The reproductive history of the unisexual, ladies-only salamander species is full of evolutionary surprises.

In a new study, a team of researchers at The Ohio State University traced the animals’ genetic history back 3.4 million years and found some head-scratching details – primarily that they seem to have gone for millions of years without any DNA contributions from male salamanders and still have managed to persist. The research appears in the journal Evolution.

First, a bit about the unisexual Ambystoma salamander: They’re female, and they reproduce mainly through cloning and the occasional theft of another salamander species’ sperm, which the males of sexual species deposit on leaves and twigs and the like. When this happens, it stimulates egg production and the borrowed species’ genetic information is sometimes incorporated into the genome of the unisexual salamanders, a process called kleptogenesis.

Scientists who study these amphibians and their relatives, which are also called mole salamanders, have theorized that the theft of sperm is part of what has kept the unisexuals around so long. If all they ever did was clone themselves, biologists reason, they’d be vulnerable to all kinds of problems that unfold when you don’t mix up the DNA pool and would disappear from the earth fairly quickly.

Going into the study, the Ohio State team figured this sperm-borrowing happened with regularity throughout history, said study co-author H. Lisle Gibbs, a professor of evolution, ecology and organismal biology.

But findings revealed that the salamanders rarely dip into the other-species pool for genetic variation.

“This research shows that millions of years went by where they weren’t taking DNA from other species, and then there were short bursts where they did it more frequently,” said Rob Denton, who led the project as an Ohio State graduate student and is currently a postdoctoral researcher at the University of Connecticut.

“Surprisingly, it doesn’t look like they’re suffering any ill genetic effects. It’s a mysterious scenario that an animal can avoid sexual reproduction for millions of years and not suffer the consequences of that.”

Using newly available technologies and a novel and complex approach to sequence and evaluate about 100 DNA samples from the salamanders, the Ohio State researchers developed a genetic blueprint for what unfolded in the last 3.4 million years.