Sudan, the last remaining male northern white rhino, has died in captivity at the Ol Pejeta Conservancy in Kenya.

He was 45 years old—elderly for a rhino—and his unexpected death may have been a result of infections and other health problems. Two northern white rhinos remain: Najin, Sudan’s daughter, and Fatu, Sudan’s granddaughter.

Northern white rhinos are a subspecies of southern white rhino, though some researchers argue they should be considered their own species. They’re hairier, smaller, and have different dental structures.

The population of northern white rhinos has been decreasing since 1960 when about 2,000 rhinos were living on the grasslands of east and central Africa. By 2008, researchers couldn’t find any. War, habitat loss, and poaching for their horns contributed to the subspecies’ extinction in the wild.