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 through the tight spots, known as “strictures”, while weaker ones were caught in oncoming currents that propelled them backwards when they got too close.
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