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 o f the testicles and epididymis have been a focus of interest among biologis ts for a long time. In this paper we use three anatomical character states of the scrotum and descensus: (1) testicles descended and scrotal; (2) test icles descended but ascrotal; (3) testicles not descended (testicondy). The se 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 scr otum is either primitive in extant Mammalia or primitive within eutherian m ammals except Insectivora; (2) evolution has generally proceeded from a scr otal condition to progressively more ascrotal; (3) loss of testicular desce nsus is less common in mammalian evolution than is loss of the scrotum. In the light of these findings we discuss some current hypotheses regarding th e origin and evolution of the scrotum. We find that these are all incomplet e in so far as it is not the presence of the scrotum in various mammal grou ps that requires explaining. Instead, it is the reverse process, why the sc rotum 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 sc rotum 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 alternati ve solution to the problem of the temperature sensitivity of spermatogenesi s is available. (C) 1999 Academic Press.