Proposed models of SSC kinetics in five different mammalian species. The calculated sperm turnover rate/generation in five different species (rats (Rattus rattus), Djungarian hamsters (Phodopus sungorus), marmosets (Callithrix jacchus), macaques (Macaca fascicularis), humans (Homo sapiens)), representing the two stem cell systems (‘direct’ and ‘progenitor-buffered’) is shown. Average numbers of type A spermatogonia (SPG), type B spermatogonia, pre-leptotene–zygotene (PLZ) and pachytene spermatocytes (PS) per testis, respectively, were used to calculate daily sperm production, lifetime sperm production (LSP) and the ratio of LSP/type A spermatogonium. The tip of the grey-shaded pyramid represents the population size of type A spermatogonia. The base of the pyramid shows that the LSP ratio is in the range of 10⁹–10¹¹ for all the species. The ratio of LSP/type A spermatogonium is in the similar order of 10⁹ for the respective species, possibly pointing to a general adaptation process in terms of testicular productivity.