- White-spotted bush frogs breed in bamboo cavities, which are highly sought-after by males for luring females.
- Males are the sole caregivers protecting the eggs from predators and parasites.
- In the absence of the male, most of the eggs are lost to other competing males, making it the first bush frog species to practise egg cannibalism.
At the southern tip of India’s Western Ghats, on the edge of a wet evergreen forest stands a tall copse of reed bamboo. Dotting the bamboo’s hollows are round, transparent eggs with a creamy white centre. In about a month, the eggs will hatch into tiny froglets. The glittery green frogs will stick around for a while and then leave the safety of the bamboo. But until they do, their father will watch over them.
Enter the secretive world of the white-spotted bush frog Raorchestes chalazodes, a ‘critically endangered’ species. Limited in its presence to the Agasthyamalai Biosphere Reserve, the frog breeds only inside bamboo stalks. After scouting out a suitable nesting site – a bamboo internode with a tiny opening to squeeze through – a male bush frog launches into a musical trill. Impressed, a female enters the hollow. She lays five to eight eggs and glues them on to the wall. The male then fertilizes the eggs and off goes the female, leaving the eggs behind in the male’s care.
The male guards the eggs alone and frequently calls from within the hollow to signal his presence, finds a study by K. S. Seshadri of the National University of Singapore. The father frog even sleeps next to the eggs with eyes half-open. If an intruder like a cockroach, katydid or another male frog enters the bamboo, he warns it off by calling aggressively and then lunging at it. This threat call is different from the mating call, says Seshadri.
The doting father apparently doesn’t even leave the bamboo to hunt for food. It probably feeds on ants and other insects that walk into the bamboo in search of food, says Seshadri. And rightly so: if left home alone, the eggs are sitting ducks for predators and parasites.
Death by predation
Seshadri learned how vulnerable the eggs were without their fathers in experiments he conducted over 2015 and 2016. He removed the parent male from 13 cavities where males guarded their eggs and, for comparison, left another 13 cavities undisturbed. Using an endoscope – a tube with a camera at its end – he peeked inside the bamboo every day to monitor the number and state of the eggs.
About 73 percent of the eggs in the 13 unguarded sites were wiped out within one to 17 days of removing the males. In the other 13 sites, where parent males were left untouched, only one egg died. About 83 percent of the total egg deaths in the unguarded sites were caused by predation; the rest by dehydration, fungal infection and parasitism by flies.
Interestingly though, other bush frog males of the same species were responsible for the majority (60 percent) of the deaths. This makes it the first bush frog species to practise egg cannibalism. The eggs are nutritionally rich, says Seshadri, so if a male finds unguarded ones, he readily eats them.
High demand, short supply
A shortage of nesting sites might also drive cannibalism. The egg-laying sites are of importance; they are not abundant and whatever is available is occupied, says Seshadri. There are definitely some males that end up not getting one.
That’s because the holes in the bamboo internodes that the frogs use as entry-exit points are made by other animals (such as squirrels that nibble on the bamboo). The hole also needs to be towards the base of the internode, so that rainwater doesn’t fill up the cavity and drown the eggs. Seshadri saw males fight over such coveted territories.
Males that did not have eggs to look after also tended to stay near the sites occupied by males guarding their eggs, he said. When these faithful fathers were removed during the experiments, Seshadri suspects, the cavities became quiet and noticing this, perhaps those other males came in to check and took over the territories.
“Many studies of parental care are just observational,” said Jodi Rowley of the Australian Museum Research Institute. “So it was nice to see the manipulative experiment proving that the parental care served a really practical purpose – protecting their eggs.” It is also a really poorly-known frog, so this is a really important contribution to our understanding of the species, which will hopefully help conserve it, she added.
Seshadri, K.S., & Bickford, D.P. (2018). Faithful fathers and crooked cannibals: The adaptive significance of parental care in the bush frog Raorchestes chalazodes, Western Ghats, India. Behavioral Ecology and Sociobiology, 72:4. https://doi.org/10.1007/s00265-017-2420-3.
Seshadri, K.S., Gururaja, K.V., & Bickford, D.P. (2015). Breeding in bamboo: A novel anuran reproductive strategy discovered in Rhacophorid frogs of the Western Ghats, India. Biological Journal of the Linnean Society, 114: 1–11. https://doi.org/10.1111/bij.12388.