Why mice have longer sperm than elephants!

Here is another story sure to be a classic in Darwin’s Book of Fairy Tales.  In Science Daily the title above leads the reader to think that something factual has been discovered: an answer to some mystery.  The headline reads this way:

In the animal world, if several males mate with the same female, their sperm compete to fertilize her limited supply of eggs. Longer sperm often seem to have a competitive advantage. However, a study now reveals that the size of the animals also matters. The larger the animal, the more important the number of sperm is relative to sperm length. That’s why elephants have smaller sperm than mice.

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Now having read the headline do you now know why elephants have smaller sperm than mice?  You know that they do but you don’t know why.  Several facts are given. Longer sperm “may” have a competitive advantage at fertilizing an egg.  Apparently  no one knows if this is true.  A study says that the bigger the animal the more the sperm delivered.  (Does this surprise anyone?)  Bigger animals have bigger heads too?  Thirdly, mice have longer sperm cells than elephants.  Why?

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The mystery remains doesn’t it?

As the article continues the comment is made: The larger the animal, the more important the number of sperm is relative to sperm length. That’s why elephants have smaller sperm than mice.

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What have we learned now?  That mice have longer sperm cells than elephants because elephants have sperm cells smaller than mice.  And we learn again that the larger the animal the more important the number of sperm cells.

Elephant sperm cells.

Elephant sperm cells.

I do understand that more sperm cells should be available in a larger animal than in a smaller animal because the animal is, well, larger!  Larger testicles, large penis, larger female, larger vagina… well blah blah this all makes sense right?  So why did evolution give the mouse and the fruitfly larger sperm than the elephant or the whale?

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