Published: December 12, 2018

Sperm Can Confirm that Lookalike Mice Are Different Species


Looking at rodent sperm shape is another way to tell closely related species apart.

A mouse perched on sticks surrounded by dried brown leaves.

Think back to health class and picture a sperm. It has a rounded head with a long skinny tail at the end, right? As it turns out, the sperm from different species of animals have different shapes—and new research shows that those shapes can be used to figure out who’s who among closely related rodents.

To reach this conclusion, Noé de la Sancha, a mammal expert and researcher at the Field, and Luis Rossi of the Universidad de Buenos Aires, had to examine a lot of sperm. They looked at 58 individual South American rodents, analyzing at least 50 spermatozoa from each animal.

But is it really necessary to study mouse sperm—aren’t there other ways to tell animals apart? Sure, but it’s not always clear-cut when you’re looking at closely related critters. Even DNA differences could be a sign of geographic variation within one species.

Many of these species are cryptic—they look nearly identical, even to experts.

Noé de la Sancha, mammalogist and Field researcher

But while two rodents’ DNA might be very close and they might look the same, their sperm could have completely different shapes—a dead giveaway that they belong to separate species. “This adds another level of evidence for telling species apart, just like the shape of their teeth or the length of their tails,” says de la Sancha.

A graphic shows three different rodent sperm shapes. Pyriform has a hooked top with a centered tail; polygonal has a hooked top with tail shifted to the right; and oval is smooth on the top with a centered tail. The species examined are C. callidus, C. venustus, C. musculinus, and C. laucha.

Scientists were able to identify different head morphologies, or shapes, among sperm from different rodent species.

Luis Rossi, Noé de la Sancha, et al.

For rodent specimens, it’s not exactly common practice to preserve the sperm. “We put the testicles in formalin, and that preserved the sperm,” says de la Sancha. “A lot of people throw the testes away—they’re an underutilized resource.”

Rossi and de la Sancha were then able to take measurements and observe differences in sperm shape and size. 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. The most variation was found in the sperms’ mid-sections, which contain energy-producing mitochondria that power the sperm to swim.

Several mouse specimens are lined up in a white box, with their tails and specimen tags pointing towards the viewer. Small glass vials are seen in a box behind them.

Rossi and de la Sancha focused on 18 different species from the subfamily Sigmodontinae, many of which look very similar in appearance but have distinct sperm. These specimens are in our collection at the Field.

Telling these rodents apart could have important implications for medicine and conservation. “Some of these species of mice are hosts of specific diseases,” says de la Sancha. “The more accurately we can determine which specimens belong to which species, the better we can fight the spread of those diseases.”

Distinguishing between species is also crucial for researchers working to protect them. “We still don’t know how many species are on the planet right now, and we’re in the sixth mass extinction. We’re losing species faster than we can identify new ones,” says de la Sancha. “This study could make us better able to understand bigger patterns of biodiversity. Using sperm to tell rodents apart is adding one more tool to the toolbox.”

This research paper was published in theJournal of Mammalogy.