The evolution of the scrotum and testicular descent in mammals: a phylogenetic view

The adaptive significance of the scrotum and the evolution of the descent of the testicles and epididymis have been a focus of interest among biologists for a long time. In this paper we use three anatomical character states of the scrotum and descensus: (1) testicles descended and scrotal; (2) testicles descended but ascrotal; (3) testicles not descended (testicondy). These states are then mapped on an up to date phylogeny of the Mammalia. Three main points arise out of this mapping procedure: (1) the presence of a scrotum is either primitive in extant Mammalia or primitive within eutherian mammals except Insectivora; (2) evolution has generally proceeded from a scrotal condition to progressively more ascrotal; (3) loss of testicular descensus is less common in mammalian evolution than is loss of the scrotum. In the light of these findings we discuss some current hypotheses regarding the origin and evolution of the scrotum. We find that these are all incomplete in so far as it is not the presence of the scrotum in various mammal groups that requires explaining. Instead, it is the reverse process, why the scrotum has been lost in so many groups, that should be explained. We suggest that the scrotum may have evolved before the origin of mammals, in concert with the evolution of endothermy in the mammalian lineage, and that the scrotum has been lost in many groups because descensus in many respects is a costly process that will be lost in mammal lineages as soon as an alternative solution to the problem of the temperature sensitivity of spermatogenesis is available.