When night falls on the military training ground of Pulau Tekong, camouflaged figures emerge from the foliage - silent and invisible. They skulk through the vegetation, each individual on its own mission.
These are not soldiers, but leopard cats - the last remaining wild cat species found in Singapore. (Leopards have gone extinct here, and the last tiger in Singapore was shot in the early 1930s.)
These nocturnal animals, which have unique coats that help them blend into the shadows of surrounding vegetation, were found here in larger numbers during the early 20th century.
But they are now critically endangered in Singapore due to the loss of their natural forest habitats.
Mammal researcher Marcus Chua, 31, estimates that there are no more than 20 leopard cats living on mainland Singapore, in the nature areas located within the Safti Live Firing Area, as well as at the Central Catchment and Bukit Timah nature reserves.
But nature is resilient, and Mr Chua's latest study, published in science journal Mammal Research last month, has given conservationists reason to cheer.
Mr Chua found a larger population of leopard cats on Pulau Tekong, a 23.5 sq km island that is 32 times smaller than mainland Singapore. Data collected shows that they are thriving there. He recorded at least 29 leopard cats on the island, identified through unique coat markings that distinguish the animals in the same way human beings are differentiated through fingerprints.
About leopard cats
• Urbanisation has made the leopard cat a critically endangered species in Singapore, but it is found widely in Asia. In Singapore, they "deserve conservation focus", said Dr Sonja Luz, director for conservation and research at Wildlife Reserves Singapore. "Leopard cats are Singapore's last extant wild feline species," she added.
• Leopard cats are common victims of the illegal wildlife trade. They are hunted for the pet trade, for their bones, which are used in some Asian traditional medicines, and for their fur. It takes around 16 leopard cats to make one fur coat, said mammal researcher Marcus Chua of the Lee Kong Chian Natural History Museum.
• Leopard cats are often involved in human-animal conflict in farming communities. The cats prey on poultry, prompting villagers to kill them.
• Leopard cats can breed with domestic cats to produce, for instance, the popular domestic bengal cat.
SOURCES: WILDLIFE RESERVES SINGAPORE, INTERNATIONAL UNION FOR CONSERVATION OF NATURE, MARCUS CHUA
Using camera trap records and mathematical algorithms, the population of leopard cats on Pulau Tekong was put at about 89 individuals for every 100 sq km - the world's highest.
Leopard cats appear to be doing better on Pulau Tekong than on the mainland as they do not have to compete with other animals, such as civets, which also prey on rats, birds and insects, said Mr Chua, who is curator of mammals and birds at the Lee Kong Chian Natural History Museum. There are no civets on Pulau Tekong.
On the importance of the animals, he said: "Leopard cats are meso predators, which means they are in the middle of the food chain. They are prey for bigger animals like pythons, and also help to regulate the population of smaller animals like rats and birds."
Another encouraging find from his study is that leopard cats are resilient animals, and able to adapt to human-modified landscapes despite the loss of their natural lowland rainforest and swamp forest habitats.
Pulau Tekong is located off the north-eastern coast of Singapore. The island is covered mainly by secondary forest. Most of it dates back 40 years, although the vegetation growing on a 6 sq km patch on the southern part of the island is younger. That area, connected to the original island by two bridges, was reclaimed in 1987.
On the eastern end of the island is an oil palm plantation, which has been changed even more by human intervention. Yet, Mr Chua's study found that most of the leopard cats - 13 of them - were recorded in the plantation. In comparison, eight individuals were recorded in secondary forests located on each of the original and reclaimed parts of the island.
Mr Chua said that the fruiting trees in the plantation attract smaller animals like rodents, turning the plantation into a "buffet table" for the leopard cats. "What I also found surprising is that the leopard cats were also found in the reclaimed part of the island," he said.
However, Mr Chua said that although oil palm plantations appear to be a good place for leopard cats to find food, forests - for shelter and breeding - are still important for their survival. His study, funded by Wildlife Reserves Singapore (WRS), was done over two years, from 2010 to 2012.
Dr Sonja Luz, WRS director for conservation and research, said his findings were encouraging.
"This research provides valuable information on their habitat use, ranging patterns, diet and basic population genetic analyses, which helps us better understand their tolerance levels and conservation requirements...," she said. "Further research to genetically assess the similarity of the Singapore population with neighbouring countries' populations will also be needed to formulate an effective conservation action plan," she added.