Hainan gibbons are the rarest apes in the world, and a contender for the rarest mammal, but this week saw their chance of survival climb, even if only by a fraction.

A recent survey has discovered a previously unknown family of the critically endangered primate. The three apes, a male and female plus their young baby, increase the known population by a shocking 12%. The finding of this fourth breeding group is welcome news to the scientists who are trying to protect the long-limbed animals, as it increases the reproductive chances of the population as a whole.

“Finding a new Hainan gibbon group is a fantastic boost for the population,” said Dr. Jessica Bryant from the Zoological Society of London, who led the expedition that discovered the gibbons. “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.”

At one time, the ape’s population spanned the whole of Hainan Island, the southernmost province of China, with around 2,000 of them thought to be swinging through the trees in the 1950s. But since then, their numbers have crashed as their forest home has been brought down around them to make room for rubber plantations. The addition of the three new apes brings their numbers up to a still distressing 28.

All the gibbons are born a beautiful golden color and darken as they develop into adolescents, but upon reaching maturity the females amazingly return to their yellow hue, although the males stay black. From the limited field studies of the apes, we know that they reproduce at a painfully slow rate, with females giving birth to one infant every two years.

Yet this reproductive technique does appear to work, as the survival rate of the Hainan gibbons (Nomascus hainanus) is actually quite high: 11 of the 12 known infants survived to adulthood. Yet how the primates disperse once they mature is less clear. Nobody actually knows what happens to the apes once they leave the family group, with only a handful of reports of solitary gibbons by locals, and none by researchers. It is this stage that could be critical to the survival of the species.

“The success of our discovery is really encouraging. 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," said Bryant. "Today is a great day for Hainan gibbon conservation."