Mice have longer sperm than elephants

In the evaluation, the researchers included factors like sperm length and even number of sperm per ejaculate.

In larger species, sperm length or speed probably comes into effect only if a sufficient number of sperm manage to get near the egg, researchers said. An elephant’s sperm has very little chance of making it to the egg no matter how long it is.

It is a freakish extreme, but as a general rule, scientists have long known that the smallest animals have the biggest sperm, and vice versa.

Sperm competition that occurs when multiple males mate with the same female is important in the evolution of sperm size, the University of Zurich reported. Interestingly, this is more common in small rodents, such as mice and rats, than in larger animals.

In small animals, larger sperm seem to have an evolutionary advantage, while in big animals it is smaller sperm in higher numbers which prevail.

So size matters for animal sperm – just not in the way you might think.

While studying the sperm science, researchers from the University of Zurich examined the sperm count and size of sperm and tried to find its link with the size of any species.

“By contrast, sperm dilution is less of an issue in small species, which creates conditions where the benefit of longer” – and faster-swimming – sperm may come into play.

The bigger the animal, the greater the selection pressure on the overall investments in ejaculates and the more important the number of sperm becomes compared to sperm length. 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.

University of Zürich, faculty John Fitzpatrick and his colleague Stefan Lüpold claimed that their research study has found that how competition between sperm from different males and the size of the female reproductive tract act upon the phylogenies of sperm in 100 mammalian species. Their approach assessed both sperm length and the number of sperm in an ejaculate. In other words – the longer sperm, the less can be produced.

Ideally, each male would produce a huge amount of impressively big 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.

“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”, Fitzpatrick said.