Delilah gave birth on her own, 10 days before her due date. She went into labor overnight and was found in the forest with her calf by sanctuary workers an estimated four hours after birth, the Indonesia Ministry of Environment and Forestry said in a statement.

The early birth surprised the sanctuary staff who discovered Delilah nursing Saturday morning, the International Rhino Foundation said. The calf is standing, walking and breastfeeding and weighs about 55 pounds, the ministry said. The mother and baby are being monitored and are in good condition.

“Both are healthy and doing exactly what they need to be doing — eating, resting, and bonding,” said a statement from the foundation, which built the sanctuary in 1996 with the Indonesian government and local groups.

Delilah, who was born at the sanctuary in 2016, is the first rhinoceros mother to be born and to give birth at the sanctuary, the government said in a statement Tuesday. The International Rhino Foundation called it “a significant milestone for the breeding program.” Two years ago, only one captive Sumatran rhino pair in the world was able to successfully breed, the group said; the sanctuary now has three pairs of successful breeders.

Delilah mated with a male rhino named Harapan, who previously lived at the Cincinnati Zoo and was moved to the sanctuary in 2015 in hopes of breeding. He was the last Sumatran rhino to live outside Southeast Asia.

Delilah immediately became pregnant after mating, the ministry said, a happy outcome after some attempts at breeding between other rhinos ended in miscarriages. It was also the first success for Harapan after eight years of attempts, the International Rhino Foundation said.

Delilah gave birth on the 460th day of pregnancy, earlier than the typical 470 to 479 days. It was the second in two months: On Sept. 30, a female calf was born to a different set of parents in the sanctuary.

“Each of these births were the result of years of hard work and international research and collaboration — and they represent our best hope of saving Sumatran rhinos from extinction,” the International Rhino Foundation said.

The species is listed as critically endangered, the last step before extinction in the wild, on the International Union for Conservation of Nature’s Red List of Threatened Species. The group estimates that wild populations are continuing to decrease, with as few as 30 living adults.

Sumatran rhinos are the smallest of the five rhinoceros species and — descended from the Ice Age’s woolly rhinos — the hairiest. They have the smallest population of the five species, though Javan rhinos also have fewer than 100 members.

Sumatran rhinos have two horns, prominent skin folds and a prehensile upper lip, according to the conservation group Save the Rhino. They are agile, fast runners and eat vegetation.

The rhinoceroses are now found only in Indonesia, where the wild populations are small and struggle to breed, according to the World Wildlife Fund. The last surviving member of the species in its native Malaysia died in 2019 at the age of 25, making the species extinct in that country.