(CNN) -- When the news came, animal conservationist Susie Ellis was ecstatic. This wasn't just an extraordinary pregnancy, but hope for the survival of a vanishing species.
Ratu, a Sumatran rhino, is pregnant and if all goes well, she will give birth at an Indonesian rhino sanctuary in May 2011. Her mate, Andalas, is the first of only three Sumatran rhinos born in captivity in 112 years. His offspring will be the fourth.
"This is a big step forward for the species," said Ellis, executive director of the International Rhino Foundation, based in Yulee, Florida.
Sumatran rhino numbers have decreased by more than 50 percent over the last 15 years, Ellis said. They are the rarest of five existing rhino species, having dwindled down to 200 in the wild and 10 in captivity.
Every individual counts; every pregnancy is momentous.
Like other threatened animal species, Sumatran rhinos began disappearing because of human encroachment on their rainforest habitats and the practice of poaching. Rhino horns are commonly sold to make analgesics in some forms of Asian medicine.
But it has been difficult to boost the population, Ellis said.
Sumatran rhinos, also known as hairy rhinos because of their hairy body and tufted ears, are solitary animals that are rarely spotted in the wild. For a long time, researchers did not understand the rhinos' breeding mechanisms.
One person who made headway is Dr. Terri Roth, director of Cincinnati Zoo's Center for Conservation and research of Endangered Wildlife. It was under her guidance that Andalas and two other rhinos were born.
Roth was able to determine when a female was ready to ovulate so she could be introduced to a mate at an optimally fertile time.
That's what happened with Ratu and Andalas.
Andalas, born at the Cincinnati Zoo in 2001, was raised at the Los Angeles Zoo. In 2007, the young pachyderm journeyed for 63 hours by plane, truck and ferry to the Sumatran Rhino Sanctuary, 250 acres in Indonesia's Way Kambas National Park.
Ratu was born in the wild and had wandered into a village near the park. Ellis said villagers did not know what to make of her; some thought she was an overgrown pig.
After months of gradual introduction by scent, sound, sight and physical proximity, Ratu and Andalas mated.
Ellis and other animal conservationists recognize that breeding in captivity can never be a substitute for protection in the wild, but at this critical point in 50 million years of rhinoceros history, it's essential to keep the species from extinction.
"It's the right thing to do," Ellis said about rhino recovery efforts. "Once they're lost, they are gone forever."