Chinese researchers have declared two gibbon species extinct because they have not been spotted in their southern China habitat for more than a decade.

Key points:
A species classified as "extinct in the wild" is now found only in captivity
The population of the white-handed gibbon has declined steadily due to habitat loss, hunting and pet trading
Both species could be found in the south-western Chinese province of Yunnan
A report into the status of endangered primates in China, released on Tuesday, says the white-handed gibbon and the northern white-cheeked gibbon are classified as "extinct in the wild", according to Chinese state media.

Extinction in the wild means the species is only known to survive in captivity.

Researchers surveyed areas where the white-handed gibbons and northern white-cheeked gibbons used to live but found no sign of the animals.

Head of the survey team at the Northwestern University of China, Li Baoguo, told local media that habitat loss, over-exploitation of resources and expanding agriculture were the leading causes of the species' declines.

"Human activities are the root cause of the unprecedented loss of biodiversity, and this has led to a serious survival crisis for primates," Mr Li said.

More likely it was climate change, ending of the major ice age.