No one can gainsay the ingenuity of some undergraduate students. When I was training as a vet, I remember medical students putting on a play and one of the cast was listed in the programme as ‘Scrotum – a wrinkled old retainer’. The writers obviously recognized a feature of the scrotum, the significance of which seems to have escaped wider attention. That is, that the scrotum is indeed wrinkled.

It is popularly held that testicles descended into a scrotum because they needed to be cooled, but this belief needs much more careful examination. In the first place, the question as to why testicles descended outside the confines of the abdomen is not a legitimate one to ask, because it is shamefully Lamarkian (teleological) and not, therefore, in keeping with Darwinian principle. We can only ask how, not why. Since in the majority of, though not in all, mammals, the testicles are scrotal, the question that needs to be put is: what advantage did the evolutionary descent of testicles confer on the animals in which it happened?

In trying to answer this question, the architecture of the scrotum and its contents might provide useful information. First, the scrotum is more than a piece of stretched skin. It is modified to become exceptionally thin and has a sub-layer of elastic fibrous tissue which contains slivers of involuntary muscle (the tunica dartos containing dartos muscle). When the dartos muscle contracts, the scrotum wrinkles; when it relaxes, the scrotum is able to stretch so the wrinkles become ironed out, either partially or completely. These changes in the configuration of the scrotum are always accompanied by movements of the contained testicles.