The World’s Oldest Preserved Sperm Are Still Swimming Strong

"What is true for the sheep is also true for humans."

On Sunday, Australian scientists announced that they’d thawed 50-year-old sheep semen and successfully impregnated living sheep with it. The semen, which was frozen in 1968, stayed frozen in liquid nitrogen as part of a project aimed at proving that sperm could remain viable after long-term cold storage. The University of Sydney researchers behind the project say that their work could expand options for young cancer patients who may want to save semen samples before starting radiation treatment. This would allow them to have children later in life, even if they haven’t found a partner yet.

And while the implications for humans are exciting, the case itself is pretty incredible, too.

It all started back in 1968, when Dr. Steven Salamon, a sheep researcher at the University of Sydney, created a time capsule. Instead of filling it with notes, books, or examples of modern technology, though, Salomon filled this time capsule with semen. Samples from four of Peter Walker’s prize rams, to be exact. Walker, a wool grower in New South Wales, was eager to help with Salamon’s research back then because he hoped it would lead to better ways of collecting semen from rams and inseminating ewes. The samples Walker gave Salamon came from merino rams, a breed of sheep well known for its high-quality wool.

When Salomon died in 2017, the next generation of sheep researchers stepped up where he had left off. In 2018, Simon de Graaf, Ph.D., an associate professor of animal reproduction, and Jessica Rickard, Ph.D., a postdoctoral research fellow, cracked open the sheep semen pellets to see how they had held up.

“When we first thawed out the semen to test its quality (motility, viability, DNA integrity) we were very excited (and a little relieved!) to see that 50 years of storage at -196°C had had seemingly no ill effects on the health of the sperm,” de Graaf tells Inverse. “That gave us confidence that if we used the sperm for artificial insemination it should still be fertile.”

And that’s exactly what they did.

Sir Freddie, photographed in 1969, donated some of the semen that impregnated 34 ewes in 2018.
Sir Freddie, photographed in 1969, donated some of the semen that impregnated 34 ewes in 2018.

The team, which undertook the research thanks in part to a grant from Australian Wool Innovation, used the semen to inseminate 56 ewes, 34 of which became pregnant — a success rate of 61 percent. They compared this number to the success rate of semen samples that had been frozen for just 12 months. Those fresher samples impregnated 618 out of the 1,048 ewes that were inseminated — a rate of 59 percent.

In short, the 50-year-old semen was almost exactly as viable as the 1-year-old semen. To the best of their knowledge, these samples were the oldest preserved semen samples in existence.

“To then see the live lambs and their characteristics was a dream come true, to be able to show that long term frozen storage of sperm is safe, and also open that window into the past and continue the research conducted by our predecessors five decades ago — just a real pleasure as a scientist,” says de Graaf. They will continue to monitor the health of these lambs over the next couple of years, and hope to publish the research soon.

Rickard (left), Walker (middle), and de Graaf (right) examine the lambs produced from the vintage semen.
Rickard (left), Walker (middle), and de Graaf (right) examine the lambs produced from the vintage semen.

In addition to this proof-of-concept, showing long-preserved semen still works, the lambs’ characteristics gave the researchers a glimpse into how merino sheep have changed over just half a century. Just as humans breed dogs to have specific physical and emotional traits, humans have also bred sheep to change their wool qualities. In the case of merinos, mid-20th-century farmers bred them to have wrinkly skin, which they thought created more surface area to produce more wool. And while this may have been true, it also created unforeseen consequences: Wrinkly skin is harder to shear, plus it creates folds that provide extremely attractive locations for parasitic flies to lay their eggs.

“Once these problems became apparent sheep producers began to breed much ‘plainer bodied’ animals with less wrinkle,” says de Graaf. But the 50-year-old ram semen resulted in lambs that were throwbacks to that time — heirloom sheep, if you will.

One of the merino lambs.
One of the merino lambs.

This research isn’t just of interest to wool enthusiasts and sheep farmers, though. For young people who may need to preserve reproductive material in the face of an uncertain future, this achievement can provide some hope.

“What is true for the sheep is also true for humans,” says de Graaf.

He points to the example of human fertility clinics, where young cancer patients about to undergo radiation or chemotherapy have to make the tough choice to preserve their semen in case their reproductive ability is affected by the treatments and they want to have children someday in the future.

“Our research shows that these men can rest assured that their semen collected in their teenage years or early 20s will remain healthy and viable while frozen until the day they need it (if they do indeed face fertility issues later in life), whether that be decades into the future,” says de Graaf.

“I’m sure their future fertility is probably not the main thing on their minds as they go through cancer treatment, but it is one less thing to worry about.”

Media via Courtesy of the Walker family, University of Sydney