The evolution of the scrotum: a new hypothesis.
The adaptive significance of the scrotum is unresolved after more than 60 years of debate and experimentation. The "training hypothesis" introduced here suggests that testicular descent is a mechanism for improving sperm quality. The hypothesis proposes that: (1) testicular descent decreases blood supply to maturing sperm cells, (2) sperm mitochondria respond to the resulting oxygen stress by enhancing their enzymatic machinery for oxidative metabolism, as do oxygen-stressed muscle cell mitochondria, and (3) the resulting increase in aerobic fitness of sperm cells is advantageous in inter-ejaculate competition. The hypothesis suggests that there is a quantity-quality trade-off in sperm production, where taxa with internal testes produce large volumes of low-quality sperm while taxa with scrotal testes produce smaller volumes of higher-quality sperm.
Freeman
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Department of Zoology, University of Washington, Seattle 98195.
D000818:Animals; D005075:Biological Evolution; D006801:Humans; D008297:Male; D008322:Mammals / Q000502:physiology*; D008954:Models, Biological; D012611:Scrotum / Q000502:physiology*; D013094:Spermatozoa / Q000502:physiology; D013696:Temperature; D013737:Testis / Q000502:physiology