Anyone who's found an old bag of shriveled, ice-encrusted broccoli in the back of the freezer knows that some things don’t last forever in a deep freeze. But a recent study found that something does outlast the cold pretty well: semen.
Researchers in Australia recently thawed a batch of sperm collected from a merino sheep that had been frozen in liquid nitrogen for 50 years, finding that the little swimmers were still strong and able. In fact, the sperm was used to sire healthy little lambs, reports Jack Guy at CNN.
In 1968, sheep researcher Steve Salamon from the University of Sydney set out to create a sheep semen “time capsule,” and so he collected semen from several prize rams—one named Sir Freddie and three others—at Peter Walker’s farm in New South Wales, reports Peter Hess at Inverse. When Salamon died in 2017, his fellow researchers decided it was time to thaw the sperm samples to see if it had passed the test of time.
“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,” Simon De Graaf, project member from the Sydney Institute of Agriculture and School of Life and Environmental Sciences at the University of Sydney tells Hess. “That gave us confidence that if we used the sperm for artificial insemination it should still be fertile.”
The team then used the sperm to artificially inseminate 56 ewes, 34 of which eventually gave birth to little lambs. Surprisingly, the pregnancy rate was 61 percent, which is practically equivalent to the 59 percent pregnancy rate for sperm that is frozen for just one year. In fact, the sperm was just as lively as batches of the stuff frozen for just 12 months. The team believes that the sperm is the oldest known frozen semen in the world, making it the oldest known sperm to produce offspring as well.
So far, the lambs sired by the frozen fathers seem to be doing well and will be monitored for the next two years to look for any health problems. While the lambs appear normal, they are a slightly different from most merino lambs born today.
“The lambs appear to display the body wrinkle that was common in Merinos in the middle of last century, a feature originally selected to maximise skin surface area and wool yields,” De Graaf says in a press release. “That style of Merino has since largely fallen from favor as the folds led to difficulties in shearing and increased risk of fly strike.”
Having a few lambs with characteristics from 50 years ago has practical applications for the sheep industry. Researchers can now compare the genetics of the old model sheep to newer vintages to better understand how selective breeding for more productive sheep has changed the animal’s genetics.
The study, not yet published, also has implications for wildlife conservation. The 50-year survival rate means researchers can confidently preserve the genetic material from endangered species to preserve gene pools that might disappear in the wild. Several institutions, including the San Diego Zoo, are collecting sperm, eggs and skin cells from endangered animals to create “frozen zoos” to do just that.
And the research is good news for humans as well, especially younger cancer patients who may lose fertility due to their treatments. “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,” de Graaf tells Hess at Inverse.