As for the existence of mysterious marks, they are actually left by white-headed langurs at their sleeping sites. These clues help researchers monitor the primates, who are now wild animals under first-class national protection, and the total population lives in the Chongzuo reserve.

The white-headed langur's population has experienced a surge from about 600 individuals at the end of the 1980s to more than 1400, by the year 2020.

Decades of conservation efforts, including habitat restoration and surveillance, have helped them bounce back, highlighting the broader significance of preserving biodiversity.

So next time you see a black streak on the cliff, that might just be a monkey's good-night note.

Li Wenzi, Yuan Ziyan, Xiao Yinuo, Zhou Xun, Wang Hong and Zhang Shenmao contributed to the story.