From 27 to 6: fewer Ugly Brown Frogs than previously thought

As to be expected from a biodiversity hot spot, the island of Borneo is home to a particularly large number of frog species. Ugly Brown Frogs, as this group of the genus Limnonectes is more or less affectionately called by its researchers, are among the most common and widespread. They can be found on Borneo in almost every body of water, from sea-level pools to streams and swamps in the high mountains. Typical features of these frogs include a somewhat stocky body, a variable brown coloration and tooth-like structures in the mouth, particularly in the males. Although the expression of many characteristics, such as color pattern, size and head shape, varies considerably across many populations, they were originally all considered to belong to one or a few species. A few years ago, initial genetic studies revealed that there are in fact many species, which, however, can hardly be reliably distinguished from each other on the basis of external characteristics. These so-called cryptic species exist in many animal groups, with this phenomenon being particularly common in tropical habitats. No fewer than 27 species have now been described from what was originally a single species of the Ugly Brown Frogs. 

Under the direction of the NMBE, very recent studies have shown that, although many of these species exhibit genetic differences, they nevertheless interbreed freely. This means that an important criterion for the recognition of new species has not been met: members of a biological species reproduce only among themselves and do not interbreed with members of other species. Consequently, the number of species in this complex of Ugly Brown Frogs is reduced again to six. This elaborate study required many years of international collaboration and fieldwork on Borneo to collect and analyze hundreds of samples of these frogs. 

This example clearly demonstrates that the emergence of new species is usually slow and gradual and always depends on external influences such as climate and geology. The evolution of this genetic diversity and the resulting species was most likely influenced by the formation of the mountains on Borneo as well as by climatic influences that have also changed the distribution of vegetation in the tropics over the last 30 million years. The precise knowledge of species, their distribution and ecology is important not only for the basic understanding of evolution, but also for the practical conservation of nature. As is the case all over the world, the habitats of Borneo are threatened by human activity. The once almost untouched rainforests have largely disappeared and have been replaced by settlements, roads and agriculture, especially oil palms.