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Two species of forest frogs of the genus Platymantis (Family Ranidae) and one species of Gekko (Family Gekkonidae) have probably evolved in forested limestone karst formations in southwestern Negros Island and the Gigante Islands off northeast Panay Island. The caves and rock crevices in these karst formations must have provided a stable, favorable environment for evolutionary processes. Limestone karst formations in the Philippines, however, deserve more studies. It is hypothesized that the ancestral stock of Philippine Platymantis species came from islands south and southeast of the Philippines, and that of Gekko gigante came from either the south-southeast or southwest source areas since the genus Gekko is distributed in the Oriental-Australian region as well as in the eastern Palearctic region.
Two species of forest frogs (Family Ranidae), Platymantis spelaeus Brown & Alcala and P. insulatus Brown & Alcala, and one species of lizard (Family Gekkonidae), Gekko gigante Brown & Alcala were discovered in limestone karst forests of southwestern Negros Island and the Gigante Islands off the northeastern tip of Panay Island in the 1960s.
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Based on current knowledge, of the seven species of Platymantis known from Negros and Panay, only these two species inhabit exclusively deep rock crevices and caves in limestone forests.
The other five species occupy various habitats in both limestone and non-limestone forests. Among the lizards of the genus Gekko, one species, G. gigante, inhabits only rock crevices and caves in the limestone forest and two other species (the Philippine endemic G. mindorensis and the non-endemic G. monarchus) are found in caves in limestone forests and in other habitats in non-limestone forests.
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The two frogs and the lizard we are concerned with utilize rock crevices and caves (entrance and part of twilight zones) as hiding places during most of the day but venture into the open at night to feed. They also use caves as egg-deposition sites. P. spelaeus lays large, yolk-rich eggs in moist (relative humidity, 90-100%), cool (temperatures, lower 20so C) rock crevices inside caves that develop directly into froglets. P. insulatus most probably also does so, like most (or all) other species of the genus Platymantis. Gekko gigante attaches its eggs to roofs or walls of caves.
Karst formations are characterized by the presence of limestone or other soluble rocks like dolomite and marble and by subterranean drainage resulting from the dissolving action of water on the bedrock. Karst topography is dominated by sinkholes, sinking streams, large springs and caves. Cool air in contact with ocean or underground streams may circulate in caves, bringing temperatures to levels lower than those above ground. Caves have therefore provided favorable habitats for the evolution of some animal species such as arthropods (e.g. crustaceans) in Hawaii and vertebrates (e.g. fish) in Mexico. Since P. spelaeus, P. insulatus and G. gigante are strictly confined to limestone karst habitats based on our extensive field observations, they most probably evolved there.
It appears that karst areas, which retain moisture for long periods of time, can confer protection to amphibians and reptiles, provided the forest cover remains intact. This is supported by the fact that two El Niño events in the 1980s and 1990s apparently did not negatively affect the populations of these three species, indicating good prospects for survival in the face of future climate changes. However, a long term study of amphibians and reptiles inhabiting rocks and caves in limestone forests is needed.
The origin of the ancestors of the three species may be related to the geology and geological history of Negros and Panay Islands. The limestone karst habitats of these three species are part of the Negros volcanic arc, dating back to the Miocene Epoch (between 5 and 24 million years before Present)
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Geological evidence supports the view that the islands of Negros, Panay, Cebu and Masbate formed one Ice-Age island mass when the sea level was 100-120 meters lower than the present level about 160,000 and 20,000 years ago during the Ice Age (Pleistocene Epoch)
The ancestral stock that gave rise to the Philippine platymantine frogs in the Philippines (present estimate of the number of Platymantis species 36-40) came from islands of the Southwest Pacific, south and southeast of the Philippines (Fig. 6). This happened during the Pliocene-Pleistocene Epochs 5-1.6 million years ago. As presently known, the distributional range of Platymantis species in the Philippines includes the Luzon, Mindanao, and Visayas Ice-Age islands as well as Mindoro, Sibuyan, Siquijor Islands. But these frogs are absent in the Palawan and the Sulu Ice Age islands
The ancestors of Gekko gigante may have colonized the Philippines in the same manner, but there is an alternative route through southwest Philippines, that is, from Borneo via the land-bridge Sunda island of Palawan and the Sulu Archipelago, the entry pathway of the ancestors of many Philippine herpetofaunal species.
A study in progress of other limestone karst formations in the Philippines may reveal findings similar to those reported in this paper.
We thank HARIBON Foundation for the financial support of our study of the unique herpetofaunal species on Negros and Gigante Islands. An earlier study supported by the ASEAN Regional Center for Biodiversity Conservation contributed basic information on Platymantis spelaeus. We also thank Jasper Maypa for preparing the illustrations
