Boreoeutherians hang low

On the podcast we asked the question of whether all mammals have external testicles. Well it turns out only boreoeutherian mammals have externalized testicles. However even within this group there are exceptions, rhinoceroses and cetaceans for example have internal testicles. The whales and dolphins get around the cooling problem by having a complex vascular network around their testes where arterial blood is cooled by venous blood form the skin.

The question is how did this split in the externalization occur during evolution and how do mammals such as elephants, rhinos, monotremes, sloths, armadillos etc... deal with the increased temperature? One theory states that those mammals have or had lower body temperatures which enabled internal testes to be just as efficient. Another theory, states that the ancestor of boroeutherians had unsually large testes, and so had to carry them outside which lead to some irreversible changes in the biochemestry and obligate externalization. Finally a third theory argues that the testes were externalized for protection (really?), mostly to shield it from changes in pressure in the abdominal cavity during intense physical activity, such as galloping or jumping. If I had to choose I'd pick the unusually large testis hypothesis...

However some mammals have evolved complex strategies to get the best of both worlds. One such unusual scrotum is found on the kangaroo:

"Unlike that of many other mammals, kangaroos' scrotum is located far ahead of the penis, almost in the middle of the belly. In hot weather it can be seen lowered by the relaxed animal to keep the testes cool, and raised when moving about. (This body part can be found in souvenir shops as "lucky pouch"...)"