Back in 1968, researchers froze ram sperm in liquid nitrogen and stored it in a laboratory. It has now been defrosted and used to get Merino ewes pregnant.
"We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring," says animal reproduction specialist Jessica Rickard with The University of Sydney.
A video view of the thawed semen under a microscope shows plenty of activity.
The project involved inseminating 56 ewes, and 34 were successfully impregnated. The researchers say the live birth rate was as high as for sperm that has been frozen for just a year.
The lambs were born with a distinctive body wrinkling seen in sheep from the father's time period. The wrinkled body style has been selectively bred out of Merino sheep over the intervening decades since it made shearing difficult.
The researchers tweeted photos of the sheep last month, with de Graaf saying they look lighter and wrinklier than most modern Merinos. "You'll see big differences next year once they're older and results are in from first shearing," he wrote.
The University of Sydney shared a photo of one of the four original sperm donors, a ram named Sir Freddie. Sir Freddie was born in 1959.
"We can now look at the genetic progress made by the wool industry over past 50 years of selective breeding," says University of Sydney associate professor Simon de Graaf. "This gives us a resource to benchmark and compare."
The sheep born from the project are living time capsules, giving researchers a unique window into the past. The last time we saw such a remarkable story involving something frozen in the '60s, it was called Austin Powers.