Evolution of reproductive strategies of the human and great apes

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To make clear the pattern of reproductive strategies in the human and great apes, we attempted to estimate sperm production in these species and to compare histological aspects of testes. We obtained testicular preparations of the human (N=7), gorilla (10), chimpanzee (11), orangutan (7) by autopsy or biopsy, and observed them using light microscopes. As the results, spermatogenesis was detected in most samples in humans (6/7), chimpanzees (10/11), orangutans (7/7), but in few individuals in gorillas (4/10). However, the values in great apes were not significantly different from that in the human. The interstitial tissue of the human was loose and scarce. Leydig cells were quite dense in gorillas and quite few in chimpanzees, and intermediate in the other two species. There were not significant differences in diameters of seminiferous tubules nor proportions of seminiferous tubules in the interstitial tissue in all species. The mean estimated total length of seminiferous tubules (TLT) in humans was significantly short, compared with that in chimpanzees and orangutans. The sperm and mature spermatid count per tubule (CN) of humans was less than chimpanzees. The mean spermatogenetic index, calculated as SI=TLT × CN in these species was significantly different from each other, namely that in humans was less than chimpanzees and orangutans, but more than gorillas. Thus, we estimated that a chimpanzee, orangutan, gorilla usually produce 60 times, 4 times and 1/4 time sperm than humans, respectively.