It is still a scientific mystery why the Romer's tree frog initially could be found on four islands in Hong Kong - Lamma, Lantau, Po Toi and Chek Lap Kok - but nowhere else in the world.
Discovered by ecologist John Romer in 1952 in an abandoned cave on Lamma Island, the frog has long been feared to be on the verge of extinction.
But scientists now believe it has successfully survived Hong Kong's booming development. Thanks to a translocation programme in the past few years, the frog can now be found in at least nine places across Hong Kong. An active, sustainable wild population now exists.
The frog is tiny - as small as a fingertip - and loves to stay in wet, shadowed forest areas close to small ponds with still water.
In the early 1990s, an extensive relocation was carried out as its Chek Lap Kok habitat was flattened and reclaimed to build the airport.
They were moved to eight sites in country parks and reserves. Some were also sent to a laboratory at Kadoorie Farm and the Botanic Garden for breeding before they were reintroduced to the wild. Visitors may be lucky to see the frog at the Hong Kong Botanic Garden where it was introduced.
It is estimated that more than 1,000 adult frogs and 1,600 tadpoles were released into the wild in 1994 and 1998. But subsequent monitoring found that at least two sites, one in Tai Po and another in Fanling, did not prove successful. Researchers have been unable to find any frogs and it is thought the environment might not have been suitable for them.
Louis Cheung Yung, a Kadoorie Farm researcher who conducts monthly surveys of the frogs released in the hills next to the farm, said the translocation programme was a success.
Last year 13 to 15 adult frogs were found from June to August, from at least five in previous years. The three-month period is the optimum time when new adult frogs can be found after the breeding season which begins in March. The number of eggs found has also increased. At least 60 pots of water are placed in the hills for the shy frogs to lay eggs in.
'They did lay eggs there. But it is more difficult to find the adult frogs which laid them. What we have to do is to go out at night and listen carefully to the frogs singing,' he said. 'It is rather loud compared to its tiny size. We can hear it even when it is 10 metres away from us. Once we get the direction, we go ahead to search with our torch.'
Simon Chan Kin-hung, wetland and fauna conservation officer at the Agriculture, Fisheries and Conservation Department, said there were no immediate threats facing the frogs which were living in country parks, conservation areas or on remote islands.
In 2002, a site in Ngong Ping on Lantau, which is home to a large population of the frogs, was also designated as a site of special scientific interest, he said.
'With all those conservation and translocation efforts, we believe the frogs now form a sustainable population, though it is very difficult to estimate their numbers,' he said.
Mr Chan said local ecologists and biologists had increased their knowledge of the frog in recent years. But some questions remain unanswered.
'How come the frog could only be found on remote islands like Po Toi, Lamma and Lantau? Why does it not exist on Hong Kong Island and Cheung Chau? How did they get to those remote islands?' he said.
Mr Chan said a leading national amphibian researcher based in Chengdu, Sichuan, found no Romer's tree frogs during studies in Guangdong. Mr Chan said more studies were needed to unravel the mysteries of the frog.
Name: Romer's Tree Frog (Philautus romeri)
Characteristics: X marks on its back, brown
or red in colour, 1.5cm to 2.5cm long
Feeding: small insects
Breeding: still, unpolluted water ponds
Mating: from March to September
Habitats: shadowed forest area
Threats: no immediate threats
Protection: protected species in Hong Kong