The tails of sea urchin and human sperm share the same bendy inner core, it seems that the tails of sperm in mammals may have evolved a reinforcing outer layer. This outer layer, or “cape”, gives them the exact amount of extra strength and stability needed to overcome the thick fluid barrier they come up against in internal fertilisation.
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Flagellar ultrastructure suppresses buckling instabilities and enables mammalian sperm navigation in high viscosity media. J. R. Soc. Interface 20180668.
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