Volume of the Sperm Mid Piece
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Human and great ape sperm share the typical paddle-shaped morphology of their sperm. Hominoid sperm also share very similar overall length and width. The only significant difference documented to date is the existence of a significantly more voluminous mid-piece (the cylindrical middle portion of sperm containing the spirally-arranged mitochondria) in the sperm of chimpanzees and bonobos. Chimpanzee sperm also contain up to twice as many mitochondria as do human sperm. It has been theorized that this difference reflects a greater need for energy and motility in the two species of Pan where multi-male mating systems leads to higher levels of sperm competition.