How Frog Fathers Keep Rival Cannibals at Bay

To get a better understanding of the white-spotted bush frogs' unique breeding behaviour, Seshadri K.S. had to see what was going on inside hollow bamboo stems without disturbing them.

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Studying frogs in the field is like watching a “typical art film” unfold, says Seshadri K.S. “For eons, nothing happens. Then you blink and miss one crucial scene that tells the whole story.” After hours of watching pretty little green frogs sitting motionless next to their eggs, he witnessed one suddenly become a cannibal. And that explained why male white-spotted bush frogs make devoted fathers.

Seshadri was one of three people to identify the species (Raorchestes chalazodes) for the first time in more than 135 years. As a novice, he had joined R. Ganesan at the Ashoka Trust for Research in Ecology and the Environment (ATREE), Bengaluru, and S.D. Biju, University of Delhi, in their quest to find the two-centimetre-long frog. Drawn by an unfamiliar rapid staccato call at night, they found the caller – a luminous green frog with yellow starburst eyes – sitting on a bamboo culm in Upper Kodayar in Kalakkad Mundanthurai Tiger Reserve, Tamil Nadu. The excitement of beginner’s luck goaded him to study frogs, and what better species to consider than the one he had helped to rediscover.

One night, Seshadri and his colleagues from ATREE stood motionless as they watched one squeeze into a small hole in the stalk. “We were all super excited,” recalls Seshadri. Until then, no other frog was known to do that. After many nights of observation, he observed these frogs calling from inside the bamboo stalks.

An endoscopy image of a male bush frog guarding his eggs. Credit: Seshadri K.S.

An endoscopy image of a male bush frog guarding his eggs. Credit: Seshadri K.S.

K.V. Gururaja had discovered a related species, the ochlandra reed frog (Raorchestes ochlandrae) in Calicut district, Kerala, which lays its eggs inside the hollow internodes of reed bamboo. Frogs typically lay their eggs above standing water. No one knew of any frogs laying eggs inside bamboo until then.

Since white-spotted bush frogs wriggled into an internode with determination and called from inside, could they reproduce like ochlandra reed frogs?

As a doctoral student at the National University of Singapore, Seshadri sliced open bamboo culms and found their eggs inside. At the start of the reproductive season in June, males look for appropriate spots in stands of reed bamboo (Ochlandra travancorica) that grow along streambanks. When they find bamboo sections with holes, they call vociferously, ballooning vocal sacs in their throats acting as amplifiers. Not any access to the internode hollow will do. If it’s at the top, then rainwater can drip inside and drown the eggs. Prime real estates are the ones with holes at the base.

When a female shows interest, the pair enters the tubular chamber, climbs up to the top of the column where she lays eggs and sticks them to inner walls of culms with a slimy substance. The male fertilises them with his sperm and then stands guard while the female departs.

Bush frogs buck the standard frog trend. They don’t hatch as aquatic tadpoles that metamorphose to terrestrial adults. Instead, they emerge from eggs as fully-formed frogs.

(D) Newly hatched froglet, (E) Froglet after emerging from internode. Note the colour change. Credit: Seshadri K.S.

(D) Newly hatched froglet, (E) Froglet after emerging from internode. Note the colour change. Credit: Seshadri K.S.

Why do the males of these green frogs stay with their eggs?

The glassfrogs of South America, for instance, keep theirs hydrated with urine and protect them from predators. Do these bush frogs pee on their eggs too?

“They might be,” Seshadri told The Wire. “The urine may be providing the much-needed moisture and may also be providing nutrition. But we do not know what is happening inside.”

He scoured bamboo thickets in an eight-hectare patch of rainforest to see where the frogs were and how many of them were about. Once he located about 134 frogs, he observed them from dusk to midnight every day.

But these nocturnal amphibians have a knack of doing things out of sight of biologists, as Jesse Delia of Boston University and his team found while working on neotropical glassfrogs. After midnight, could male bush frogs trade the egg-tending job with females, for instance?

A stand of reed bamboo where white-spotted bush frogs breed. Credit: Seshadri K.S.

A stand of reed bamboo where white-spotted bush frogs breed. Credit: Seshadri K.S.

Seshadri installed ‘Gardenwatch’ cameras that took a photograph every five seconds throughout the night. From this time-lapse sequence, he noticed the males didn’t leave their eggs, so he felt confident he didn’t have to continue observing into the small hours.

The air temperature inside the 60-centimetre column of the internodes was cooler and more humid than the outside, providing ideal conditions for the embryos to develop. Since the eggs don’t have a shell to hold moisture in, they could dry up, killing the developing froglets. The high humidity within the hollow makes up for the lack of standing water.

To get a better understanding of the species’ breeding behaviour, Seshadri had to see what was going on inside the hollow stems without disturbing them.

“Perhaps the biggest challenge was the lack of any similar work in this region,” says Seshadri. He could have opted to observe captive behaviour. “We did not know when the frogs breed, how many were out there, do they stick around for the whole period of breeding, where do they go when they are not breeding, etc.” So studying them in captivity was out of the question. He had to figure out ways of investigating frog behaviour in the wild by himself.

A female white-spotted bush frog. Credit: Seshadri K.S.

A female white-spotted bush frog. Credit: Seshadri K.S.

“I spoke to a few surgeons, and the endoscopes they use are either expensive or unavailable,” says Seshadri. “I went to a hardware store mall in Singapore with a printout of an endoscope. It did the trick, and I bought two kinds within an hour. We used the two endoscopes for a month, and both broke. Then I got a new, cheaper one but had to buy a field computer to use it. Things fell into place eventually and we were able to observe the frogs without harming them or altering their behaviour too much.”

He widened the hole in the bamboo so the probe of the endoscope could fit and peered inside. He found 43 clutches of eggs at 32 sites and monitored them every day. During the day, the males sleep next to the eggs with their eyes half-closed. But at night, as soon as Seshadri poked the equipment into the bamboo, most of them stopped calling and hid behind the eggs or at the base of the internode. Some brave ones attacked the endoscope or called aggressively to it.

To investigate how critical the male was to egg survival, Seshadri experimented. Katydids and snakes are known egg-eaters. The tiny slit in the bamboo would be too small for these predators. Seshadri caught the guardian males of 13 clutches and released them on the other side of the stream, leaving the eggs with no protection. He compared the fate of this group with another group of 13 clutches that had paternal care.

In less than three weeks, about 70% of the fatherless clutches fared poorly. Ants, flies and fungal infections killed some of them. But the main threat to offspring survival was other male frogs that ate about 80% of the lost eggs. Only 34 of 129 orphaned eggs hatched.

Compared to this dismal rate, froglets emerged out of almost all the eggs that had a guardian. Only one egg rolled down into water collected at the base of the internode and drowned. Other species of frog parents prey on flies and ants that come for their eggs and eat infected eggs before they can contaminate others. Perhaps white-spotted bush frog dads maintain the health of their eggs in the same manner.

A male guarding eggs inside an internode of reed bamboo. Credit: Seshadri K.S.

A male guarding eggs inside an internode of reed bamboo. Credit: Seshadri K.S.

“This work provides solid experimental evidence demonstrating the functional benefits of male-only care in an endemic Rhacophorid [tree frog family] frog,” Jesse Delia told The Wire. “I think this species association with bamboo is really fascinating! The authors use ‘spy’ technology to get a peek at the everyday life of father frogs as they care for eggs inside living bamboo. I think these details are wonderful – it can be extremely challenging to get basic behavioural observations on secretive species.” Delia wasn’t involved in the study.

Could this effort come at a cost to the males? They could be out feeding and mating during the time they spend guarding the eggs. But these bush frogs don’t seem to suffer much from lost opportunities.

They spend time every evening finding prey before returning to egg-sitting duties. Since the frogs are particular about what kind of internode makes a good nursery, ideal spots are in short supply. A male in possession of one is a catch. He sings to advertise his asset, wooing females to spawn in his lair. He doesn’t have to abandon his eggs to go looking for a mate.

Seshadri had observed that for every male that guarded eggs, two to three others hung around with no eggs of their own. Fathers call incessantly through the night to ward off these rivals with cannibalistic tendencies. When such prized locations fall silent after the researcher removed them, the other males probably realised the hollow was theirs to take. In addition to gaining a breeding spot, they get nutrition by eating eggs.

Did cannibalistic frogs then proceed to serenade females and tend their own eggs?

“We did find the cannibalistic males in the usurped internode,” says Seshadri. But he captured the males to take measurements. Although he released them, they abandoned the internodes. “We did not really observe them attracting a mate and attending to eggs after cannibalising an existing clutch.”

All this parental devotion pays off when, approximately 37 days later, froglets hatch. They hang around their natal spot for up to a month, absorbing the yolk reserves in their bellies. Their fathers remain with them but don’t seem to take particular interest in their young anymore. Eventually, they lose all interest in their prized bamboo stem territories. When Seshadri hunted for them in December and January, outside the breeding season, he couldn’t find any.

This is the first member of the tree frog family to show paternal care that prevents cannibalistic rivals from causing damage.

Biologists know of only five locations where the white-spotted bush frog lives. People harvest reed bamboo during the amphibian’s breeding season, a potential threat for the future of species. Indeed, one researcher sipping tea at a roadside stall heard the call of a new frog species (Raorchestes manohari) from a lorry piled high with harvested bamboo culms.

“In addition to expanding a small but growing literature on male parental care in frogs, this study provides key ecological information on the importance of native bamboo to the health of this species,” says Delia. “Both humans and frogs enjoy bamboo, so having these types of data can be extremely useful for conservation planning.”

The study was published in the journal Behavioral Ecology and Sociobiology on December 14, 2017.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.