The galloping hypothesis

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. … 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. 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.

—Liam Drew, *Slate*[^1]