At the start of the 20th Century, northern white rhinos were a common site on the savannahs of eastern and central Africa, but then poaching, habitat destruction and armed conflict caused populations to crash.

By the 1980s, there were just 15 animals left in the wild. When they died, conservationists hoped that the small number of animals left in captivity would be able to kickstart the population, but the rhinos didn’t get the memo. Northern white rhinos don’t breed well in captivity, and the last male, known as Sudan, died in 2018.

However, before the final few males died, Hildebrandt and colleagues started to collect and freeze their semen.