Why many mammals keep their testicles outside the body, and whales don't
Sperm production works best a couple of degrees cooler than the rest of the body, so many mammals hang the testes outside in an adjustable sac. Muscles raise or lower it as the air warms or cools. Yet plenty of mammals, from whales to hedgehogs, later gave the arrangement up entirely.
In humans the scrotum keeps the testes at about 35 degrees Celsius, two or three below core body temperature, because warmer conditions impair sperm production. Two muscle systems do the adjusting. The cremaster muscle draws the testes up toward the abdomen when it is cold, and the dartos layer beneath the skin contracts, which is why the sac tightens and wrinkles in chilly air. A central membrane divides it into two chambers, and blood vessels including a mesh of veins called the pampiniform plexus run down into it from the abdomen.
Temperature may not be the only benefit. Sitting outside the abdomen spares the testes from internal pressure that could squeeze sperm out before they mature, and some researchers think the frequent lopsidedness, with the left testis usually hanging lower in men, reduces the chance of both being crushed at once or improves cooling.
It forms early. Testosterone production in a male embryo begins around week eight, peaks near week 13, and transforms the same folds of tissue that become the labia majora in females into the scrotum, which is why the two are considered equivalent structures. A visible seam, the raphe, marks where an embryonic groove closed by week 12.
Evolutionarily, the external scrotum is a defining trait of the boreoeutherians, the large branch of placental mammals that includes primates, rodents, carnivores and hoofed animals; every other vertebrate keeps its testes internal. Several lineages later abandoned it, including hippos, rhinos, moles, pangolins, many bats and rodents, seals, and whales and dolphins. Some male marsupials have a sac positioned in front of the penis, but it evolved separately.
Source: Scrotum
