Now 34, Ang has been fond of monkeys for a long time. When she was 10 years old, her family was given a juvenile African vervet monkey. Not grasping the difference between wild animals and domestic pets, the young Ang raised the monkey – affectionately named “Ah Boy” – as if he was a pet dog.

She took him on daily walks around her Singapore neighbourhood after school and fed him home-cooked food. Ah Boy would climb on her shoulders to groom her scalp. But as the monkey grew bigger, he became increasingly miserable chained up in her flat and Ang made the painful decision to let him go.

With the help of the Animal Concerns Research and Education Society (Acres), a Singapore-based animal conservation group, in 2004 she raised the funds to repatriate Ah Boy to a sanctuary in Zambia. It broke her heart to bid farewell to her beloved friend, says Ang.

“The last memory I have of him was at the Singapore Zoo where he was housed for a while before being sent back to Africa. I sneaked in a chance to get close and he reached out his hand towards me.”

Ang has travelled across Asia to study a variety of monkey species: Tonkin snub-nosed monkeys, Indochinese silvered langurs, black-shanked doucs in Vietnam and white-handed gibbons in Thailand. But a species closer to home fascinated her.

Ang first became intrigued by the little-known Raffles’ banded langur in 2008. Among the rarest of Singapore’s native species, these furry black-and-white creatures with white rings around their eyes were ‘discovered’ by Sir Stamford Raffles in 1821. Once thriving in Singapore, now just 60 of these monkeys are still left in the wild, living high in the forest canopies of the Central Catchment Nature Reserve.

Time is running out for these creatures as the city encroaches on the island’s last vestiges of wilderness. It has been announced that the upcoming mass transit Cross Island Line will run under the Central Catchment Nature Reserve, which Ang fears may make the langur population even more vulnerable. The monkeys are also showing troubling signs of inbreeding.

There are an estimated 250 to 300 langurs left in the Malaysian states of Johor and Pahang, says Ang, who visits Malaysia every month, but illegal poaching and logging to make way for palm oil plantations threaten the habitat of this langur population.

To ensure long-term survival of the species, Ang says it is critical to consider genetic exchanges between the langurs in Singapore and Malaysia, or a “Tinder for monkeys”, as she wryly calls it. But species translocation for conservation is a delicate exercise, and high-level, country-to-country discussions can become mired in politics.

“You want to take a monkey from one country and put it into another,” she says. “What if it dies in the process? Who’s going to be responsible? It might affect relations between the countries because we’re talking about endangered species.”

Elsewhere in the region, Ang has been involved in efforts to relocate endangered Indochinese silvered langurs in Kien Luong in southern Vietnam, a district known for its spectacular rock formations.

With construction companies driving demand for limestone, developers are mining the caves and hills where these monkeys live, all but pushing them out into the danger of open sea and roads.

Fearing the langurs might fall and die on the rocks if they were tranquillised for relocation, Ang, the Vietnamese authorities and agencies including the Southern Institute of Ecology have been negotiating with the mining companies since 2014. The idea was to encourage the companies to plant mangrove trees on the coastline below the hills, and construct rope bridges to serve as crossings for the monkeys to move from hill to hill and on to other protected areas.

The first rope bridge was finally set up last month and camera traps will help determine how many langurs and other animals use it.

Creative solutions were also needed for problems connected to Singapore’s most commonly seen monkey, the long-tailed macaque. With a population of 1,500 to 1,800 individuals, wild macaques have become accustomed to being fed by people and now they sometimes climb into homes in search of food, leading to complaints of aggressive behaviour.

“People want to be close to nature, but they are selective about what kind of nature they want to see,” Ang says. “They want butterflies and hornbills, but not pythons, mynahs, monkeys and wild boars. It takes a lot of understanding, effort and collaboration to raise more awareness about living in harmony with wildlife.”

Now a group of residents, security guards and volunteers helps patrol the residential areas visited by the macaques. They know how to identify the troupes’ alpha males, and if they see one, they repeatedly hit the ground with a stick to make a noise that creates an “invisible barrier” so the primates learn not to approach the area again.

Ang’s colleagues at the Jane Goodall Institute drew up a macaque family tree listing each macaque by name and noting their unique personality traits, so people would “learn to see them as individuals and not as pests”, she says.

Security guards had regarded these monkeys as a problem because of residents’ complaints, but these days they have a new-found affection for them, says Ang.

“One time, a macaque got hit by a car and was injured and the security guards were very concerned,” she says. “They would tell us, ‘Hey, Leo got knocked down’. Or they would say things like, ‘Finally, Leo didn’t come into the condo today. He’s learning; he’s just sitting on the palm tree’.

“They feel invested because they see the monkeys every day,” she adds. “They’ve become primate watchers.” Attitudes need to change more broadly in Singapore, Ang believes.

“People always think, ‘Monkeys in my trash bin, monkeys in my backyard, monkeys taking my plastic bag’. But on nature walks, you see them just feeding on plants from the forest, not your McDonald’s, having fun, grooming each other, and not attacking people.

“It can get depressing sometimes when people don’t value wildlife and natural habitats. We often hear, ‘The monkeys are everywhere’, ‘they should be ‘removed’ and ‘what’s so important about protecting monkeys?”

“When the otters crossed the road in Shenton Way, many said, ‘So cute’!” she points out, referring to a busy road in downtown Singapore. “But if the ones crossing the road were monkeys, you can imagine the headlines: ‘Monkeys terrorising the Central Business District’ freaking everybody out.’”

Ang has one important wish for Singapore’s primates: “For our monkeys not be treated as second-class citizens, and that we can be more appreciative of what we have.”