Why some mammal species don’t have descended testicles, but most do

Scientists have long wondered what the earliest mammals’ balls were like. After all, a few species today live with theirs swaddled safely up by the kidneys, like elephants do. Most other mammals drop their testes to the lower abdomen to a spot under the skin — like seals — or into an extra-abdominal sack called the scrotum — like humans. What style came first has been a topic of debate.

Now, new research on the genetic underpinnings of testes locations suggests that the male ancestor of placental mammals sported one of the descended modes. “For sure, the [ancestor’s] testes wouldn’t be close to the kidneys,” says Michael Hiller, a computational biologist at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany.

The evidence comes from comparing the genetic instruction books, or genomes, of 71 mammalian species, scientists report June 28 in PLOS Biology. Hiller, a coauthor of the study, says the team wasn’t looking for DNA evidence of ancient testicles, but kept seeing broken versions of two genes, INSL3 and RXFP2, popping up just in the lineage that includes golden moles and manatees. These genetic fossils, vestiges of once active code, suggested that certain Afrotherians, a grouping of mammals that includes elephant shrews, aardvarks and elephants, had lost something.

Those genes program the development of the gubernaculum, a ligament that holds dropped testes. “[It’s] necessary for testicular descent to happen,” Hiller says. As it turns out, all of the species with nonfunctioning versions of the two genes also had undescended testes. “You only see remnants of the genes required for testicular descent in species with no descent. The only way to explain that is that this is an ancestral trait that was later lost” in these species, Hiller says. Otherwise, the genetic fossils shouldn’t exist.

Hiller doesn’t think that there was one Afrotherian ancestor that lost descended testes and passed on the trait to subsequent species in its lineage. Rather, because different Afrotherians have different INSL3 and RXFP2 mutations, the shift happened at different times for different species, he says. Rock hyraxes and elephants still have completely intact copies of the genes but no descended testicles, for instance. Hiller says that means the animals probably lost the trait relatively recently.

Rock hyraxes and elephants may still use the genes for some other function. “Sometimes genes have multiple functions. If one [function] is no longer necessary, that doesn’t mean the gene will become inactivated,” says Mark Springer, an evolutionary biologist at the University of California, Riverside. It’s also possible that there’s another, shared mutation in a third gene that inactivates testicular descent that researchers haven’t spotted yet, he says.

Still, Springer says he applauds the study authors. Such genetic sleuthing is one way to tackle the evolution of soft body parts that don’t preserve well in the fossil record. “The study attacked this problem from a new angle we haven’t seen before,” Springer says. But whether that ancestor’s descended testes rested in a scrotum or in the lower abdomen under the skin “is still an open question.”

Somewhat surprising, too, is why a species would evolve to have its testicles in such a vulnerable position. “Can you imagine having descended ovaries in a thin, unprotected sack?” asks Gordon Gallup, an evolutionary psychologist at the University at Albany in New York who has studied testicular evolution “It’s unthinkable. It’s equally unthinkable for testes. Protection of the gonads is paramount.”

But sperm need a signal telling them they’re in a female reproductive organ. That signal is temperature, Gallup says. Sperm begin swimming vigorously when the heat goes up — when, say, they move from a cool descended testicle into a warm vagina.

Before descended testicles evolved, Gallup thinks that the temperature signal came largely from a rise in female body temperatures during ovulation. “It’s referred to by vets as an animal in heat,” he says. Keeping semen stored a bit cooler would make that change in body temperature more dramatic and help activate the sperm even more after coitus, Gallup says. “That may have led to the evolution of descended testicles in the first place,” he says. “It makes for a much more efficient use of semen.”

At least two of the species with undescended testicles have work-arounds. Both tenrecs and golden moles keep their body temperatures below 37° Celsius, the cutoff temp for good sperm storage. And elephants’ and rock hyraxes’ sperm ductwork detours close to the body’s surface, which may act as a cooling mechanism. Researchers may soon get on the ball with exploring those mysteries.