Reason for externalization of the testis of mammals

Testes externalized in a scrotum are found only in those mammals whose present lifestyle (or that of their ancestors) involves jumping, leaping or galloping, thereby creating concussive hydrostatic rises in peritoneal pressure. This would be expected to expel the contents of the reproductive tract since it does not possess a sphincter. The lower temperature of the externalized testes is then seen as a secondary adjuvant adaptation to a cooler location than within the body cavity. The theory also explains the close phylogenetic relationship between the Proboscidea, Hyracoidea, and Sirenia, if it is assumed that their common ancestor was not endowed with the genes necessary for externalization.