With animal sperm, size matters | Science| In-depth reporting on science and technology | DW | 18.11.2015
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With animal sperm, size matters

The bigger the animal, the smaller the sperm. Sounds paradoxical? According to a new study, it makes perfect sense. Researchers have shown that the size of the animal determines sperm size and sperm numbers.

According to new research published in "Proceedings B," a journal by the UK's Royal Society, it is the size of the female reproductive organs that determines whether males produce lots of tiny sperm - or fewer sperm that are much larger.

The paper by researchers from Stockholm and Zurich Universities explains why the size of the sperm is related to the animal's size. The researchers analyzed sperm samples by 100 different mammals.

"If you're a species where the female reproductive tract is really large, like say in an elephant, an ejaculate gets diluted within that tract," Sean Fitzpatrick from Stockholm University and one of the authors of the report told DW.

So, sperm can get lost - they can't find the way to the eggs potentially, or there's just such a large volume that they have to work their way through that, in those cases, it pays to produce a lot of sperm," he explained.

By contrast, in smaller species, there is less dilution, making it easier for sperm to get to the egg. However, if the female mates with more than one male, sperm risk being "elbowed" out of the way, so it pays to produce bigger sperm that can beat down a path to the egg.

Competition is, of course, also an issue for larger species, but there, the reproductive tract is so vast that size really does not matter. There, it is more a case of finding strength in numbers.

Fitzpatrick compares the tract to a huge sports pitch, where the sperm are the players. "If you have a huge pitch, the sperm are unlikely to interact much with one another. But if you have a really small pitch, it's more likely that the sperm are going to physically interact as they move towards the goal."

In some species, like fruit flies for example, sperm can be up to six centimeters (2.25 inches) when stretched from end to end. That's several times larger than the actual animal. But because the sperm curls up in a ball, it is able to enter the female reproductive tract, where it then tries to beat rival sperm to it.

In mammals, the house mouse (Mus musculus) has the longest sperm relative to its body weight; the Asian elephant has the shortest sperm, similar in size to human sperm.

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