- A new study shows how the Rhacophoridae family of Old World tree frogs has come to occupy a wide variety of environments across Asia and Africa, thanks to adaptations in its breeding methods.
- Frogs typically spend their larval stage in water as tadpoles, but different Rhacophorid species have also adapted other methods: gel nesting, foam nesting, and direct development.
- Foam and gel nests help the frogs lay a large number of eggs in more open and drier habitats to keep the eggs from getting dry, while direct development gives them an edge in spreading into warmer and drier areas where there are no water bodies.
- The study authors say the revelations about the frogs’ evolutionary past will be useful in predicting their responses to current and future climate changes, and hence their conservation.
COLOMBO — Amphibians evolved from a fish ancestor that had functional lungs and bony lobed fins, which helped them become the first vertebrates to conquer land. Keeping these earlier trails, today’s frogs and toads also have larvae with gills that need an aquatic environment until they transform into air-breathing adults. But this unique feature hasn’t restricted members of the frog family Rhacophoridae from branching out into a wide range of environments, thanks to the adaptation of different reproductive methods, a new study shows.
With 22 genera and 432 species, the Rhacophoridae family of Old World tree frogs represents about 6% of the world’s frog and toad species. They’re believed to have evolved from a common ancestor that lived in northeastern Asia about 68 million to 53 million years ago.
In their native ranges today in Asia and Africa, Rhacophorid frogs are found in a rich variety of biogeographic regions, from strips of continental mainland to archipelagos, and from continental islands in East and South Asia, to montane regions such as the Himalayas.
Diverse breeding methods
“We tried to answer the key questions of evolution including whether reproductive mode evolution in these frogs is an adaptation connected to specific climatic events,” said study lead author Gajaba Ellepola, an evolutionary ecologist at the University of Peradeniya and Ph.D. candidate at the College of Forestry at Guangxi University, China.
Rhacophorid frogs’ early ancestors could have used the main aquatic breeding method of laying eggs directly on water bodies, with their larvae spending a completely aquatic life until their transformation. But Ellepola and his team now show that these frogs use three additional reproductive methods: laying eggs in gel nests, in foam nests, and a method known as direct development.
Gel nests and foam nests are built on the edges of water bodies, allowing the hatched tadpoles to slip into the water with ease. In the case of gel nests, the female frog secretes a gelatinous substance after depositing the eggs. It’s the same with foam nests, except that the male frog, while clinging on to the female’s body during copulation, beats its legs so that the gelatinous secretion turns foamy.
Direct development is when the froglets emerge directly from the eggs. The Rhacophorid frogs that use this method lay their eggs on a leaf or on moist soil; the tadpoles that emerge from the embryo are held captive inside the egg and emerge as froglets once ready.
“Foam nests help the frog to lay a large number of eggs in more open and drier habitats to keep the eggs from getting dry, but the ‘direct development’ is an adaptability that gives an edge to the frog to spread into warmer and dry areas where there are no water bodies,” Ellepola told Mongabay.
Researchers identified these methods as key evolutionary innovations, or KEIs, that have allowed Rhacophorid frogs to diversify and establish themselves in niches that were not previously available to them. When the researchers mapped the areas where these tree frogs dwell, they found that the typical method of laying eggs directly in water was restricted to those species found in East and Southeast Asia.
“This indicates that the other species adapted to ‘direct developing’ and ‘foam nesting’ which could spread to other regions, while those that needed water to lay eggs were restricted to the area of the family’s origin,” Ellepola said.
The researchers also constructed a phylogenetic tree to study the history of the evolution of Rhacophorid frogs. They used a new method to map out this family tree of 415 Rhacophorid species.
“This phylogeny or the mapping of history of evolution is the most comprehensive species tree for rhacophorid frogs to date,” said study co-author Madhava Meegaskumbura, an evolutionary biologist at Guangxi University’s College of Forestry.
The study confirms that the ancestral members of the Rhacophoridae family lived in northeast Asia. They then began to spread out to cool and wet areas from their center of origin by adapting to new climatic conditions or by dispersal during periods of favorable climate, the study shows.
“We found near-constant diversification rates across the phylogeny, but a highly uneven, phylogenetically clustered, distribution of species richness throughout Rhacophorids’ range,” Meegaskumbura told Mongabay.
The study also confirmed that montage regions on islands and some mainland regions with cool-wet climatic conditions have higher genetic diversity and endemism. These include the montane islands of Sri Lanka and Indonesia, as well as elevated regions around the Himalayas — regions where Rhacophorids emerged as endemics more recently.
The results of the genetic study indicate that after the early ancestors of Rhacophoridae colonized the East and Southeast Asia region, their descendants adapted to rising temperatures during a global warming event that occurred about 40 million years ago, during the Middle Eocene era. The researchers’ mapping finds that these adaptations coincided with the evolution of these frogs’ terrestrial reproductive modes, i.e. the use of foam nests and direct development. A subsequent global cooling event followed, reducing sea levels and allowing these warm-adapted ancestors to jump into adjacent areas between climatically harsh regions.
Meegaskumbura said the significance of this study is that they tested how lineages or sections of family trees used optimal climatic conditions and also how they adapt to new conditions. The knowledge generated, he said, will be useful in predicting the responses of these frogs to current and future climate changes, and hence their conservation.
Ellepola, G., Pie, M. R., Pethiyagoda, R., Hanken, J., & Meegaskumbura, M. (2022). The role of climate and islands in species diversification and reproductive-mode evolution of Old World tree frogs. Communications Biology, 5(1), 347. doi:10.1038/s42003-022-03292-1
Banner image of a pair of mating round-snout pygmy frog (Pseudophilautus femoralis), a species found only in Sri Lanka, while laying eggs on a leaf using the “direct development” method. Image courtesy of Madhava Meegaskumbura.