand tied behind his back, she’d go for him because he’s obviously tougher still. It is controversial, but it does offer explanations for a number of problematic biological phenomena, such as male birds’ colorful plumage and songs that should attract predators. If the handicap theory is right, the scrotum exists to let its possessor say, “I’m so able to look after myself, I can keep these on the outside!”
In the mid-1990s, Michael Chance, a professor of animal behavior at the U.K.'s University of Birmingham, came across a newspaper story about the Oxford-Cambridge University boat race that piqued his interest in testicles. He learned that after the race, the rowers’ urine contained fluid from their prostates.
The oarsmen's exertions, the cyclic abdominal straining, had deposited prostatic fluid in their urethras because there are no sphincters in the reproductive tract. Without such valves, squeezing of any of the sacs and tubes that make up this system is liable to empty it, or at least rearrange its contents. In 1996, in what has become known as the galloping hypothesis, Chance argued that externalization of the testes was necessary when mammals started to move in ways that sharply increased abdominal pressure.
A survey of how mammals move reveals a good deal of variety. And when Chance listed animals with internal testicles, he didn't find many gallopers. The elephants, aardvarks, and their cousins on the undescended branch of the mammalian tree don't bound or jump around. On the other side, the creatures such as moles and hedgehogs that reabsorbed their sexual cargo seem to have evolved away from internally disruptive types of movement. Among mammals that have returned to the sea, the few that have retained scrotums are the only ones who breed on land, such as elephant seals, who fight vigorously to defend their territory during rutting season.
One might argue that evolution could surely have thrown in a sphincter or two, or some internal shielding, but besides the possibility that the mechanics of ejaculation would struggle with such things, another argument supports Chance’s thinking. In 1991 Roland Frey of Germany's Freiburg University reported a number of features of blood vessels of scrotal testes that ensure more constant pressure, possibly to avoid impaired blood drainage during galloping. The specific adaptations are different between marsupials and the rest of us but seem aimed at the same goal.
The galloping hypothesis would be a case of evolutionary compromise—the dangers of scrotality being a necessary price for the greater advantages of a new and valuable type of movement.
There are many theories in evolutionary biology. Often there's great pleasure in the detectivelike process of piecing together the available, incomplete evidence into a coherent story, but the big challenge for this science is actually testing these ideas. One exciting recent development that might provide relevant evolutionary data has been the identification of the signal that controls the testicles’ initial descent from the kidney region to the undercarriage.
When the testes and ovaries are young, they are held in place by the so-called cranial suspensory ligament, while holding on loosely is a second, measly ligament termed the gubernaculum. To begin their roller-coaster ride, testicles secrete a signal that causes the suspensory ligament to degenerate and the gubernaculum to grow capable of dragging them to the base of the abdomen.
To study the evolution of this signal, a molecule related to insulin, Teddy Hsu and colleagues at Stanford University turned to the duck-billed platypus. They found that the platypus has a single gene for the prototype version of the signal, and that it was this gene's duplication in subsequent mammals that allowed one version to evolve a function in testicular descent and the other in nipple development.
It’s a beautiful example of a genetic event in biological history that produced mammalian specialization. However, elephants and their nondescended cousins all have the duplicated genes, so the story's not complete. A crucial next step will be determining the genes required for forming the inguinal canal and making the scrotum. Probably the best place to look will be in those mammals that have backtracked on externalization, where these genes have likely changed.
It's rather humbling to realize that this basic aspect of our bodies remains a mystery. The fact that such a ridiculous appendage evolved twice surely means we should be able to get a handle on it. A successful theory will have to explain the full diversity of mammalian testicle positions, not just the scrotum’s existence. I like Chance and Frey's galloping hypothesis, but could a scrotum really be the only way to deal with undulating abdominal pressure? In addition, do scrotal sperm really differ fundamentally from internally generated tiddlers? Can we definitively prove temperature sensitivity evolved after the expulsion of the scrotum? And signaling is still an outside bet, but if scrotums were really sexually selected, where's the mammalian peacock, some species toting a pair of soccer balls?
Talking of which, while we wait for a final answer, the scrotality totality, us soccer goalkeepers should probably look to our baseball-playing friends who use evolution's gift of a large brain and opposable thumbs to don a protective cup.
Correction, July 9, 2013: This article originally misspelled the name of the vervet monkey.