Rare, 1-Inch Frogs Raise Babies Inside Bamboo Stems

A male Raorchestes chalazodes, aka white-spotted bush frog. (Photo: Seshadri K.S./National University of Singapore).

Frogs can perform some amazing feats, like hearing with their mouths, using concrete storm drains as megaphones, raining from storm clouds and preventing old milk from going bad. Just when we think we've seen it all, though, these innovative amphibians surprise us with yet another biological leap.

Take India's white-spotted bush frog. First discovered in 1876, it was presumed extinct after nobody saw it again for 125 years. The species was rediscovered in 2003, then listed as critically endangered due to habitat loss and fragmentation. Only now, however, are we learning one of the weirdest quirks about this 1-inch frog: It breeds, lays eggs and raises its babies inside hollow stalks of live bamboo.

This is a previously unknown mating strategy, or "reproductive mode," but a new study reveals how it's been mastered by Raorchestes chalazodes. Scientists had documented a total of 40 reproductive modes used by frogs and toads — including 17 aquatic modes and 23 on land — so this represents the 41st, "which is different from all other known modes," according to the study's authors.

First, an adult male finds an internode on a bamboo stem with an opening near the bottom. (A high opening could let the stem segment fill with rain and drown the froglets.) Even though these frogs are only about 1 inch (25 mm) long, getting inside the bamboo can be a challenge since the openings are often less than 0.2 inches (5 mm) long and 0.1 inches (3 mm) wide. See this video for an example:

Once inside the bamboo, the male frog calls out to attract mates. These calls may draw more than one female, according to the researchers, yielding up to eight eggs per clutch. The male stays inside his bamboo to care for the eggs, which skip the tadpole stage and develop directly into froglets. He only leaves the bamboo for a few hours each evening to feed, then returns to look after his young.

"Amphibians are among the most threatened creatures on Earth, and yet we know very little about them," says lead author Seshadri K.S., a Ph.D. student at the National University of Singapore, in a statement. "I was enthralled when we observed this behavior and it opened a whole new world for me. There are several evolutionary questions that could be answered by studying this fascinating group of frogs. For example, what transpires inside the bamboo internodes is still a mystery."

R. chalazodes is actually one of two frogs that use this novel reproductive mode. One of the study's co-authors, Gururaja K.V. from the Indian Institute of Science, had previously seen the related Ochlandra reed frog (R. ochlandrae) breeding in bamboo internodes, but it had been ascribed to an existing reproductive mode that involves building nests. The researchers saw no nest-making behavior in this study, however, so R. ochlandrae was reclassified to the same mode as R. chalazodes, even though the frogs' ranges don't overlap and they rely on different species of bamboo.

bamboo frog eggs
R. chalazodes eggs inside bamboo, with some already hatching directly into froglets. (Photo: Seshadri K.S./NUS)

Both species live in India's Western Ghats mountain range, and the white-spotted bush frog was found within the wet evergreen forests of Kalakad Mundanthurai Tiger Reserve. That doesn't necessarily means it's safe, though — the species is critically endangered because it's scattered among small populations that occur in just five known sites, all dependent on plentiful bamboo. Unregulated overharvesting of bamboo for paper and pulp could destroy vital breeding habitat, Seshadri says, and even threaten the long-term viability of entire populations. More research is thus needed, both to shed light on the frogs' biology and to develop frog-friendly techniques for harvesting bamboo.

"The Western Ghats is a well-known hotspot for amphibian diversity which is facing threats primarily from habitat loss," says Seshadri, who's studying the frogs as part of his doctoral thesis. "If we do not initiate conservation efforts, we may lose everything before we even document anything."