Nesting Inside Bamboo : A Novel Reproductive Strategy Discovered in Rhacophorid Frogs


Raorchester chalazodes; Courtesy: David V. Raju

Frogs belong to the order Anura. Anurans have an immense number of reproductive modes amongst all vertebrates with numbers as high as 40. Frogs are usually known to undergo larval development through a free-living aquatic tadpole. But this is not always the case as a direct development of frogs has also been observed in a few species. Direct development is when the tadpole stage is completely bypassed and the embryos develop within the eggs from which froglets hatch directly. But with this mode or any mode that involves terrestriality comes the increased risk of predation. Thus, to combat this factor, diverse species of anurans have increased their survival offspring survival chances by providing care to eggs and offspring. For instance Eleutherodactylus coqui from Puerto Rico, Aromobatid frogs from Peru, and Hyalinobatrachium valerioi from Costa Rica. Anurans parental care is divided into 8 different types, i.e, egg attendance, egg transport, egg brooding, tadpole attendance, tadpole transport, tadpole brooding, tadpole feeding, and froglet transport. Amphibian evolutionary ecology is attracting costs and benefits of parental care as a reproductive tactic. Factors that determine whether or not parental care will be prevalent are stable and structured habitat, unusually stressful environment, scarce or specialized food resources, and predation pressures. Parental care has been observed to increase survivorship of organisms. These constraints thus affect reproductive fitness and thereby result in a new adaptation by the means of natural selection.



One of the biodiversity hotspots of India, Western ghats, has become profoundly important for studying and understanding amphibian diversity, evolution, and biogeography. Nest building in Nyctibatrachus kumbara and Rhacophorous lateralis and foot flagging behavior in Micrixalus aff. saxicola Jerdon are some of the recent discoveries in anurans. Habitat diversity and geological history have led to high diversity in several families. Western Ghat is considered as a hub of adaptive radiation and endemism. Over 180 species of anurans are found in the Western ghats. Members of the family Rhacophoridae form a significant part of this figure. There is a great diversity of the modes of reproduction as well with 15-40 known from the region. Four species are found to be having direct development mode. 



A finding by K.S. Sheshadri et al. observed a novel mode of reproduction in an arboreal frog, Raorchester chalazodes from the forests of the Western Ghats. This species was presumed to be extinct until it was rediscovered recently from the wet evergreen forests of Kalakad Mundanthurai Tiger Reserve.



This new mode of reproduction can be summarised as male paternal care for developing eggs. These eggs are laid inside a hollow bamboo internode which does not contain stagnant water. The shoot must also contain a narrow exit/entry opening. These openings are presumably made by insects or rodents. The males usually vocalize near the internode nocturnally in the bamboo Ochlandra travancorica. The males and presumably the females enter the bamboo by these small holes. These openings are usually located towards the base of the internodes. About 5-8 eggs are deposited roughly 25 cm above the opening. It was found that the adult males remain inside, vocalizing, until the eggs hatch. Thus the male presumably mates and results in the presence of multiple clutches near the internodes. The froglets after hatching were found to reside near the capsules. The eggs are attached to the inner walls of the bamboo using a mucilaginous strand. Understanding of anuran behavior in an evolutionary framework is rather inadequate due to a lack of life history data. 


R. chalazodes entering the bamboo internode through an opening; Courtesy: K.S. Seshadri

Females are presumably polyandrous and perhaps shift from one internode to another in search of more mates. If amplexus is present it must occur inside the internode. Frogs prefer the bamboo where the holes are below the internodes because this will prevent water from accumulating and flooding the internode thereby drowning the froglets.



This mode of reproduction is similar to that of another species, R. ochlandrae. This similarity suggests that these two species may be a result of parallel evolution or maybe ancestral lineage. Nevertheless, the phylogeny is not currently available to ascertain this theory.



Thus it can be assumed that parental care enhances reproductive success. There are typical characteristics and trajectories in anurans having parental care. These include smaller clutch sizes, larger eggs, and egg attendance. Parental care is also observed more for those frogs living in lotic environments or areas with seasonal small water bodies. The studies and data that consider costs, benefits, and fitness effects of such parental care behavior are very sparse.



R. chalazodes are critically endangered and are found in only five localities. Bamboo nesting frogs like R. chalazodes face an ominous threat from habitat loss as bamboos are harvested extensively outside protected areas for pulp and paper. Often it is observed that harvesting accords with the breeding seasons of frogs which results in the decimation of the frog population. Therefore unregulated harvesting might negatively impact the long term viability of frogs. Consequently, studies are required to find measures to counter this kind of harvesting and find new ‘frog friendly” harvesting methods.



REFERENCE

[1]K. Seshadri, K. Gururaja and D. Bickford, "Breeding in bamboo: a novel anuran reproductive strategy discovered in Rhacophorid frogs of the Western Ghats, India", Biological Journal of the Linnean Society, vol. 114, no. 1, pp. 1-11, 2014. Available: 10.1111/bij.12388.

[2] K. V. Gururaja, K. P. Dinesh, H. Priti, and G. Ravikanth, “Mud-packing frog: A novel breeding behaviour and parental care in a stream dwelling new species of Nyctibatrachus(Amphibia, Anura, Nyctibatrachidae),” Zootaxa, vol. 3796, no. 1, p. 33, 2014.

[3]S. Biju, "A novel nesting behaviour of a treefrog, Rhacophorus lateralis in the Western Ghats, India", Current Science, vol. 97, pp. 433–437, 2009. [Accessed 30 August 2020].




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