The new species of frog was discovered when researchers came across a ‘strange’ looking tadpole. After continuously searching for an adult for 2 years, they spotted around 100 adult frogs around a roadside puddle. Photo by SD Biju.

Hidden spots and an insect-like call

The researchers observed that the frogs started congregating in hundreds around temporary muddy puddles, two to three days after the first monsoon showers. After four to five days of intense breeding, the frogs disappeared completely, leaving the researchers mystified.

“After we first discovered this frog in 2015, we carried out several surveys in and around the region over a period of three years to study this frog. However, due to its secretive behaviour we were only able to locate it during a very short window of less than four days,” said the authors in an interview.

After the frogs disappeared, the researchers were not able to locate “even a single individual” at any other time of the year. “We don’t know where it hides, lives, and what it does for the rest of the year. The frog’s external appearance does not show morphological adaptations for burrowing. At the same time, we doubt that it simply hides under leaf litter, rocks and stone (the usual hiding places for frogs during the non-breeding time). It’s still a mystery for us,” they said.

When the males called to attract females, they raised the hind part of the body to show off “a pair of black false-eye like spots.” The frogs did the same when the researchers tried to approach them, said Garg.

“The effect is quite startling,” she said. “When the animal is sitting down, the spots are hidden. When we were close to the animal, the frog raised the hind part of its body. This movement really made the spots very visible.”

“The best guess we have is that it’s a defensive mechanism,” she said.

The call of the frog is also quite distinct, observed the researchers. “It resembles an insect chorus,” write the authors in the press release.

The frogs probably had such a unique call to attract females in the most efficient manner, said Garg. “Even if a puddle is crowded with multiple frog species and multiple individuals of the same species, even if it’s pitch dark, the female needs to be able to find her way to the male,” she said. “This is one reason to have unique calls. Also, for this species, with such a short breeding window of four or five days, the pressure to get it right is much higher,” she added.

New-frog-species-false-eyes
The frog has 2 distinct eye-like spots on its rear-end which probably serve as a self-defence feature. Photo by SD Biju.

A mysterious past

A phylogeny (a sort of family tree) of all known genetic data known from the family Microhylidae shows that the closest relative of the subfamily Microhylinae is the subfamily Dyscophinae, which is restricted to Madagascar.

The available molecular evidence gives us this story. The family Microhylidae would have originated on Gondwanaland, the ancient supercontinent which would eventually break up to form most of South America, Africa, the Indian subcontinent, Australia and Antarctica. When Gondwanaland broke up, the subfamily Dyscophinae took up home in Madagascar and subfamily Microhylinae moved on toward Asia on the Indian subcontinent. The split between Dyscophinae and Microhylinae happened about 67 million years ago, giving Microhylinae enough time to diversify in the Indian subcontinent as it drifted along towards Eurasia. Once the Indian subcontinent docked at Eurasia, frogs that make up the Microhylinae subfamily spread all over Asia.

For M. franki in particular, the closest relative on the family tree is the genus Micryletta, also belonging to subfamily Microhylinae but found in the Indo-Burma and Sunderland biodiversity hotspots in Southeast Asia and China.

Using algorithms that can parse out evolutionary timelines by considering the rate at which DNA changes over time, the researchers were able to give tentative dates to different nodes of the phylogeny.

“Our study shows that the common ancestors of Mysticellus and Micryletta diverged about 40 million ago. Most likely they originally inhabited the Indian Peninsula and later diverged to give rise to both these genera,” said the authors.

The authors posit that the two genera are likely to have split when the Indian landmass moved close to mainland Southeast Asia through the Myanmar-Malay Peninsula during Middle/Late Eocene.

Karthikeyan Vasudevan from LaCONES said, “Recent evidence from the study of arthropods in amber suggests that prior to the final collision at around 55 million years ago with Asia, India moved close to or had land connections with Africa and Europe.” Some models of continental drift show that species could have moved between the Indian subcontinent and Southeast Asia prior to the former’s collision with Asia, he added.

“This might help explain the presence of genera that are not present in the Eastern Himalayas, but are found in South India, Sri Lanka and Southeast Asia,” he added. He gave examples of the skink genus Dasia and the pit vipers (genus Tropidolaemus).

David Blackburn from the Florida Museum of Natural History agrees. “Clearly, some lineages must have survived on India as it moved across the Indian Ocean during the late Mesozoic and early Cenozoic, but several studies, including this one, now support that colonisation of the Indian subcontinent by animals from Asia before India had fully collided with the Asian mainland,” he said.

The secretive frogs vanished after appearing for the breeding season which lasted for around four days. Photo by SD Biju.

With inputs from Sahana Ghosh.

CITATION

Garg, S., & Biju, S. D. (2019). New microhylid frog genus from Peninsular India with Southeast Asian affinity suggests multiple Cenozoic biotic exchanges between India and Eurasia. Scientific reports9(1), 1906.

Banner image: Mysticellus franki sp., a new species of frog discovered in the southern Western Ghats. Photo by S.D. Biju.

Article published by Sandhya Sekar
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