Conservationists are mourning the death of the world’s last remaining male northern white rhinoceros. The 45-year-old rhino died Monday at a nature reserve in Kenya, where he’d been under armed guard to protect against poachers. In a last-ditch effort to save the species, zoologists are now attempting to use in vitro fertilization to impregnate the last two remaining female northern white rhinoceroses, who are unable to produce offspring naturally.