The Hainan gibbon is the rarest ape on Earth, with its entire population squeezed into a single nature preserve on a single island off the coast of mainland China.

The island of Hainan was home to about 2,000 of these gibbons in the 1950s, but they were obliterated over the next few decades by rampant hunting and habitat loss. Although they're now protected under Chinese law, their population is still down to just 25 individuals living in three social groups — or so we thought.

A research team has found a fourth group of Hainan gibbons in the Bawangling National Nature Reserve, according to a news release from the Zoological Society of London (ZSL). This group only has three gibbons, but that's enough to increase the critically endangered species' total population by 12 percent in one fell swoop.

And there's even better news: The three gibbons in this group are a family, consisting of a mother, a father and a young baby. The discovery of a breeding group was unexpected, says ZSL researcher and expedition leader Jessica Bryant, and it provides badly needed optimism for a species on the brink of extinction.

"Finding a new Hainan gibbon group is a fantastic boost for the population," Bryant says. "We had hoped to locate at least one or two solitary gibbons, but discovering a whole new family group complete with a baby is beyond our wildest dreams."

Hainan gibbonOne of the newly discovered Hainan gibbons peers down at a researcher in June. (Photo: Chanee Kalaweit/ZSL)

Gibbons are apes, not monkeys, that inhabit forests across southern Asia from India to Borneo. (They're known as "lesser apes," due to smaller bodies and less sexual dimorphism than great apes like chimpanzees, gorillas and orangutans.) They're divided into 15 species, all but one of which are listed as endangered or critically endangered. Many fall victim to poachers, but perhaps their greatest threat comes from habitat loss and fragmentation due to deforestation.

Gibbons are monogamous, which is relatively rare among primates. They live in family groups of an adult pair and their offspring, staking claim to a territory with loud, complex songs that can echo for miles. These calls help researchers track the apes through dense forest, but Hainan gibbons' low population density leads them to sing less, the ZSL notes, since there are few neighbors around to hear them.

That can make them difficult to find, so Bryant and her colleagues used new acoustic techniques that prompt gibbons to make investigative calls. That's how they discovered this previously unknown family of three, raising hopes that Hainan gibbons won't be the first ape species wiped out by human activity.

The sighting of a baby is especially good news, since female Hainan gibbons only give birth to one infant every two years. That's a low reproductive rate, but the species seems adept at parenting when given a chance: Research suggests about 92 percent of infants survive until they're subadults. Little is known about what those subadults do after leaving their birth groups, one of many questions researchers hope to answer as they continue searching for signs of this elusive ape.

"The success of our discovery is really encouraging," Bryant says. "We now want to learn more about this new group, and also hope to extend the investigation to perhaps even find additional solitary gibbons or other groups. Today is a great day for Hainan gibbon conservation."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.