Sudan, the last male northern white rhino, is seen at Ol Pejeta Conservancy in Kenya on May 3, 2017. Keepers at the conservancy announced Tuesday that Sudan’s health had worsened this week and he was put to sleep Monday. (Dai Kurokawa/EPA-EFE/Shutterstock)

The world’s last male northern white rhino, Sudan, has died after “age-related complications,” researchers announced Tuesday, saying he “stole the heart of many with his dignity and strength.”

The 45-year-old rhino was euthanized (yoo-thuh-nized), or put to sleep, Monday after his condition “worsened significantly” and he was no longer able to stand, according to the Ol Pejeta Conservancy in Kenya. His muscles and bones had weakened, and he had a deep infection on his back right leg.

“He was a great ambassador for his species and will be remembered for the work he did to raise awareness globally of the plight facing not only rhinos, but also the many thousands of other species facing extinction as a result of unsustainable human activity,” said the conservancy’s CEO, Richard Vigne.

The only remaining northern white rhinos are females Fatu, front, and Najin, who are seen grazing in a paddock Tuesday at the Ol Pejeta Conservancy. Scientists haven’t given up on saving the subspecies from extinction. (Tony Karumba/Agence France-Presse/Getty Images)

Only two northern white rhinos remain: Sudan’s 27-year-old daughter, Najin, and her 17-year-old daughter, Fatu. Some scientists are hopeful that they may develop a technology to save the subspecies while the two females are alive. Others have said that it’s probably too late to save the northern white rhino, whose natural habitat has faced severe limitations, and that efforts should focus on other critically endangered species with a better chance at survival.

The rhino was named after his birth nation, where he was born in the wild. He was taken to a zoo in the Czech Republic and then transferred to Kenya in 2009.

Northern white rhinos once roamed parts of Chad, Sudan, Uganda, Congo and the Central African Republic. They were particularly vulnerable because of the armed conflicts that have swept the region over decades.

Other rhinos, including the southern white rhino and the black rhino, are under heavy pressure from poachers who kill them for their horns to supply illegal markets in parts of Asia.

About 20,000 southern white rhinos remain in Africa. Their numbers dipped below 100 around a century ago, but an intense effort started by South African conservationist Ian Player in the mid-20th century turned things around.

— Associated Press