The last male northern white rhinoceros — his name was Sudan — died in March, leaving only two members of the subspecies behind: his daughter and granddaughter.

In the past, those stark facts would have spelled the end. But researchers at the San Diego Zoo’s Institute for Conservation Research — home to a frosty menagerie known as the Frozen Zoo — are working to give northern white rhinos a second chance. Since 1975, the institute has been collecting tissues from creatures, some endangere