THE SPERM OF different species of animal have different shapes and according to a new study those shapes can be used to tell closely related species apart.

Researchers in the Field Museum in Chicago looked at the shape of sperm of 18 different species of rodents. Their report is published in the Journal of Mammalogy. 

They analysed upwards of 50 individual spermatozoa [mature sperm cell] from 58 individual South American rodents from the subfamily Sigmodontinae of the family Cricetidae.

“I trapped a lot of these mice when I was working on my dissertation. When we prepared the male specimens, we put the testicles in formalin, and that preserved the sperm,” said researcher Noé de la Sancha.

A lot of people throw the testes away – they’re an underutilised resource.

They discovered that the sperm varied quite a bit, even in very closely related species. Some had hooked heads, like the top of a soft-serve ice cream cone, while others were rounded and smooth, and their tails were different sizes.

Sperm from three closely related species of vesper mice. While these mice look very similar and can be hard to tell apart with DNA, their sperm shapes are radically different. Luis Rossi, Noé de la Sancha, et al. Luis Rossi, Noé de la Sancha, et al.

The most variation was found in the sperms’ mid-sections, which are packed with energy-producing mitochondria that power the sperm to swim.

“The biggest surprise was that the sperm shapes didn’t group the way we thought they would. You’d expect the sperm of closely related species to be really similar to each other, but they discriminate really nicely. Sometimes the sperm from distantly related species looked more alike than the sperm from close relatives,” de la Sancha said. 

It’s not exactly clear why the different species have such differently-shaped sperm, but co-author Luis Rossi hypothesized that the different sperm shapes must be conferring some sort of advantage to the rodents.

“Sperm competition is a form of post-coital sexual selection that influences the evolution of the semen characteristics of animals,” he said.

This work represents an important step in our understanding of how evolutionary advances can maximise reproductive success. Looking at the sperm also lets us see if the shape can be traced back to the evolutionary relationships between species– what biologists call a phylogenetic signal.

The researchers said many of these species look almost identical, even to experts and  DNA isn’t a perfect solution.

“You can tell by DNA that two specimens are different from each other, but how different is different enough to be a separate species? Is it just geographic variation? For many closely related species, it’s not that straightforward,” de la Sancha explained.

While two rodents’ DNA might be very close and they might look the same, their sperm might have completely different shapes – a dead giveaway that they belong to separate species.

Being able to more easily tell these rodents apart could have important implications for medicine and conservation, researchers said. Some are hoses of specific diseases and the more accurately they can determine which specimens belong to which species, the better scientists can fight the spread of those diseases.