Why Mouse Sperm is So Much Longer Than Elephant Sperm

Why is the sperm of rodents so much longer than the sperm of large mammals, like primates? That’s a question that researchers have long struggled with. Now, they may have the answers. The sperm of the Asian elephant has the shortest sperm relative to its body size, whereas the house mouse (Mus musculus) has the longest. 

Why is the sperm of rodents so much longer than the sperm of large mammals, like primates? That’s a question that researchers have long struggled with. Now, they may have the answers.

Scientists examined how competition between sperm from different males and the size of the female reproductive tract influence the evolution of sperm in 100 mammalian species. They assessed sperm length and the number of sperm in an ejaculate. This is important since resources used to produce sperm must be shared between number and size.

The researchers found that the size of the female influences whether it’s better for a male to produce longer or more sperm. In larger females, with bigger reproductive tracts, sperm can be lost or diluted on their journey to the egg. This is why in larger species males produce a lot of tiny sperm. In small species, though, males produce comparatively longer sperm.

The researchers also found a connection between sperm size and monogamy versus multiple mates.

“We show that males invest more in sperm size and number in species where sperm from multiple males compete to fertilize eggs compared to monogamous species,” said John Fitzpatrick, one of the researchers, in a news release. He went on to say that “This demonstrates that the location where sperm compete-inside the female’s reproductive tract in mammals-determines when it pays a male to produce longer or more sperm.”

The findings are published in the journal Proceedings of the Royal Society of London.

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