A male's sperm has one job -- to fertilize a female's eggs. So why is sperm from rodents so much longer than that of larger mammals, such as primates, tigers and even whales? Researchers from Stockholm University may now have the answers.  

In a recent study, scientists took a closer look at how sperm has evolved in 100 different male mammals in relation to competition from other males along with the size of their female partners' reproductive tract. In addition, according to a news release. Essentially, size of sperm was compared to number of sperm because the longer the sperm, the less of them that can be produced.

So what did they find? Their analysis revealed that the size of the female directly influences whether a male produces longer or more (and shorter) sperm. This is because larger females have bigger reproductive tracts, so larger males produce a lot of tiny sperm to ensure it is not lost or diluted en route to fertilizing the female's egg. In contrast, the sperm from smaller male species has an easier time finding the egg, so males are more likely to produce sperm at least double in size, comparatively, as the release confirmed.

"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," John Fitzpatrick, associate senior lecturer at Stockholm University, explained in the university's release.

For the study, Fitzpatrick worked alongside his colleague Stefan Lüpold, from the University of Zürich. Together the researchers also discovered males invest more in sperm size and number if they have to compete with others to fertilize eggs, compared to monogamous species, the release noted.

Their findings were recently published in the journal Proceedings of the Royal Society B. 

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