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Why does sperm size vary so much between animal species?

Sperm size varies greatly between different animal species. But how can there be such a big difference, when the task to be performed is the same in all cases?

Why does sperm size vary so much between animal species?

0.002 millimeters to 6 centimeters: The size of sperm cells varies greatly between animal species. While they all really have the same mission: to fertilize eggs. That’s why the scientists wanted to see if they could figure out where this difference came from. As it turns out: it’s at the site of fertilization.

Fertilization can be carried out in three ways: internally, as in mammals, external, as in fish, and in a hybrid form. They call it: sperm. The male releases the eggs into the water and the female collects them for internal fertilization.

After studying a huge amount of data about the sperm cells of animals, the researchers saw that the large sperm cells occur mainly during internal fertilization. In external fertilization, the male must compensate for the dilution of sperm. Then you want to make as many as possible (even if it comes at the expense of size). Large sperm cells arise precisely when they have to survive in a narrow space of the female reproductive system.

Oh, and in case you were thinking: Who is really attributed to the largest sperm cell? 6 cm? You will never guess it. It’s the fruit fly.

In this audio you can hear researcher Ariel Kehrl from the Department of Zoology at Stockholm University. You can read more about the research here: Females charge the development of sperm in animals.

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The paper can be found in Nature’s ecology and evolution: Fertilization mode drives the evolution of sperm length across the animal tree of life.

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