Top Guides, a tour company started by Harvard anthropologist Kathryn Levenson, also takes visitors to search for the Vietnamese langur in the country's northern rainforest region.

CNN: ASIANOW - TIME Asia | Travel Watch

After admiring the lion pride, we crisscross the park's dirt roads, passing birds (there are 300 bird species in Gir), langur monkeys with whip-like tails and docile chital (Bambi-esque spotted deer).

WSJ: Jeeping Up on India's Wild Lions

The golden-headed langur of Vietnam and China's Hainan gibbon are down to a few dozen and the Horton Plains slender loris of Sri Lanka has been spotted only four times since 1937.

BBC: NEWS | Science/Nature | Quarter of primates face abyss