This is how the female human body uses obstacles to block weak sperm, like salmon running into a waterfall

For millions of sperm it is the end of the road. Scientists have found evidence that the female reproductive tract is shaped in such a way that stops poor swimmers from reaching their goal. Researchers used small-scale models and computer simulations to show that pinch points that behave like gates along the sperm’s arduous path from cervix to egg allow only the fastest ones through. Tests with sperm from men and bulls revealed that the strongest swimmers were most likely to make it…

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