"Hush! They can hear us," Li Jiahong presses his hand to his lips, whispering.
Just meters above, two monkeys dangles on their long arms from the branches for a while, and then stop to rest under the cover of the thick canopy.
Li, in his mid-50s, and his team are patrolling the Nankang Station area of Gaoligong Natural Reserve, in southwest China's Yunnan Province. The two monkeys they have been following all day are eastern hoolock gibbons, an endangered primate only found in southwestern Yunnan, northeast India and northern Myanmar.
A ranger for 19 years, Li could still pass as a school teacher once he's taken off he's green hat.
Li became fascinated with tales of the "black monkey", the local name for the hoolock when he began work as a principal at a primary school in a village outside Baoshan City in 1980.
"Local lore had it that the black monkeys were monogamous, pairing for life, and the locals even believed their brains could cure epilepsy," Li recalls.
He went to the Gaoligong Mountains during school vacations to watch the hoolocks. Until in 1995, he quit his school job and went to work at Longyang Station in Gaoligong Natural Reserve.
The reserve, on the southeast edge of Qinghai-Tibet Plateau, covers more than 4,000 square kilometers and was listed by UNESCO as a World Biosphere Reserve and World Natural Heritage site in 2003.
Tracking hoolocks is never easy. Li planned the current excursion with a group of visitors a week in advance. Thanks to the fine weather and a bit of luck, he found signs of hoolocks near his current base at Nankang Station. A few hours' walk in the forest was enough to find them.
When he first started as a ranger, Li had no GPS or climbing gear, just cheap rubber shoes and ex-army camouflage fatigues. The station was a rundown shed made from bamboo and sticks.
"Finding a flat and dry place to rest during a patrol was a luxury," he recalls.
More challenging were the mountains. The ever-changing weather and varied terrain made every patrol unpredictable. Rangers could be dripping sweat in hot and damp subtropical forest in a river valley, and frozen two hours later on a snowy mountain ridge.
But to find the hoolocks, he had to get familiar with its habitat with days on the trail. Sometimes he set out at midnight, sleeping under trees where the hoolocks might be. More than once, he heard their howls and watched them from afar.
