Banner image of an adult hourglass tree frog (Taruga eques) with yellow dorsal coloration, from Sri Lanka’s central montane forests

• A study into an endemic tree frog in Sri Lanka, Taruga eques, has found wide variations in both color and dorsal marking.
• The authors of the study say this high variance, or polymorphism, squares with what researchers already known about other tree frog species, and makes T. eques’s common name — hourglass tree frog — a misnomer.
• Polymorphism is important in amphibians to help regulate body temperature and evade predators, but hasn’t been researched in frogs in Sri Lanka until this new study, despite the island being known as a biodiversity hotspot.
• Amphibians like T. eques are threatened by habitat loss, forest fragmentation and climate change.

Is an hourglass frog still an hourglass frog if it doesn’t have the distinctive marking on its back that gives it its name? That’s one of the interesting questions thrown up by a recent study that looks at, among other things, the various forms of a species of hourglass frog found only in Sri Lanka.

Taruga eques, also known as the montane hourglass tree frog or Günther’s whipping frog, was described in 1858 and inhabits high-elevation forests, including in Horton Plains National Park, a UNESCO World Heritage site. Like other amphibians on this Indian Ocean island, no studies have ever been carried out into the physical variations, or polymorphism, in individuals of the species — an omission that researchers Praneeth Silva and Dharshani Mahaulpatha sought to address with their study. They looked at nearly 500 hourglass frogs from both inside Horton Plains and outside, as well as other frogs from the same region between January and November 2017.

Their subsequent findings are that there’s considerable variation between individuals within the same species. Only 77% of the hourglass frogs found inside the national park had a pattern that could be considered a distinct hourglass shape, while the rest ranged from an “aberrant” shape to not having an hourglass pattern at all (11%).

A juvenile hourglass tree frog (Taruga eques) from Sri Lanka’s central montane forests, featuring a deviant hourglass pattern.

Distinct hourglass pattern

Outside the park, the variations were more pronounced: only 52% of the frogs had a distinct hourglass pattern, while nearly 20% had none.

“Present study was … the first to discover Taruga eques without characteristic hourglass pattern,” Silva and Mahaulpatha write in their study. “These discoveries indicate that common name of Taruga eques has not been convincingly entitled and it cannot be broadly classified or confined as an ‘hour-glass frog.’”

A juvenile Taruga eques without the hourglass pattern.

The variations didn’t end there. The individual frogs also ranged in color from green to yellow to purple, and several shades in between. Again, those differences were more marked outside Horton Plains National Park, where nearly two-thirds of the frogs studied were dark brown; inside the park, the vast majority, 93%, were green.

But far from prompting the naming of a new species based on these differences in appearance, the findings  square with what researchers already know about the high incidence of polymorphism in other tree frog species.

An adult Taruga eques with dark brown dorsal coloration.

“The present study indicated that endemic Taruga eques possess diverse colour and pattern polymorphism as other tree frogs in the world,” the study says. Effectively, the hourglass pattern is itself just one of the many markings (or lack thereof) that this species can exhibit, rather than an inherently identifying trait.

Silva said polymorphism is likely the reason that may have led to T. eques being named an hourglass tree frog — that is, the type specimen first described in 1858 had an hourglass pattern — but that in reality there’s sufficient diversity in both color and pattern that the name doesn’t do it justice.

An adult Taruga eques with dark orange dorsal coloration.

Advantage against predation

Polymorphism is important to many species, and particularly to amphibians, where coloration can help with regulating body temperature, the researchers say. It’s also influenced by other environmental factors, such as the need to evade predators and blend into the vegetation.

“Presence of dorsal pigment patterns may be a selective advantage for amphibians specially tree frogs for disrupt the expression of bright dorsal colouration for visually oriented predators,” the study says. It calls for further studies to be carried out on the subject to determine whether frogs with certain colors and patterns have a selective advantage against predation.

A highly distinct hourglass pattern on the back of an adult Taruga eques.

The researchers made similar findings of polymorphism among a smaller sample of three other frog species from inside and outside Horton Plains National Park: Minervarya greenii, Fejervarya kirtisinghei and Fejervarya limnocharis. All three species were previously thought to have a yellow line running down the middle of the back. But the researchers found that 17% of their M. greenii specimens either had a deformed line or none at all, while more than a third of the F. kirtisinghei specimens had a deformed line, and more than half of F. limnocharis had no vertebral line.

Even as these polymorphic variations abound to give the frogs an evolutionary edge against predators, the main threat to the amphibians of Sri Lanka is the loss of their habitat due to human activity.

The dorsal spot pattern of an adult Taruga eques.

Thasun Amarasinghe, a research associate at the Research Center for Climate Change, University of Indonesia, who was not involved in the recent study, said almost all amphibians in Sri Lanka are under threat, especially high-elevation species like the hourglass tree frog.

“These highland species are extremely sensitive to temperature increase and humidity, so in addition to habitat loss and forest fragmentation, their collective future is also threatened by climate change,” he told Mongabay.

Globally, threats against amphibians have escalated so rapidly that conservationists are concerned about mass extinctions. According to Defenders of Wildlife, a Washington, D.C.-based organization that campaigns for species and habitat protection in North America, amphibians are also threatened by their use as exotic pets in domestic and international trade, pollution, pesticides, road building through their habitat, competition from invasive species, and climate change.

Citation: 

Silva, P., & Mahaulpatha, D. (2019). Distribution of colour and pattern polymorphism in montane frogs of Sri Lanka. Journal of Entomology and Zoology Studies, 7(2), 486-491.