How India’s shrub frogs crossed a bridge to Sri Lanka – and changed forever

Hindu legend has it that the deity Rama built a bridge from the southern tip of India to the island of Sri Lanka, for his army of ape men to cross and help him rescue his wife, Sita, from the clutches of the Ravana.

It’s a story immortalized in the Sanskrit epic the ‘Ramayana’ and venerated as myth. But the idea of creatures crossing from one side to the other over a land bridge is far from a myth: A recent study shows a group of frogs made the crossing some 27 million years ago, settling and evolving in Sri Lanka’s highlands, before some crossed back again about 8.8 million years ago.

The study used time-calibrated multi-gene phylogeny, a theory that sheds light on how organisms are related to one another through evolutionary time, to analyze the tempo of diversification, or the rate at which a species evolves. By studying diversification in the context of past climate and geography, the researchers hope to identify historical reasons that may be driving current trends of diversity and distribution.

“The isolation of islands contributes to high endemism,” taxonomist Rohan Pethiyagoda, a co-author of the study and former deputy chair of the IUCN’s Species Survival Commission, told Mongabay.

That’s also the case with Sri Lanka, where the “three main mountain ranges of this region constitute ecological islands of cool, wet habitats separated by low, warm and often drier valleys,” according to the paper, published in the March issue of the journal Molecular Phlyogenetics and Evolution.

However, the habitat associations of these species also indicate that a cooler climate itself is not enough for their survival. “They also need specific habitat conditions, and the most of important of these is forest cover. Forest cover coupled with a cool wet climate effectively serve as a refugium,” said Madhava Meegaskumbura, an evolutionary biologist and lead author of the study.

Climate-driven evolution

The descendants of those frogs that made the original crossing — during a period of climate cooling and drying that exposed the land bridge of low islands today known as Ram Setu (Rama’s Bridge) — constitute a group of shrub frogs from the genus Pseudophilautus. Today they’re endemic to the Western Ghats of southwestern India and to Sri Lanka and exhibit a wide range of diversification, though dozens of known shrub frog species are now considered extinct.

After arriving, their endemic diversification largely took place within the wet tropical regions of Sri Lanka — a vivid story about species evolution, geography, geology and historical climate change.

The study highlighted how the hills of Sri Lanka served as “species pumps as well as refuges” throughout 31 million years of evolution, highlighting the importance of tropical montane regions for both the generation and maintenance of biodiversity.  Pseudophilautus today occupy several habitat types, ranging from open grasslands, rock substrates along streams, and rainforest canopy, to shrubs, open grasslands and human-influenced habitats.

While the frogs’ initial habitat during the Oligocene epoch some 27 million years ago was the mountainous central hills, they later repeatedly colonized areas beyond. There was also a frequent pattern of sister species being distributed across the three mountain regions and diversification within a given geographic region.

Changes in the climate played an important role in the geographic distribution, dispersal and process of speciation, providing an opening for the frogs’ colonization of Sri Lanka.

The gradual diversification of their lineages began during the Oligocene, as the Indian subcontinent pushed up against the Asian land mass, creating the Himalayan mountain range and thus initiating the monsoon cycle.

This was followed by a period of glaciation in Antarctica, ushering in a climate suitable to the spread of species adapted to cool and wet conditions, such as those that would have existed in Sri Lanka’s central hills.

Further up the geological time line, lowland lineages arose as the Asian monsoon cycle strengthened, giving rise to lowland rainforests that would have enhanced the frogs’ opportunity for diversification.

While there was nothing to indicate a rapid initial burst in the Sri Lankan diversification of these shrub frogs, there was a constant rate of diversification over the entire history of the clade, the study shows.

Sri Lanka’s habitat complexity provided opportunities for isolated populations of the same species to evolve in different ways. The current exceptional diversity of Pseudophilautus within Sri Lanka owes much to the long period of time available for diversification, abundant ecological opportunity, habitat, and fewer competing lineages, Pethiyagoda said.

Favorable period

The traffic wasn’t one-way, though; during the late Miocene epoch, about 8.8 million years ago, another glaciation event once again exposed the land bridge between India and Sri Lanka. This allowed the migration back to the Indian mainland of some of the frogs, now vastly different from the species that made the first crossing all those millions of years earlier.

Those back-migrating Pseudophilautus were possibly lowland forms that were probably adapted to drier conditions. Today, the lowland regions of India and Sri Lanka adjacent to the shallow Palk Strait that separates the two land masses are warm and dry, with scrub forest habitats in which a single species of Pseudophilautus occurs on either side: P. regius in Sri Lanka and P. kani in the Western Ghats.

The period between the initial migration and the subsequent back migration also coincided with the intensification of the monsoon, indicating a period favorable for diversification of the species in Sri Lanka. The initial migration to Sri Lanka may have provided abundant ecological opportunity to the early colonizers in the form of habitat unexplored by other frog species, the study says.

When diversification of these shrub frogs began to take place, a process facilitated by climatic events, suitable habitats likely became available gradually over time. The species that were adapted to lower elevations — and warmer and drier conditions — were more likely to be sources of dispersal among mountain ranges, unlike those species restricted by elevation, the study says.

During climatically favorable periods, the species crossed the valleys between mountain ranges, giving rise to montane communities made up of members derived from disparate clades.

These features highlight the importance of montane regions in generating and sustaining the remarkable diversity of Sri Lankan shrub frogs through time.

“It is important however, to note, while there is high endemism, there is also high extinction, at the higher elevations,” Pethiyagoda said.

And this happens, according to Meegaskumbura, when lowland rigorous species move up the mountains displacing the more vulnerable Pseudophilautus species. “As climate warming continues, unable to find high enough mountains to track their optimal climatic conditions, many of these species risk going extinct.”

Citation

Meegaskumbura, M., Senevirathne, G., Manamendra-Arachchi, K., Pethiyagoda, R., Hanken, J., & Schneider, C. J. (2019). Diversification of shrub frogs (Rhacophoridae, Pseudophilautus) in Sri Lanka — Timing and geographic context. Molecular Phylogenetics and Evolution, 132, 14-24. doi:10.1016/j.ympev.2018.11.004

Banner image: Pseudophilautus femoralis, commonly known as the round-snout pygmy frog, belongs to the Rhacophoridae family and is endemic to Sri Lanka. Its natural habitat is subtropical or tropical moist montane forests and is threatened by habitat loss. Photo by Hiranya Sudasinghe.