THE TESTIS - THE WITNESS OF THE MATING SYSTEM, THE SITE OF MUTATION AND THE ENGINE OF DESIRE

There is now abundant evidence in a wide range of mammalian and non-ma mmalian species to show that the relative size of the testis and the m orphology of the spermatozoa are infallible predictors of the mating s ystem. Species with the largest testis/body weight ratios and the best spermatozoa have a multi-male or promiscuous mating system in which s perm competition operates. Judged by these criteria, men were not desi gned to be promiscuous. There is increasing evidence in humans to show that most spontaneous mutations of the germ line occur in the testis. Because these provide the variability on which natural selection can operate, the testis holds the key to evolution. Genes on the Y chromos ome that control male fertility are particularly prone to mutations, p erhaps because of the mutagenic metabolites produced by the metabolica lly active testis. Testicular descent into a scrotum, and cooling by c ountercurrent heat exchange between the spermatic artery and vein may have evolved as a way of holding the mutation rate in check. The hormo nes secreted by the testis, which control libido and aggression, ensur e that these male mutations are disseminated as widely as possible thr oughout the population.